Chapter I (2)

The influence of the opposite conditions in reference to ocean currents experienced by the eastern and western borders of the continent are even more marked in the life of the adjacent waters than in the vegetation and fauna of the land itself. The plant and invertebrate life of the shoal waters of the Pacific coast, consisting largely of southern species, is exceedingly rich and varied, even to the inlets of the Alaska coast, where glaciers come down to the sea; while on the Atlantic border, northern species occur on the New England coast, and even farther south. The contrasts in temperature between the waters of the Atlantic and Pacific which cover the submerged border of the continent are well shown by the distribution of the cod, the most valuable of all fishes to man, which, as is well known, belongs to the northern fauna and ranges from the north Atlantic about the arctic coast of both the Old and the New Hemispheres, to the north Pacific. On the east coast of America this circumpolar fish, of which several species are known, is found occasionally as far south as Cape Hatteras, but the most southern "bank" on which it is extensively taken is off Cape Cod, in latitude 42 deg.; on the west coast it travels perhaps as far south as the mouth of the Columbia, but the most southern locality where it occurs in commercial quantities is off the Shumagin Islands, in latitude 55 deg.. The life of the continental shelf, as well as of the adjacent land, thus bears testimony to the vast importance to North America of the great ocean currents washing its shores.

_Tides._--The waters of the ocean are subject to wave-like undulations, caused by the attraction of the moon and sun, termed the tides. Every day, at the average interval of twelve hours and fifty-one minutes, the "tide rises," and with equal regularity intermediate between these periods it "falls." This rise and fall of the waters along the coast, accompanied frequently by strong currents, are produced directly by the arrival in the shoal water of a pulsation of the ocean, which becomes a true onward-moving gravity wave as it nears the land. In the open sea the amplitude of the tidal undulations is but two or three feet, and their rate of travel in general 700 to 800 miles per hour. On reaching shoal water, however, the onward movement is decreased by friction on the bottom, the waves become higher, and when they meet an outward-flowing bottom current, their bases are still more retarded and the slope of their fronts increases until the water falls forward and breaks into foam. On the Atlantic coast, each tidal wave reaches the land broadside on, as it were (Fig. 4), and at the outer capes high water occurs at practically the same time from Florida to New England, but its farther landward progression is greatly modified by the shape of the coast and the depth of water in the estuaries and other indentations. When the wave as it rushes landward enters a broad water body through a narrow entrance, as the Gulf of Mexico, for example, it spreads, and as the impulse is transmitted to larger and larger volumes of water, it decreases in height. (In a critical study the tides originating in the Gulf itself should be considered.) At Galveston, Texas, the mean range between high and low tide is less than one foot. When, however, an estuary with a broad mouth receives a tidal wave from the ocean, the impulse is more and more concentrated and the wave rises higher. At the head of the Bay of Fundy the difference between high and low water is from 50 to 60 feet. For the reason just stated, the tidal wave is generally higher in the Atlantic coast estuaries than on the ocean capes, and under favourable conditions may be transmitted for long distances up the rivers emptying into such estuaries, and may be felt where the mean elevation of the stream is several feet above the mean level of the sea on the neighbouring open coast. Tidal waves pass up the Hudson to Troy, a distance of 150 miles from the Narrows, where the mean range is 2.3 feet. In St. John River, New Brunswick, the tidal impulse is felt at Frederickton, 70 miles from the Bay of Fundy, and at an elevation of 14 feet above its surface. In the St. Lawrence estuary and river the tidal waves ascend 283 miles to Three Rivers, a few miles below Montreal, where the mean elevation is about 11 feet and the mean range of the tide 0.9 foot. In the Columbia the range of the tide is about 6 inches at a distance of 140 miles from the ocean.

In the north Pacific (Fig. 4) the tidal waves come from the south and expand much the same as the corresponding waves do in the north Atlantic, but instead of striking the coast broadside on, sweep along the shore from south to north.

There are two localities on the coast of North America, one at the head of the Bay of Fundy and the other at the head of Cook's Inlet, Alaska, where the tides present especially interesting features. In each of these inlets the incoming tidal wave meets an outward-flowing current which tends to hold it back. The incoming waters are thus piled up until sufficient head is established to cause them to advance as a steep-fronted wave termed a _bore_, which curls over and breaks in a long line of foam as it rushes along. At the head of the Bay of Fundy the bore travels at the rate of 6 or 7 miles an hour, and has a height of from 4 to 6 feet. The great disturbance produced by the strong current and breaking waves causes the mud of the bottom to be disturbed and the waters to be charged with sediment. Much of this mud is deposited during the interval of quiet water at high tide, and as the outflow is not so impetuous as the inflow, broad mud-flats are formed. At certain localities about the Bay of Fundy artificial dikes have been made, which admit the mud-charged waters at high tide, and retain them until much of their freight is deposited. In this manner, large areas of rich lands have been secured.

