The Geology of Darling State Park

Part 1

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. 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 seen in the park were originally sedimentary rocks. These rocks belong to the Gile Mountain Formation which was deposited during the Devonian Period some 300 million years ago (see Geologic Time Scale, 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).

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