Chapter I

THE GEOLOGY OF
DARLING STATE PARK

_By_
HARRY W. DODGE, JR.

VERMONT GEOLOGICAL SURVEY
Charles G. Doll, _State Geologist_

DEPARTMENT OF FORESTS AND PARKS
Robert B. Williams, _Commissioner_

DEPARTMENT OF WATER RESOURCES

1967

Jay Peak 39 MILES 3861′
Gore Mtn. 25 MILES 3330′
Monadnock Mtn. 29 MILES 3140′
Haystack Mtn. 35 MILES 3223′
Belvidere Mtn. 34 MILES 3360′
Willoughby Lake 15 MILES
Bold Mtn. 15 MILES 3315′
Mount Mansfield 44 MILES 4393′
Stone Mtn. 11 MILES 2753′
Mount Washington 35 MILES 6288′
Camels Hump 50 MILES 4085′
Mount Ascutney 80 MILES 3144′

THE GEOLOGY OF DARLING STATE PARK

_By_ HARRY W. DODGE, JR.

INTRODUCTION

Darling State Park, located in northeastern Vermont (see map, “Burke Mountain,” Fig. 1), offers outstanding opportunities to the camper, picnicker, hiker, view-seeker and winter sportsman. On a clear day the top of Burke Mountain offers a most spectacular view of northeastern Vermont and such distant points as Mount Ascutney (located on the Connecticut River, some 80 miles as the crow flies, to the south). Other prominent peaks that may be seen are Camels Hump (50 miles southwest), Mount Mansfield (44 miles west), Jay Peak (39 miles northwest), and Mount Washington (the highest Peak in the New England States and the northeast, which is located in New Hampshire some 47 miles southeast of Burke Mountain). For the traveler, the view from Burke Mountain reveals “where he has been” and where he might “next go.” See Figure 1 for the location of points which can be seen from the overlooks atop Burke Mountain.

Both the professional and amateur naturalist will find Darling State Park extremely interesting. This pamphlet is devoted primarily to the geology of the park, but the fauna and flora of this area present the visitor with days of interesting studies. It is hoped that in the near future pamphlets describing these aspects of Darling State Park will be published.

THE GEOLOGY OF THE PARK

Before discussing the more detailed aspects of the geology of Darling State Park, certain basic geologic concepts must be explained. But, even before such a discussion, it might be best to clarify the position of geology among the many other, and oftentimes interrelated, sciences.

The basic reason for the science of geology might be said to be twofold; one is economic, the other related to Man’s basic curiosity. In the first, the geologist through the use of his knowledge of the earth’s rocks, locates those indispensable minerals and fuels without which our advanced society and technology could not exist. In the second, the geologist tries to unlock the many mysteries within the earth’s crust merely to satisfy a thirst for knowledge and to pass such knowledge on to his fellow man. These two basic reasons complement each other and allow continued advancements in geology, both as a pure science and as a primary economic aid to the nation.

As found in most spheres of present-day scientific endeavor, the geologist relies heavily on other related sciences for insight into problems at hand. A basic knowledge, and oftentimes an advanced knowledge, of physics, chemistry, mathematics and zoology, to name only some, are needed before the geologist can approach many of his own problems. It might be obvious to you by now, but a geologist will be certainly included in the first scientific party to journey to the moon and planets.

Within the general science of geology are several branches, to name only a few; paleontology, sedimentology, mineralogy, petrology, stratigraphy, petroleum geology, and structural geology. Each of these branches or specialty-areas contributes basic data for the overall interpretation of the past geologic history of any given geographic area. The historical geologist takes all these clues and attempts to fit the pieces of information together into a picture of past events.

