View of Purgatory Chasm
Theory 1: Written by Professor Crosby of MIT, Crosby did not see any smooth rocks at the chasm, just angular rocks and so he concluded that there would have been no water action in forming the chasm. He also believed that there would have been no glacial action because the chasm was transverse to the glacier movement. Basically this means that the chasm is not parallel to the movement of the Laurentide Glacier, the glacier that went over the area in the past. Therefore, Crosby believed that a violent earthquake caused a keystone fault, a drop in a part of the bedrock that covers the area, which created the chasm. Problems with this theory: A fault could not have been formed at this scale or in this type of rock. Also, the glacier does not have to flow parallel as it often plucks rocks out, meaning that glaciation could have been the cause.(Source 1).
Theory 2: Written by Professor RJ Lougee. Professor Lougee believed that the chasm is an ice marginal channel that was cut through the granite bedrock. Water would have flowed from an ice marginal lake in the Casey Brook Valley. This would have flowed along the ice edge and then would have have entered the location where the chasm is now. This flow of water would have cut out the chasm that is seen today. Problems with this theory: As Professor Crosby observed, there is no evidence of erosion of water as the rocks are angular. Also, it is unrealistic that water would have flowed from the Casey Brook Valley to the chasm because if the low areas were filled with ice it would be almost impossible for this to have happened.(Source 1)
Theory 3: Written by Professor Mauri Pelto. At the front of the chasm there is a glacially polished rock. For this reason glacial action was part of the creation of the chasm. Also the joints on the walls of the chasm do not match up. This only occurs on a fault plane. Tectonic forces in the area have been silent for 200 million years so the fault would have created a weakness in the rock. The rock was then separated into the chasm by glacial activity. The glacier would have plucked bedrock while moving over the pre-existing weakness from the fault.(Source 1)
Conclusion: Theory 3 made by Professor Pelto is the most logical and realistic theory. Pelto was able to combine parts of different older theories to make an ideal single theory with no major flaws. Taking into account the fact that there is both evidence of faulting and glacial presence, Pelto allowed for both to appear in a theory with no other major ideas needed. Joints on either side of the chasm do not match up which occurs when there is action on a fault plane so there must have been an earthquake that initially formed what is now the chasm. As well, there are signs like glacial striations indicating that at least part of the formation of the chasm is due to glacial movement. While there is evidence that the chasm is transverse to glacier movement which makes the glacier a less powerful tool in creating the chasm, there is still the possibility that the glacier could have plucked rocks from the chasm as it moved over the chasm. Finally there is no evidence of water being used in the creation of Purgatory Chasm so the only forces needed to create the chasm were glaciation and faulting. It is therefore understood that Purgatory Chasm was first weakened by tectonic activity and then plucked out by a glacier.