The geographical influences of the tides and of the currents produced by them are of interest in many ways. To navigators they are of special importance. Even in this age of steam, the arrival and departure of vessels from harbours is regulated so as to take advantage of the incoming or outgoing tidal currents. Many harbours can be entered by deep-draft vessels only at high water, for the reason in general that sand-bars are frequently formed at the mouths of tidal estuaries. One of the most marked illustrations of the influence of the rise and fall of the tide on navigation occurs at St. John, New Brunswick, where the tide flows in and out of St. John River so as to form a cascade each way, dependent on the direction of the current. At low water the level of the river is from 11 to 15 feet above the Bay of Fundy, and at high water the level of the bay is from 8 to 12 feet above that of the river when not affected by the tide. There are four periods of from ten to fifteen minutes each during each twenty-four hours when vessels can pass in and out of the river's mouth.

In the Arctic Ocean the tides are small. At Point Barrow, the most northern locality on the coast of Alaska, the difference between high and low water is but 6 or 7 inches. The tide comes from the southward and westward, and there is a prevailing current setting to the eastward. At Herschel Island, near where the east boundary of Alaska reaches the Arctic Ocean, the mean range of the tide is but 1.8 foot. At Cape Sheridan, the northeast point of Grinnell Land, north latitude 82 deg. 25', there is a range of 2.6 feet during two periods each month when the tides are highest, and but 1.2 foot at the lowest or neap tide periods.

In addition to the weakness of the tides and tidal currents along the arctic coast, there is an absence or great diminution of the influence of wave and currents, owing to the prevalence of ice on the sea. Shore erosion is there at a minimum in spite of the abrasion produced by the ice-packs when forced landward by the wind.

_Islands._--In the classification of islands used by A. R. Wallace in his Island Life two primary divisions are recognised, namely, continental and oceanic islands.

Continental islands are land masses which have been separated from continents and are rarely far removed from their borders, and, besides, are composed of rocks similar to those of the neighbouring mainland and inhabited by terrestrial animals which are related to the fauna of the larger land area. Ancient and modern continental islands have also been recognised, their age being indicated by the degree of similarity between their faunas and the fauna of the continent with which they were formerly connected. Those of ancient origin are commonly surrounded by deep water, while those which are more modern usually rise from continental shelves, the channels intervening between them and the mainland being less than 100 fathoms deep. Oceanic islands rise from deep water, are either volcanic or so far as their emerged portions are concerned composed of coral rock, and are without warm-blooded terrestrial animals.

About the borders of North America there are islands belonging to each of these classes. The numerous examples rising from the continental shelf all about the margin of the land, but in the Atlantic most numerous from New York northward, and in the Pacific from the Strait of Fuca northward, are plainly recent continental islands. The larger of the West Indies and the group of small islands off the California coast are also continental islands, but show by the character of their faunas and the depth of the water about them that they have been long separated from the main mass of the continent. Typical examples of oceanic islands are furnished by Bermuda, in the Atlantic, and Guadalupe, in the Pacific. In this same class, but less remote from the mainland, and in their faunas and floras showing a nearer relationship to South than to North America, belong the Caribbees.

TOPOGRAPHY OF THE COAST

The generalized coast-line of North America measures about 35,000 miles in extent, and presents a great variety of scenery. The range in diversity embraces all classes of coast topography from the low, sandy mangrove-fringed borders of Florida and the Gulf of Mexico, to the magnificent sea-cliffs of Labrador and British Columbia and the marvellous ice-walls of Greenland and Alaska where tide-water glaciers enter the ocean.

Like nearly all the features of the earth's surface, this narrow intricate belt where the sea and land meet is constantly undergoing changes. The principal processes which lead to alterations in the coast-line may be considered as forming three groups: First, the wearing away of the land through the action of waves and currents and the deposition of the _debris_ thus produced so as to make additions to the borders of the continent; second, the upward and downward movements of the land; and third, the changes produced by glaciers, ice-flows, and icebergs.

With these more active agencies by which the coast-line is being modified may be included chemical solution and deposition, the influence of plants and animals, the weathering of the margin of the land, etc.; but a critical review of all these processes is impracticable in the present treatise.

_Changes in the Coast-Line due to Waves and Currents._--The waves of the sea beat on the land with never-ceasing activity, but exert the greatest force during storms. The blow which a great surge strikes when it breaks at the base of a cliff, amounting in many instances to 3 or more tons to the square foot, tends to disrupt the rocks both directly by its impact and by the compression of air and water in their interstices. The greatest work of the breaking waves is performed, however, with the aid of the stones which accumulate on the beaches. These are hurled against the land by the force of the landward-rushing waters and break and abrade the rocks with which they come in contact. The friction produced by the impact of waves charged with sand, pebbles, and boulders against the land leads to its removal along a horizontal belt with a narrow vertical range. The waves of the sea, in fact, act like a horizontal saw, the edge of which slowly advances landward. As a result of this process of under-cutting, highly characteristic and frequently most picturesque forms are given to rocky coasts. Whenever the sea is bordered by hard rocks standing well above the surface, but not rising too precipitously from deep water, we find cliffs facing seaward. At the base of each of these sea-cliffs there is a shelf or terrace which records, in part at least, the advance that the sea has made inland.