The concept of Geologic Time must be understood before the history of Darling State Park can be unraveled. Usually we think of time in terms of minutes, hours, days, weeks, months and years. The geologist thinks and talks in terms of millions or even billions of years. Time units as short as hundreds of years are impossible to distinguish in the past history of the Earth. When it is realized that the earth is probably 4 to 6 billion (4,000 to 6,000 million) years old, and the record of these years is incomplete, it is easy to understand why the geologist speaks in terms of millions of years instead of years. With modern methods of radioactive dating the geologist hopes for finer time definitions in the future.

In short then, the geologist interprets and puts order into millions of years of history which can only be “read” as recorded in the rocks beneath our very feet. Of course, just looking at the rocks does not magically open the book of geologic history. This pamphlet is designed to sharpen your powers of observation and to help you in your interpretation of these observations.

THE ROCKS AND THEIR HISTORY

The most conspicuous rock found in the park is _granite_.[1] Along the road which winds to the summit of Burke Mountain you will see several outcrops of the white or pinkish biotite granite (Fig. 4). This granite is well displayed in the summit parking area and along the trail to the observation tower (Figs. 3 and 5). A walk down the Bear Den Ski Trail also shows an abundance of granite outcrops (Figs. 6, 7, and 8).

While looking at some of the above-mentioned photographs, a second family of rocks is discovered (Figs. 3, 5, 6, 7, and 8; also, Figs. 9, 10, 11, and 16). In many places these rocks have a layered or banded appearance and in other places large lath-like crystals are common in some of the layers. In some areas these rocks are very heterogeneous in appearance and display distorted layers and profuse development of lath-like crystals (Figs. 12 and 13). These rocks belong to the second major family of rocks, the _Metamorphic_ rocks. The metamorphic rocks[2] seen in the park were originally _sedimentary_[3] rocks. These rocks belong to the Gile Mountain _Formation_[4] which was deposited during the _Devonian Period_ some 300 million years ago (see Geologic Time Scale[5], Fig. 14). So much for the two major families of rocks present in the park, the igneous and metamorphic rocks, and how to distinguish one from the other. Let us assume that you can now distinguish between the granite and the metamorphic rocks.

Now, what is the relationship of one to the other? That is, where you can see both of these rock types exposed together in one outcrop, can you describe the physical contact of one with the other? For instance, look at Figure 6, which was taken about midway down the Bear Den Trail, here you see the granite (the white speckled igneous rock which cuts horizontally across the picture) cutting across the distinctly layered or banded metamorphic rocks. The granite is said to have a cross-cutting relationship to the metamorphic rocks. In some outcrops the granite is more or less parallel to the layers of metamorphic rock (Fig. 7). Here, the granite is said to have a conformable relationship with the metamorphic rocks. Still another relationship between the granite and the metamorphic rock is seen in Figure 8. Here, blocks of metamorphic rocks are inclosed by granite. These inclosed blocks are called inclusions and are pieces of invaded rock which fell into or were encircled by the invading granite.

From these relationships, what can be said about the relative ages of the two rock types? Which is the older, or first formed? Which is the last formed? If you study the above relationships for a minute or so, it will become obvious that the layered rock had to be formed _prior_ to the emplacement of the granite. Some of the minerals now seen in the layered or banded metamorphic rocks were formed at the time of granite intrusion, but the basic “stuff” or partially metamorphosed sedimentary rock was present before the granite entered the area from beneath. So, the knowledge of the two rock types present and an understanding of their relationship to one another tells us a story of at least two events which occurred in the park area hundreds of millions of years ago.

Can we find other facts in these rocks which might add to the above-mentioned events? The answer to this question is, yes! The types of minerals found in the metamorphic rocks coupled with the inherited layered structure so common in these rocks, tells us that they were once sedimentary rocks. There is other evidence which indicates that these sedimentary rocks were slightly metamorphosed and folded prior to the invasion of the granite. Added information indicates that these same rocks were subjected to increasing temperatures with the invasion of the granite and another metamorphic mineral change took place. Thus far, the rocks have told us about four distinct events; the deposition and hardening of the Gile Mountain Formation of sedimentary rocks, the first period of wide-spread metamorphism, accompanied by broad folding, the invasion of the granite, and a second phase of metamorphism with the increased temperatures produced by this invasion (see cross-sections illustrating the geologic history of the park area, Fig. 17).

