Figure 16. Midway between the second and third turns, descending on the Burke Mountain summit road. Metamorphic quartzite and phyllite showing at least two prominent joints. Layers are vertical and parallel to the front joint (one which hammer handle touches). For the more advanced student of geology, note the lineations parallel to the hammer handle and on the front surface. For scale, hammer handle is about one foot long.
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?
Figure 17. Geologic cross-sections illustrating the geologic history of Darling State park. (For explanation of cross-sections see top of [page 19].)
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.