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Part 2

The Geology of Darling State Park · Harry W. Dodge — chapter 2 of 3 · ~1,391 words · public domain

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, 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