The four events which are mentioned in the preceding paragraph took place hundreds of millions of years ago. What has happened in the park since these events? Take a look at Figure 15, which was taken along the road to the summit of Burke Mountain (coming down from the summit, this outcrop is located on your right, midway between the second and third turns in the road). Here the granite exhibits linear scratches or striations which trend about 40 degrees east of south (general direction in which the hammer handle points). Again, just down the road from the midway picnic and camping area, and on your right, striations can be seen. Here they trend about 45 degrees east of south or approximately in the same direction as the first series of striations mentioned. These scratches or striations occur in many places throughout the park, and in most cases their orientation is about the same. What caused these numerous striations?

ERAS PERIODS—YEARS AGO DARLING STATE PARK EVENTS
EPOCHS

CENOZOIC CENOZOIC GLACIAL STRIATIONS—“SHEETING STRUCTURE”
Pleistocene
Pliocene EROSION, JOINTING
Miocene
Oligocene
Eocene
Paleocene
70 MILLION
MESOZOIC CRETACEOUS
JURASSIC
TRIASSIC
200 MILLION
PALEOZOIC PERMIAN
PENNSYLVANIAN
MISSISSIPPIAN EROSION, JOINTING
Invasion of Granite with second stage
of metamorphism.
DEVONIAN Regional folding and first episode of
metamorphism.
SILURIAN
Deposition of the Gile Mountain
Formation.
360 MILLION
NO RECORD IN PARK
ORDOVICIAN
CAMBRIAN
550 MILLION
PRECAMBRIAN TIME——ORIGIN OF EARTH, 4 TO 5 BILLION YEARS AGO.

Since they are still preserved in the rocks for us to see, they must have been formed quite recently, that is, geologically speaking. What can explain these striations and their common orientation? Did you ever hear about the Great Ice Age, or the Pleistocene Epoch? Less than one million years ago, in fact, some 12,000 years ago, an ice sheet many thousands of feet thick rode over Burke Mountain in a southeastward direction. The many boulders frozen to the underside of the ice sheet tended to scratch the rocks over which they rode. The scratches or striations seen in the park rocks were caused by these attached boulders. The ice sheet also plucked and rounded Burke Mountain into the shape it possesses today.

A look at Figure 4 shows still another event which occurred during recent geological time. The prominent smooth fracture-surface seen to slope or dip toward the road is called “sheeting structure” which has its origin in post-glacial time. It is thought by many geologists that these flat surfaces or _joints_[6], which are generally parallel to the ground surface, were formed with the release of the weight of the overlying glacial ice when the glacier retreated northward. So, here we have evidence displayed in the rocks which tells of still another event in the park’s history. It should be mentioned here, while still on the subject of joints, that other joints do occur in the park rocks. Figure 16 shows joints which were formed earlier than the “sheeting” and which are not parallel to the surface of the ground. These joints were probably formed as a result of the removal of the overlying rocks through erosion, thus releasing long-continued pressures produced by the weight of the overlying rocks, and movement of the earth’s crust. We now have the story of two main episodes in the park’s geological history; one took place many millions of years ago, the other within the last 12,000 years. What happened between these two rock-documented episodes?

1. Deposition and hardening of the Gile Mountain Formation. At this
stage the layers of rock were more or less horizontal.

2. The horizontal and parallel layers of the Gile Mountain Formation
were gently and broadly folded and regionally metamorphosed. This
is the first stage of metamorphism in the park area.

3. Invasion by granite. This invasion was accompanied by local
metamorphism of the invaded rocks. This is the second stage of
metamorphism in the park area. Note the inclusions of first stage
metamorphosed Gile Mountain rocks in the granite.