A cross profile of a wave-cut seashore (Fig. 5) shows two prominent features, namely, a sea-cliff with a horizontal base, and a terrace sloping seaward from the foot of the cliff. Of these, the cliff is by far the more prominent as it stands up boldly to view, while the terrace is in large part and perhaps wholly submerged. These two leading characteristics in the topography of wave-cut shores are shown in the following diagram:

The water carried landward by each wave as it rushes up the sloping surface of a terrace again finds its way seaward, either wholly or in part, as an "undertow." Much of the rock _debris_ ground fine by the ceaseless beating of the surf is separated from the coarser material, thus leaving the latter free to be moved by succeeding waves, and is carried seaward by the bottom current or undertow. During storms especially there is usually to be seen a belt of discoloured water seaward from the white breakers which margin the land. The finer _debris_ carried away from the shore by the undertow is sooner or later deposited, and much of it is laid down on the terrace bordering the land and serves to build out its seaward margin. A normal sea-terrace is thus in part the result of the cutting away of the land, and in part of the deposition of the material removed. The sea not only cuts away the land, however, but at many localities makes important additions to it.

Where the water is shallow the larger waves break at a distance perhaps of several miles from the coast-line, and build up long narrow bars, usually of sand, which form barriers, more or less parallel with the shore, and shelter it from further encroachments of the sea. Again, when the wind from the sea blows obliquely to the coast, currents are established in the water which sweep along the loose material on the beach and on the submerged portion of the terrace of which the beach is a visible part, and cause it to travel in the general direction of the prevailing on-shore winds. This action also leads to the building of bars more or less parallel with the coast and at the extremities of capes, particularly where the shore currents enter deeper water and give origin to spits of various shapes, which are frequently curved towards the land and at their extremities become hooks and loops.

There are thus two important processes, one destructive and the other constructive, by which the sea is continually modifying the border of the land.

When once the underlying principles on which depend the characteristics of coastal topography are suggested, any observant person can apply them for himself and thus be able to read the history as well as admire the beauties of seacoast scenery. It is not necessary, therefore, to attempt to present a detailed account of the coasts of North America from a purely geographical point of view; there are certain results of the processes just referred to, however, which are of wide-reaching economic, and especially of commercial interest.

From Central America northward to Cape Cod the rocks bordering the sea are soft or easily soluble, and the adjacent land of low relief. Throughout this section the work of the sea is mostly constructive, and the margin of the land is sheltered by sand-bars from the attack of waves and currents. Where the waves of the open ocean do reach the land, as on the coast of New Jersey, the sea-cliffs are low and the topography of a mild type. Very generally, as along the coast of Mexico and Texas, and from Florida to Long Island, there are long narrow bars adjacent to the shore, with lagoons intervening between them and the mainland. These features are well illustrated on the accompanying map (Fig. 6) of a portion of the Atlantic coast where long narrow bars, sometimes forming skeleton capes, are a characteristic feature. On the middle Atlantic coast of the United States the prevailing winds blow southward and there is a general southward flow of the shore currents, which carry with them the sand on the beaches and bars. An interesting fact in this connection, pointed out by N. S. Shaler, is that although the sands are continually being moved they are not worn out. After the sand-grains have been reduced to a certain size they retain films of water which separate them one from another, and act as cushions which prevent the grains from coming in contact, thus greatly retarding further comminution. But for the protection thus afforded the sand-bars would be removed and the border of the land exposed to the attack of the waves and cut away; whereas under existing conditions lagoons are formed, which in many instances are utilized as harbours or are filled by wind-blown sand, the sediment brought by streams, plant growths, etc., and valuable additions are made to the continent.

The sand-bars just referred to frequently cross the mouths of rivers, and in such instances a struggle ensues between the currents moving along the shore and the outflowing river-waters aided by the currents produced by the tides. This conflict leads to the formation of sand banks and bars, generally submerged, across the entrances of bays and inlets and to the building of sand-spits from the seaward capes. A typical instance is furnished at the entrance of Mobile Bay (Fig. 7), where a spit from each side has been built by shore currents so as to greatly contract the tideway between. Similar features are presented by Sandy Hook and Coney Island, each of which has been built of sand deposited by shore currents at the seaward entrance of the lower New York Bay. Another illustration of this same general character is furnished by the curved extremity of Cape Cod (Fig. 8), which is a sand-spit of large size with a hooked extremity. Spits of this nature are common on our coasts, and in many instances themselves form harbours, as at Coney Island and near the extremity of Cape Cod. Many other similar examples of the importance of lagoons, sand-bars, spits, etc., to shipping, which occur, especially along the Atlantic coast of the United States, may be studied to advantage on the admirable charts of the United States Coast and Geodetic Survey.

The sand-bars, spits, and other similar structures along the Atlantic coast are also of strategic importance, for the reason that they afford advantageous sites for fortifications, as is illustrated by the strong forts at Sandy Hook which guard the entrance of New York Bay. These sea-built foundations are also utilized in a large number of localities for lighthouses. The waterways shut off from the sea by off-shore bars in some instances permit of the passage of vessels from one harbour to another. In this connection it is of interest to note that an important system of canals is under consideration for making a continuous waterway for deep-draft vessels, some 700 miles long, which will connect the estuaries and lagoons from New York to the Carolinas.

