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. [Illustration: 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.] [Illustration: 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.