4. Many millions of years of erosion took place, the forces of nature
finally exposing the granitic rocks at the surface of the earth.

5. Continued erosion caused the metamorphically reenforced Gile Mountain
rocks to wear down more slowly than the surrounding weaker rocks.
For this reason, these strengthened rocks stand higher than the
weaker rocks.

6. Less than one million years ago the glaciers advanced over the park
area. The glacial ice plucked and scratched (striated) the
underlying rocks as it slowly advanced southward. During the
retreat (northward) certain deposits were left. Present-day Burke
Mountain is much the same as it was when the glaciers left, but,
some added erosion has taken place and, because of uplift, the
Mountain stands a bit higher than it did some 10,000 years ago.
Some soil, much of which was removed by the glaciers, has since
formed on the mountain.

There are no rocks present in the park which were deposited during this interval of time, therefore, no rock record. If no rocks representing this time interval are present, one of two reasons must be responsible. Either the park area was undergoing active erosion (wearing down) during this period, or sediments were deposited during part or all of this time interval and subsequently completely removed by erosion. Most probably, the intervening time found the park area above the depositional environment of the sea, when its rocks were being worn away by the erosional forces of nature. Again, see Figure 17 for a diagrammatic representation of the geologic history of the park.

WHY IS THERE A BURKE MOUNTAIN?

Granite is a very resistant rock, that is, it wears away very slowly under the forces of nature. The granite is worn down more slowly than the metamorphic rocks which it has intruded. The granite has been squeezed between and across the layers (bands) of the metamorphic rocks (previously sedimentary rocks) now found on Burke Mountain. In a very true sense, the granite forms a skeleton framework for the metamorphic rocks of Burke Mountain. In other words, it holds these metamorphic rocks up above the surrounding area of metamorphic rocks.

HIKES TO TAKE

A very interesting hike, both geologically and for nature hunting in general, is the old fire road which begins just above the old C.C.C. camp and the present Bell Gardens. This trail cuts off to the right, if ascending the summit road, and runs completely around the mountain. Shortly after the fire trail intersects the Bear Den Ski Trail, and on the left, granite with obvious drill holes is seen (Fig. 18). Most of the granite used as curb stones, culvert headers, and islands along the park summit road was obtained from these small quarries. A few miles walk along this trail proves quite rewarding to the adventurer; an old lean-to demonstrates what a bear can do while sharpening his claws.

The Devil’s Den Trail leads down the east side of Burke Mountain from the observation tower at the summit. This trail is rather poorly marked past a certain point, but if you wish to strike off on your own and see some wonderful country, a hike down this trail with a swing to the north will bring you back to the summit road.

The several ski trails on Burke Mountain are all walkable during the summer, but they will not appear as smooth as when covered with snow, and the trip down will take considerably longer on foot than on “boards.” The Bear Den Ski Trail is especially good for geological sightseeing.

NEARBY AREAS TO VISIT

While at Darling State Park a visit to Lake Willoughby is well worth the trip (Figs. 19, 20). A glance at Figure 1 will show you its general location. Once you visit this lake, you might ask “why so beautiful a Lake Willoughby?” There is still some question as to the origin of the lake basin, but a combination of oriented joints and recent glacial movement seems to fit the picture. Deeper and faster weathering along parallel joints together with glacial movement and scour in the same direction as the joints probably dug the elongated trench which, when filled with water, became Lake Willoughby.

It is hoped that this pamphlet has given you the desire to enlarge your knowledge of the science of geology. Geology is all around you wherever you might travel and a knowledge of geology will open new roads even while traveling old ones. Here’s luck to you in your travels; try to _see_ what you look at.

Granite-Metamorphics
Observation Tower
Burke Summit Rd.
Bear Den Trail
Fire Trail
Park Boundary
Contour Line
Dip and Strike: Bed
Dip and Strike: Bed?
Dip and Strike: Joint

SUGGESTED READING

Leet, L. Don and Judson, S., 1965, _Physical Geology_, 3rd Edition,
Prentice-Hall, Inc., Englewood Cliffs, New Jersey. This is a
starter for the geology student.

