  The most conspicuous rock found in the park is granite. Figure 3. Speckled granite with inclusion of metamorphic rock. Black specks in granite are flakes of black biotite mica. Metamorphic inclusion, located just above the hammer head shows some reaction with the invading granite. Pieces of metamorphic rock were undermined by and dropped into the granite as it worked its way upward into these rocks. Picture taken a few yards west of the tower on top of Burke Mt. 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 Figure 4. Outcrop of biotite granite located on the summit road between the second and third turns from the summit of Burke Mountain and on the right side of the road if descending. Note the “sheeting structure” or flat joint surface which slopes or dips into the road. This flat break in the rock was probably caused by the release in pressure of the overlying glacial ice when it melted from this region. Figure 5. East side of parking area, summit of Burke Mountain. Outcrop of granite with many metamorphic rock inclusions (hammer, center of picture, rests on large inclusion). Layering or banding in the inclusions is almost vertical. Figure 6. Outcrop located about midway down the Bear Den Ski Trail. Alternating metamorphic quartzite and phyllite invaded by lighter colored and speckled biotite granite. Note how the granite cross-cuts the layered or banded metamorphic rocks. This cross-cutting points out the fact that the layering or banding was present prior to the invasion of the granite. Figure 7. Outcrop located about midway down the Bear Den Ski Trail. Metamorphic quartzite and phyllite (darker color) and invading biotite granite (light speckled appearance). Here the granite has a more or less conformable relationship to the layers or bands in the metamorphic rock. Compare this relationship with the cross-cutting relationship in Fig. 6. For scale, the handle of the geologic hammer or pick is about 12 inches long. 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. Figure 8. Inclusion of layered or banded metamorphic rock (hammer is resting on this inclusion) in lighter colored biotite granite as seen along the Bear Den Ski Trail. The metamorphic rocks were invaded and undermined by the granitic rocks, with the result that pieces of the metamorphic rock were surrounded by granite. For scale, the handle of the geologic hammer or pick is about 12 inches long. Figure 9. Picture taken along the Burke Mountain summit road of typical Gile Mountain metamorphic rock. Here the rocks dip almost vertically. For those more advanced in geology, note the pillow-like segments or boudinage structure about one foot to the left of the chisel point of the hammer. This structure is due to a stretching of the rock. For scale, the hammer handle is about 12 inches long. Figure 10. Banded or layered metamorphic rocks with inter-squeezed granite (lighter colored material). This outcrop is located on east side of the Bear Den Ski Trail and quite close to the Burke Mountain summit road. The hammer handle, center of picture, is about one foot long. Figure 11. Picture taken only a few yards from the Burke Mountain observation tower, along the path to the summit parking lot, looking northwest. Here you see metamorphic rocks (quartzite and phyllite) with some inter-squeezed granite. Note how the nearly vertical metamorphic rock layers bend or “wrap-around” to the right. The highly resistant inter-squeezed granite actually holds Burke Mountain up, or to be more scientific, it prevents these rocks from being worn down as fast as the surrounding rocks. For scale, see the clip board in the center of the picture. 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 Figure 12. Outcrop on south Lookout, summit of Burke Mountain. Distorted layers of Gile Mountain metamorphic rock. Lath-like crystals developed along some of these layers during the second period of metamorphism, that is, when the granite invaded the metamorphic rocks. For scale, the hammer handle is about one foot long. 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 Figure 13. Photograph of the outcrop beneath the observation tower, summit of Burke Mountain. Note the heterogeneous appearance of the granite-infiltrated metamorphic rock. Here the metamorphic rock approaches granite itself in composition and if the process had progressed a bit more, it would be said to be granitized rock. Large lath-like crystals are very prominent in the rocks of this outcrop. 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? Figure 14. Geologic Time Scale. The main Darling State Park geologic events are noted on the right, opposite the approximate geologic time when each occurred.
Figure 15. Glacial striations or scratches on outcrop midway between the second and third turns in the road down from the summit area of Burke Mountain. Striations trend about 40 degrees east of south or in approximately the same direction that the hammer handle is pointing. Hammer handle is about one foot long. 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 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. 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. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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