While the islands of sand referred to present many conditions favourable to commerce, fisheries, and other industries, their apparent durability is deceptive, and in some instances faith in their permanence has led to disastrous results. They owe their existence to the action of waves and currents, and unless blown sand is heaped upon them are raised but a few feet above mean sea-level, and are liable to inundation if a high tide is accompanied by an on-shore gale. A sad illustration of this plain conclusion is furnished by the disaster that overwhelmed the city of Galveston on the night of September 8, 1900, during which some 3,000 people perished and $20,000,000 to $30,000,000 worth of property was destroyed. This great loss was in large part due to the fact that the city was inundated by the advance over its site of the storm-driven waters of the Gulf of Mexico. The island on which Galveston stands (Fig. 9) was built by the waters of the Gulf, and during the hurricane referred to they again claimed their own.

Northward of Cape Cod, the rocks adjacent to the ocean are mostly hard and resistant, consisting largely of schist, gneiss, granite, trap, etc., which when undercut by the waves stand as bold cliffs and headlands. This portion of the continental border abounds in picturesque scenery and is abundantly supplied with fine harbours and well-sheltered havens in which boats may take refuge. Typical portions of this rugged coast are furnished by the magnificent sea-cliffs of Mount Desert and Grand Manan islands, the bold shores of Newfoundland and Labrador, and the precipitous border of Greenland. The scenery throughout nearly all of this vast extent of wave and storm beaten rocks is in striking contrast to the mild and generally monotonous sand-built shores to the south of Cape Cod. Between the angular headlands and rugged capes at the north, with their white girdles of surf, there are frequently curved beaches and numerous spits and bars of yellow sand which connect the salients of the shore or extend from them so as to furnish safe anchorages.

On the arctic coast of North America the action of the waves and currents on the land is greatly retarded by ice, and the tides are small, but to what extent these conditions unfavourable to the work of the sea are counterbalanced by the abrasion performed by ice-floes is unknown. The northern border of Alaska, as well as the shore of Bering Sea, is mostly low and the rocks soft, although certain of the sea-capes are bold and are evidently composed of resistant material.

The Aleutian Islands present a peculiar exception to the general coast topography of the rest of the continent. Although this region has not been studied in detail, it seems to furnish an example of a rugged mountain range that has been partially submerged at a comparatively recent date. The rocks in many places descend precipitously into deep water, leaving no room for the formation of beaches, and hence the waves, to a great extent, are without tools with which to cut away the land. At the heads of the many bays and inlets, however, one finds beautiful sand-beaches with gracefully curving lines, in striking contrast to the dark, rugged cliffs bordering their seaward extensions.

The southern and southeastern borders of Alaska are exceedingly bold, and present some of the most sublime coast scenery to be found in the world, but to the geographer the greatest interest of this portion of the continental border, as is true also of the entire Pacific coast of North America, centres in its relation to up and down movements of the land.

_Changes in the Coast-Line due to Oscillation of the Land._--Land areas are exposed to the erosive action of wind, rain, streams, etc., and are sculptured by these agencies into valleys, canyons, peaks, ridges, and other familiar topographic forms. The various processes by which land areas are modified lead in general to a roughening of the surface. As an extreme illustration, a high plateau becomes dissected by streams so as to form an intricate system of rugged mountain ridges and peaks, with deep, steep-sided valleys between. The degree of this roughening depends principally on the elevation of the land, together with contrasts in the resistance of the rocks due mainly to variation in hardness, climatic conditions, etc., but in general one may say the higher the land is raised above the sea the more rugged will be its topography as the process of wearing away progresses. It is to be remembered in this connection, however, that land areas pass through a somewhat definite series of changes, from topographic youth to topographic old age, each stage being accompanied by changes in the relief. It is during topographic maturity that the greatest roughness of the surface of a land area is produced.

Land areas are continually wasting away, owing especially to the attacks of the streams, and the material removed is deposited in the sea. The _debris_ brought from the continents by streams is laid down in shallow water--about the shores of North America almost entirely on the surface of the continental shelf--and in this region of deposition the hollows are filled and a generally smooth surface given to the sea-floor.

The topography of the land, for the reason stated above, is nearly everywhere uneven; while the topography of the sea-floor is characterized by uniformity. We can easily predict, therefore, the general character of the changes in a coast which would result from either a subsidence of the land, thus allowing the sea to encroach upon it, or of an elevation, which would expose a portion of the sea-bottom, thereby increasing the area of the land. A subsidence of the land adjacent to the sea permits an extension of the waters landward; the sea will enter the valleys so as to form estuaries, bays, straits, etc., while the high land between the partially water-filled depressions will rise above the water-level and appear as peninsulas, capes, and islands. A bold, deeply sculptured coast when depressed will give origin to an intricate, and what may be termed a ragged shore-line; while a lower region crossed by large river-valleys would be changed to a system of broad estuaries.

