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]).
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.
| 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. | ||||
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.