Dunbar, C. O., 1959, _Historical Geology_, John Wiley and Sons, New
York. Also a beginning book, read after above book.

Dennis, John G., 1956, _The Geology of the Lyndonville Area, Vermont_,
Vermont Geological Survey Bulletin 8. This is for the more
advanced student and relates the geology of the area adjacent to
Burke Mountain and Darling State Park.

Jacobs, Elbridge C., 1941-42, _The Great Ice Age in Vermont_, Report of
the State Geologist, Vol. 23; pp. 27-47.

Stewart, David P., 1961, _The Glacial Geology of Vermont_, Vermont
Geological Survey, Bulletin No. 19.

Woodland, B. G., 1963, _A Petrographic study of Thermally Metamorphosed
Pelitic rocks in the Burke Area, Northeastern Vermont_, American
Journal of Science, volume 261, pages 354-375. For the advanced
student.

Woodland, B. G., 1965, _The Geology of the Burke Quadrangle, Vermont_,
Vermont Geological Survey Bulletin No. 28. This is a comprehensive
study of the Burke Mountain area and a must for those interested
in Darling State Park.

Footnotes

[1]The granite found in Darling State Park is white or pinkish in color
and most times is speckled with shiny black mica flakes. On close
inspection, grains of smoky to clear color are seen within the rock.
The white and pink grains are the mineral, feldspar; the shiny black
flakes, biotite mica; the smoky to clear grains, quartz or silica. A
magnified picture of a slice of granite (see Fig. 2) shows the
individual mineral grains and their interlocking nature with each
other. Granite belongs to a major family of rocks, termed _Igneous
rocks_. Igneous rocks are formed through the hardening or
lithification of molten rock-material when subjected to the cooler
temperatures at or near the earth’s surface. The molten rock
material formed at some depth beneath the surface of the earth,
where temperatures were many hundreds of degrees hotter than at the
surface.

[2]Metamorphic rocks are either sedimentary (this, a third major family
of rocks which is characterized by a layered appearance that has
been retained by many of the altered park rocks) or igneous rocks
(the granite) which have been under the influence of pressure, heat,
and chemically active fluids, oftentimes resulting in chemical and
structural changes. Most of the metamorphic rocks seen in the park
are either schist, phyllite, slate or quartzite. For the benefit of
the more advanced student, the rocks of the park area are considered
a granite-hornfels complex (see Bertram G. Woodland’s paper of 1963,
“A Petrographic study of Thermally Metamorphosed Pelitic rocks in
the Burke Area, Northeastern Vermont,” in the American Journal of
Science, volume 261, pages 354 to 375).

[3]Sedimentary or layered rocks, the third major rock family, are
composed of pieces, grains and other materials from older
metamorphic, igneous and sedimentary rocks. These fragments have
been carried by rivers and streams to some resting place at the
bottom of a sea, lake, or stream channel. This mud, sand and gravel,
under the weight of steadily increasing overburden, and the presence
of cementing materials, slowly hardened into rock which we now call
limestone shale, sandstone, and conglomerate.

[4]A geologic formation consists of a sequence of rock layers which were
deposited under essentially the same conditions, or a series of
alternating conditions, and which can be easily distinguished and
mapped as a unit by geologists in the field.

[5]Geologic time is divided into four Eras which are designated from
oldest to youngest: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
Each of these Eras is divided into geologic periods of time. The
Devonian Period is in about the middle of the Paleozoic Era and
began some 330 million years ago and ended approximately 290 million
years ago (see Geologic Time Scale, Fig. 14).

[6]A joint is a break in a rock mass which interrupts its physical
continuity. A group of more or less parallel joints is known as a
joint set.

Transcriber’s Notes

—Silently corrected a few typos.

—Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
_underscores_.