An upward movement in the earth's crust along the ocean's shore would expose a portion of the sea-floor and add a strip of generally level country to the previous land area. The boundary between the old and new topography in such an instance would be the upraised coast-line with its sea-cliffs, wave-cut caves, terraces, beaches, and other characteristic features of coast topography.

There are thus two strongly contrasted types of coast scenery, produced by oscillations of the earth's crust where ocean and continents meet. In each class there is a wide range in details, which vary in harmony with the amount the land rises or falls in reference to sea-level.

When one has these general laws in mind a map of the coast-line of North America acquires great significance.

From about the latitude of New York southward to Central America many comparatively small oscillations of the land have occurred in recent geological time, and what was formerly a portion of the continental shelf is now exposed and forms a coastal plain. This plain, in general from 50 to 100 miles broad, slopes gently seaward, and its continuation under the sea forms the present continental shelf (Fig. 2). Evidently a slight up or down movement or a gentle tilting of this partially submerged plain in an east and west direction would cause a marked advance or recession of the sea. Each time the sea advanced the country submerged would be smoothed over by the action of the waves and currents and a sheet of sediment laid down upon it; and each time the sea receded the emerged land would be trenched by the rivers flowing across it. The records show that many such changes have occurred.

The Gulf border of Mexico and Texas, composed of soft marine sediments, forms a gently sloping plain bordered on the west by a roughened upland, and illustrates the general feature of a recently emerged coastal plain (Fig. 9). The same is true also of the entire coast from Texas to New York, but it happens that a recent movement through this region was of such a nature as to allow the sea to encroach on the land, and the previously excavated stream valleys are now, in part, occupied by the sea. This feature is most marked from the Carolinas to New York (Fig. 6), where there are several great estuaries and drowned river-valleys which extend far into the land. The best examples are Albemarle Sound and Chesapeake and Delaware Bays. The James River channel is submerged as far as Richmond, the Potomac to Washington, the Susquehanna to Harrisburg, the Delaware to Trenton, and the Hudson to Troy. These are typical illustrations of what geographers term drowned river-valleys. They are evidence that the land formerly stood higher than now, was trenched by the rivers that flowed across it, and was then depressed or tilted so as to allow the sea to encroach upon it. The importance of these events in the settlement of North America by Europeans and on the subsequent development of commerce, manufactures, the location of cities, etc., needs only to be suggested to permit the reader to fill in the details for himself.

On the Gulf coast and about Florida the later movements of the land have been less than in the region from Albemarle Sound to New York, and estuaries are there absent, with the somewhat marked exception of Mobile Bay. Certain secondary conditions need to be introduced here, but space will not permit of more than a brief presentation of them. Not only have the recent movements of the land been less about the shores of the Gulf of Mexico than in the middle Atlantic region of the United States, but the rivers at the south are in general smaller and less swift than those farther north, and hence are less able to excavate broad valleys. The Southern rivers, such as the Alabama, Mississippi, Rio Grande, etc., are silt-laden and tend to fill their estuaries, while the weaker streams are unable to resist the encroachments of sand-bars and spits built by shore currents, and their mouths have been practically closed. The coast of Texas gives evidence of slight modern subsidence, but the small estuaries formed have, for the most part, been separated from the Gulf by sand-bars.

Northward of the middle Atlantic region the recent oscillations of the land continued to increase and reached a maximum about the shores of the Arctic Ocean; on the Pacific coast also there is similar evidence of an increase in the recent earth movements from the south northward.

In an outline sketch of the present coastal topography of the continent we can generalize, and say that the whole continent during the late Tertiary, glacial, and recent times has swayed up and down about a hinge-line situated in the region of the Gulf of Mexico, and the movements, although not uniform, have increased in amount from the south northward. Let us glance at the evidence on which this broad statement, involving the up and down surging of a vast continent, is based.

The Hudson, as stated above, is a drowned river as far as Troy, a distance from the present land margin of 160 miles. In the next great river to the northward, the St. Lawrence, the tide rises and falls nearly up to Montreal, a distance of about 800 miles from the general shore-line. Still farther north are Hudson Strait and Hudson Bay, which, although but imperfectly explored, seem to be an example not only of the drowning of a river-valley, but of the largest part of a river-basin. The geography of the arctic archipelago fringing the north shore of the continent also suggests that a strongly stream-cut plateau has there been deeply submerged.

In addition to the drowned river-valleys and ragged coasts which record a subsidence of the land, there are raised terraces and beaches which begin at the south near New York and increase in elevation above the present sea-level, when followed northward, all the way to the arctic region, and have in the far north an altitude of about 1,200 feet. These old beaches and terraces show that the land was formerly depressed and has since risen; but, as shown above, has not regained the elevation it had previous to the glacial epoch.

The marked differences in the geography of the coast from New York northward to the Arctic Ocean, and from the same locality southward to Central America, are due primarily to the fact that the oscillations of the land have been such that at the north the continental shelf is entirely submerged and the sea has encroached on a rough land; while at the south the recent oscillations have been less and a broad margin of the continental shelf is exposed and forms the coastal plain.

