Various Theories About the Formation of the Chasm There are several different theories about the formation of Purgatory Chasm. All of these hypotheses agree that the Chasm was formed approximately 14000-16000 years ago, near the end of the last ice age. One theory, proposed by Professor R.J. Lougee of Clark University, is that when the ice border of the Laurentide glacier that covered this area during the ice age had receded, a marginal lake formed in Casey Brook Valley due to damming of the run-off water. When the dam broke for some unknown reason, that supposedly created a torrent that flowed into Purgatory Brook Valley, through the Chasm, and created the rock formation that people hike through today. The first problem with this theory is that there is no evidence to show that water erosion occurred in the Chasm or on its walls. There are no rounded rocks; all the rocks and boulders in the Chasm are angular. Another problem is that for the water to have reached the chasm from Casey Brook Valley, the river would have had to drop 100 feet and flow down a ridge. If that were true, there would be a clear path of erosion from Casey Brook Valley to Purgatory Chasm that would survive to the present time. No evidence of such erosion remains. Professor W.O. Crosby of MIT and his son Irving B. Crosby proposed that the Chasm was not formed due to glacial action. Their main reason for this assertion was the observation that the chasm (which runs north east to south west) is transverse to the direction of glacial movement (north west to south east). Instead, they believed that a violent earthquake created the Chasm. They proposed that a large wedge-shaped rock between the tops of the two Chasm walls fell approximately one hundred feet into an underlying hollowed-out volume, cutting the shape of the chasm and leaving the broken remnants visible today. The rock formation leading to a collapsed region like the Chasm under this theory is known as a keystone fault. The first problem here is that keystone faults cannot occur on such a large scale. A second problem is that the orientation of the Chasm is actually not relevant. It doesn't matter whether the Chasm oriented along the direction of glacial movement or not since a glacier could pick up and move rocks in any case. A third theory has been proposed by Professor Mauri Pelto of Nichols College. According to Professor Pelto, as the Laurentide glacier flowed over the area now making up the Chasm, water at the wet bottom surface of the glacier flowed between cracks in the rocks, then refroze to the bottom of the glacier. As the glacier continued to flow over the rock base, it ripped apart some of the rocks at weak points such as joints between different segments of metamorphic rocks such as gneiss (see below). The glacier then carried chunks of rock some distance before they were deposited in their current locations. This theory currently seems to agree with the physical evidence better than the others since it accounts for the angular shape of the rocks in the chasm, their apparently random deposition in the Chasm, the abrasion of the rock walls, and for the lack of evidence of water erosion. Rock Formations in Purgatory Chasm Many of Purgatory's smaller rock formations (i.e. other than the large central Chasm itself) were created as a result of frost wedging--the process of rocks splitting apart due to the freezing of water. The crack known as "Fat Man's Misery" was likely formed by many winters of freezing and melting water, gradually separating the rock to form an almost perfectly straight cut through the rock. Lover's Leap, an overhang formed by the breaking off of large chunks of rock, is located at the top of the Chasm. Several deaths (both from accidental falls and suicides) have occurred there over the years. Rocks in Purgatory Chasm Granite is formed when magma from volcanoes cools and solidifies. Varied in color, granite is composed primarily of potassium feldspar and quartz crystals. It is one of the most well known and abundant igneous rocks and this makes it a prime material for use in construction and decoration (such as for outer walls of buildings and in granite counter tops). Gneiss is a metamorphic rock, banded with patterns formed by the combination of dark and light silicates. It is formed by heating of granite, Muscovite/biotite mica, quartz, potassium feldspar, and other rocks to very high temperatures (over 500 degrees C) under high pressure. Heat and pressure segregate the silicates from each other, creating the banding patterns. Gneiss sometimes breaks into irregular pieces, but it can also break along the lines of different component minerals in the rock. (Source: Information from https://sites.google.com/site/purgatorychasmearthscience6, rewritten by JL)