At the north, we find innumerable islands, bold, rocky shores with many capes and headlands, separated by deep inlets, sounds, straits, bays, etc., or, in brief, a ragged coast such as finds typical illustration on the shores of Maine (Fig. 10), while at the south (Fig. 6) the shores are low, sandy, remarkably uniform in trend, and without islands, excepting such as are built by the waves and currents. The West India Islands will, no doubt, be recalled by the reader, but their history is again different. Intermediate between the land that has experienced great oscillation at the north and the region of less energetic movements at the south is the series of large estuaries mentioned above, in the narrower portions of the coastal plain.

The northern and western coasts of Alaska are mostly low, and correspond in a general way with the coastal plan of the Carolina region. The last well-marked movement of the land in that region has been in the direction of an elevation, and we find low shores, with but few harbours, similar in many ways to the coast of Texas.

It is probably true, as already stated, that the Aleutian Islands, although in part the result of recent volcanic activity, owe their peculiar and exceptional characteristics to the partial subsidence of a deeply sculptured mountain range. On the south coast of Alaska, in the region of Mount St. Elias and Mount Fairweather, a recent and extensive elevation has occurred, which, however, did not bring the bottom of the adjacent portion of the ocean above the sea-level. This apparent anomaly seems to be due to an uprising of the rocks along the north side of a break, or belt of branching fractures, which closely approximates to the coast-line and has determined the position of the continental border in that region. The facts, so far as known, appear to show that we have here what geologists term a fault, the north or landward side of which has been raised at least 5,000 feet in very modern times, but, so far as we can judge, without disturbing the seaward border of the break. The coast between Mount Fairweather and Mount St. Elias is by far the boldest, and from a scenic point of view the most impressive, portion of the entire shore-line of North America. The mountains are young and among the highest on the continent. They rise precipitously from the margin of the sea, and are sheathed in snow and ice from base to summit throughout the year.

The margin of the continent southward from Mount Fairweather to the Columbia River, a distance in a straight line of about 1,200 miles, furnishes some of the best illustrations of the changes in coastal geography due to subsidence that our continent affords (Fig. 11). This wonderfully irregular coast is fringed with a belt of mountainous islands from 50 to 100 or more miles broad. The inlets between the bold capes and the straits separating the numerous islands are deep. The rugged, forest-clothed slopes with precipitous, and in many instances nearly vertical walls, descend into water that is frequently from 50 to over 200 fathoms deep. In brief, a deeply dissected mountain range more than 1,000 miles in length has there been depressed at least 2,000 feet below its former altitude, thus allowing the sea to flood its deep, picturesque valleys.

Puget Sound, with its numerous and frequently narrow arms (Fig. 23), is the southward extension of the partially inundated country considered above. To the west of this magnificent sound rise the Olympic Mountains, which barely escape being an island at the present stage of the swaying of the land. On the west, as on the east border of the continent, there are drowned river-valleys, such as the Stikine, Frazer, Columbia, and Sacramento. It is not to be understood, however, that the entire Pacific coast region has been raised or depressed as a unit. There have been differential movements in some of its parts, but these are not as yet well known. In southern California, for example, raised beaches and a narrow coastal plain about Los Angeles give evidence of a modern rise of the land.

In reference to the broad generalization that the continental mass of North America has undergone up and down movements, greatest at the north and decreasing southward, as if moving on a hinge-line running east and west in the region of the Gulf of Mexico, it is of interest to note that the ragged coasts of Maine, Nova Scotia, Newfoundland and Labrador, due to the partial submergence of a rugged land, lie in the same latitudes as the equally ragged coast of Washington, British Columbia, and Alaska. This is more than a coincidence. The rocks on the two coasts are similar, being for the most part resistant crystalline schists, gneisses, granites, etc., and in each instance stood high above the sea for a long period during which they were deeply trenched by streams and by great glaciers, and then at about the same time, as nearly as can be judged, each region was depressed so as to allow the sea to encroach upon it.

While a deeply sculptured land when partially submerged gives origin to a ragged coast, a region of similar elevation, but not cut by streams or other agencies so as to have deep valleys, when subsidence occurs produces a bold, harbourless shore without islands. The striking contrast between the deeply indented border of the continent, with its broad fringe of islands, from Mount Fairweather southward to Mount Olympus, and the remarkably uniform although bold coast-line from Mount Olympus southward to Mexico, and indeed nearly to Cape Horn, has much significance in this connection.

The mountains bordering the Pacific coast of the United States are among the younger on the continent. These coast ranges, largely on account of their youth, have not been deeply sculptured, but rise boldly from the ocean's shore throughout nearly the entire distance from the Strait of Fuca to the end of the peninsula of Lower California. The mountains of Central America, although but little known, are of comparatively recent date, but differ from the coast ranges in being more largely built of young volcanic rocks. Both the coast ranges and the mountains of Central America are much less deeply sculptured than the mountains bordering the Pacific to the north of Puget Sound, and a subsidence along this shore would produce but moderate changes in the coast-line. In this great extent of coast, measuring nearly 5,000 miles, there are but few harbours; in the portion belonging to the United States the generally bold coast-line is broken but in two places, one where the Columbia reaches the sea, and the other where the Sacramento finds an outlet through the portions of its drowned valley known as the Golden Gate.

The Bay of San Francisco owes its origin to a subsidence of the land which has admitted the sea into the valley of the Sacramento, but this valley, which, uniting with the one at the south drained by the San Joaquin, forms the Great Valley of California, is not due to stream erosion, as in the case of the drowned valley of the Hudson or of the St. Lawrence, but to the upraising of the mountains bordering it. During a former time of greater subsidence than at present the Bay of San Francisco was larger than now, and has been contracted both by the deposition of sediment and by a partial re-elevation of the land. The exceptional character of the Bay of San Francisco and its marked excellence as a harbour give to the city on its shore promises of marvellous development.

The Gulf of California is due, in a general view, to what may be considered as a departure of the Coast mountains away from the general trend of the continental border. We have but little detailed information concerning this region, however, and the studies of modern geographers have likewise been meagre throughout all the coast-line farther south.

The Pacific coast of Mexico is geographically similar to that of California, but instead of a single great harbour there are four of moderate size and excellence, the histories of which have not been studied. Farther south, along the Central American coast, the shores are bold, but several indentations, due in part at least to volcanic agencies, furnish shelter for vessels and offer encouragement to commerce.

The bold and not deeply sculptured mountains along the nearly unbroken coast from the Strait of Fuca to Panama, rise close to the true border of the continent. The continental shelf of this portion of the shore of the Pacific is narrow. An elevation of 100 fathoms would add scarcely more than 10 miles to the extent of the land. This narrowness of the continental shelf seems to be due to the recency of the uplifting of the Coast mountains, and the lack of time for the _debris_ from the land and the organic refuse of the sea to shoal the water. The Pacific basin is deep close to the land bordering it, thus restricting the seaward extension of the continental shelf.

_Changes in the Coast-Line due to Ice._--It is now well known that glacial ice many hundreds of feet thick formerly covered the northern half of North America and flowed outward across the present position of the coast-line throughout all of the northern border of the continent from Staten Island in the east and Puget Sound in the west, with the exception of the Arctic and Bering Sea coasts of Alaska.

The effects of this outward-flowing ice on the topography of the continental border crossed by it were in general in two directions. Where the land was rough or moderately so previous to the coming of the ice-sheets the inequalities of surface were increased; but where the land was smooth or but gently undulating its elevations were planed away by the glaciers and made still more smooth. The reason for these differences is that when the land from which the ice flowed was rugged or had previously been deeply trenched by streams, the valleys gave direction to the ice currents and the margins of the continental ice-sheets became divided into separate ice-streams, as is the case in Greenland at the present day. This localization of the ice currents served to deepen and broaden the pre-existing valleys, and especially on the bold coast of Alaska and British Columbia increased in a marked way the inequalities of the surface and favoured the production of a ragged coast-line when the ice melted and was replaced in part by the sea. When, however, the topography of the land was not sufficiently accented to cause the ice flowing over it to gather into well-defined currents the general surface was worn down, thus favouring the production of an even coast-line after the melting of the ice-sheets.

Where the coast-lands were high and rugged, the deepening and broadening of the valleys led to the origin of deep, narrow, canal-like waterways termed fiords (_fjords_), when the ice withdrew from the partially submerged land or when subsequent depression carried the glaciated troughs below sea-level. On the Atlantic coast from Maine to Labrador, and thence northward to the Arctic Ocean, there are numerous examples of fiords, as is also the case on the Pacific coast from Mount St. Elias to Puget Sound. At the present time the localized ice-streams from the great central ice-sheet of Greenland are continuing this process of fiord excavation. The same is true also, but on a much smaller scale, of the tide-water glaciers of southern Alaska.

The shores of the northern portion of the continent from New England to the Aleutian Islands are now being modified by the grinding of ice-floes, which are driven against the land by the wind. This process, however, although locally important, need claim but little attention in a general view of the geography of the continent.

_Changes in the Coast-Line due to the Deposits made by Streams._--The visible loads of silt and sand in suspension carried to the ocean by streams, as well as the material the streams roll and push along their bottoms, is delivered to the waters of the ocean and deposited in various ways. Much of this material, notably the coarser portion, is dropped near land and the finer portion floated far out from the coast before settling to the bottom. Two classes of deposits made in this way may be recognised, namely, those laid down by the streams themselves as they drop their loads on entering still water, or delta deposits; and those spread over the sea-floor by waves and currents after receiving the _debris_ brought from the land. Which of these two modes of deposition will prevail depends on whether the waters of the ocean at the localities where the streams deliver their loads are essentially still or are affected by strong currents. In the former instance all but the finer of the _debris_ derived from the land is quickly dropped and deltas are formed; and in the second instance the currents bear the material away and deposit it either in the shallow water adjacent to the neighbouring shore, forming shoals, bars, embankments, spits, etc., or spread it in a sheet over the sea-floor. The most notable changes in the coast-line resulting from this general process occur where silt-laden streams enter still water and form deltas.

On the coast of North America many of the streams which enter estuaries deliver their loads to waters which are agitated, especially by tidal currents, and ill-defined shoals, sand-banks, etc., are produced. In three conspicuous instances, however, large rivers are engaged in building deltas, and thus producing well-marked changes in the coast-line.

At the north, the Mackenzie enters the nearly tideless Arctic Ocean, where floating ice almost completely counteracts the tendency of the wind to produce currents, and a great delta is being extended seaward. The river divides on its delta into many _distributaries_ and enters the sea by several mouths. The sea near the mouths of the river is reported to be shallow, and obstructed by many sand-banks and islands. No survey of the Mackenzie delta has as yet been made, and but little definite information concerning it is available.

The Yukon on entering the shallow eastern portion of Bering Sea, where the influence of the tides is small and floating ice is present throughout about nine months each year, is also engaged in building a great delta which projects into the sea and gives the coast-line a bold outward curve. The Yukon begins to divide into separate channels, several of which enter the sea as independent distributaries at a distance of about 150 miles from the outer border of its delta. The distance between the outer finger-like division of the stream is about 90 miles. The Yukon is a graded stream--i. e., is able to carry material in suspension, but not to deepen or fill its channel--in the lower portion of its course, and is making an important addition to the land owing to the dropping of its burden of silt as soon as the still water into which it flows is reached. The stream is thus being extended, and in order to enable it to continue its task of transportation and the delivery of its load to the sea, the extended portion of its channel is built up so as to give a slope down which the waters can flow--that is, the beds of the distributaries are raised, and they also shift their positions from time to time and make additions to the entire surface of the delta. This extension of the stream and deposition of silt by its distributaries have added about 1,000 square miles to the land. Although the delta of the Yukon presents an admirable example of the change in a coast-line produced by the sediment dropped by a great river, the partial surveys of it that have been made are not as yet available for study.

Fully as characteristic of the modification of coast-lines made by a stream as any in the world is the well-known example of the delta of the Mississippi. This classical instance illustrates not only the manner in which coast-lines are modified, but the behaviour of a large silt-laden stream which has reduced its valley to a low gradient, and throughout hundreds of miles of its lower course is spreading out a wide flood plain. The extension seaward of this flood plain forms the broad delta at the river's mouth.

During high-water stages the Mississippi widely over-spreads its banks and during such inundations of its valley drops much of the silt it previously held in suspension. The material deposited is laid down most abundantly on the immediate border of its low-water channel. Each side of the channel is thus raised so as to form what is termed a natural levee. During this process also the bed of the stream is raised by the deposition of sediment upon it, thus tending to cause the stream to flow on a raised ridge and producing an unstable condition which from time to time enables the river to break across its confining levees and divide into two or more separate channels. In the lower portion of the river some of the new channels thus formed reach the sea and furnish independent outlets for its waters. The first of these distributaries now departs from the main channel at a distance of 200 miles from the Gulf of Mexico, and farther seaward several other divisions occur (Fig. 12). The area of the delta is about 1,230 square miles. Each distributary is engaged in building a pair of embankments, or natural levees (although this process in recent years has been modified by the construction of artificial embankments for the sake of improving navigation), and each subdivision of the river is also building a delta. Each of the finger-like extensions of the delta, shown on the accompanying map, is due to the prolongation of a pair of embankments into the Gulf by each distributary and the growth of a secondary delta at its mouth. The river is thus building a highly compound delta, composed of the secondary deltas formed at the mouth of each of its distributaries. A conspicuous modification of the otherwise generally evenly curved border of the Gulf of Mexico is thus produced, a result that could only be reached in a water body but little disturbed by wind or tidal currents.

ESTUARIES AND HARBOURS

The features of a coast of greatest importance to civilization are its harbours. A coast without harbours is like a Chinese wall, and tends to isolate a people inclosed by it. An indented coast with numerous havens for the shelter of vessels fosters the interests of navigation, including sea fisheries, invites commerce from other lands, and stimulates its inhabitants to explore and travel. A diversity of industries is thus favoured and the people adjacent to an indented coast with good harbours tend to become more progressive and more cosmopolitan than if intercourse with other communities is confined to overland routes.

The Atlantic border of North America is abundantly supplied with fine harbours, which not only favour communication with distant countries, but are within easy reach of agricultural and forest lands and important coal and other mineral deposits adjacent to the coast or in the interior, and are near extensive and valuable fishing grounds. The best of these harbours are at the mouths of rivers which have been depressed so as to form estuaries with wide entrances. These sea-gates, however, are frequently contracted, owing to the presence of sand-bars and spits deposited by shore currents.

The great St. Lawrence estuary reaches to Montreal, and beyond lie the Great Lakes, the rich lands of Ontario and New York, and the now highly productive States of the Middle West. Two geographical features in this basin detract from the conditions otherwise highly favourable to commercial development, namely: the rapids in the St. Lawrence between Montreal and Lake Ontario and the fall in the Niagara, and the winter climate of Canada, which causes the rivers and estuaries to be ice-bound for a considerable part of each year. To obviate the first of these unfavourable conditions far-reaching plans for a deep waterway between the Great Lakes and the Atlantic are now being matured. The splendid harbours from Nova Scotia southward are never seriously obstructed by ice, and south of Virginia ice is practically unknown.