This theory came to be from the efforts of W.O Crosby and his son Irving B. in 1925. This theory states that there is a keystone fault located directly underneath the present site of the chasm. The fault is “master joints converging downward and bounding a V-shaped mass of rock” (Smith). When an earthquake vibration hit the area where the master joints are located, it caused a tensile force in the earth’s crust resulting in the joints pulling apart and the V-shaped mass of rock dropping down. Then, once the shock wave passed, the earth’s crust tried to return back to its original position. However since the mass of rock blocked it, the V-shaped mass went through extreme compressive force (Smith). Since the rock was unable to withstand this pressure, it was crushed. Through many earthquakes hitting the area, the rock would become extremely damaged, and thus creating the chasm (Smith). Crosby also stated that there was no way for glaciation to have caused the chasm because there is no evidence of glacial drift and the direction of the chasm is perpendicular to the direction of glacial drift ("Purgatory Chasm State Reservation"). 

 

Problems:

Professor Mauri S. Pelto pointed out that although Crosby was correct in that there is not water erosion, and the chasm does look like a fault plane, there is no way that a Keystone Fault could occur at this scale and with this type of rock because it would require the entire earth’s crust to drop under the chasm. However, Crosby was not correct in saying that glaciation could not have been a cause. There is, in fact, a lot of glacial drift present in the chasm, and glaciers pluck rocks out of valleys so there is no need for them to have been running parallel to each other ("Purgatory Chasm State Reservation").

 

 

 

 

 

 

WATERFALL THEORY

 

A second theory of the formation of this chasm is the “Waterfall Theory” which was created in 1950 by R.J. Lougee. Lougee examined aerial photos of the chasm which showed that it is part of an “ice-marginal channel system cut through a ridge of bedrock in an area of strongly jointed granite surrounded by regions of schist and gneiss” (Smith). He believes that there was a waterfall that descended from 550 ft. elevation at the “ice-polished saddle” to 475 ft at the base. The top of the waterfall eroded headward through the granite until it reached an elevation of 530 feet where washed surfaces of “bouldery till” were found. Lougee thinks that when the last glacial event retreated to a location which met the north side of the col in the granite ridge, the ice-marginal lake located in Casey Brook Valley was released suddenly (Smith). This resulted in large quantities of water flowing along the edge of ice until they were able to enter Purgatory Brook Valley through this col in the granite. During this event, large boulder which rolled over the falls broke off of their joint blocks and everything except the largest of these blocks was washed away. In the meantime, expanding rock from this continues to disrupt the granite of the area. This theory provides an explanation for the reason by the floor of the chasm is completely hidden from view; the weathered rock from the walls of the chasm accumulated at the base (Smith). Lougee gave further evidence to support this theory. First, the channel of Purgatory Brook has many potholes which show how large amounts of rock were released through the chasm. Also, the great amount of digging that occurred at this location indicate that a very large volume of water worked very quickly to create the chasm. Lougee also stated that this process would have been completed before all of the ice melted from the upper end of the chasm (Smith).

 

Problems:

There are many issues with this theory. First is the lack of evidence of water erosion on the walls of the chasm. It is physically impossible for the rock to have been in contact with water and have not been smoothed and polished in the process. Next, there is the issue of how the water could have traveled from Casey Brook Valley into the start of the chasm. At the beginning there is a ridge, meaning that the water must have flowed over a ridge. The only way for that to occur would be for the areas at a lower elevation to have been full of ice, which is very unrealistic and there is no evidence of a river leading from Casey Brook Valley to the chasm ("Purgatory Chasm State Reservation").

 

 

 

 

 

 

Dr. Mauri S. Pelto, a professor of geology at Nichols College has his own idea of how the chasm formed. He used lots of evidence seen at the chasm to try and piece together an idea of its formation. First, glacially polished rock seen at the front of the chasm indicate that it could not have been polished and then have been exposed to all of the water from Casey Brook Valley. There is a lot of evidence showing that the chasm formed from a fault plane. Grooves in rocks in the chasm show that the rocks were scraping against each other on a fault. Pelto also found that joints which point north on one side of the chasm do not match up with ones found on the opposite side which means that they must have been moved over time by a fault. However, tectonic activity in this area has been quiet for over 200 million years, so although it formed the initial crack in the rock of this area, the chasm itself was created through ice and water. By the southern end of the chasm, the ground is covered with large boulders which came from the chasm. However, rockfall in the area is rare, so Pelto was left with the question of how these rocks ended up here. Water is out of the question because it would have rounded them and scattered them evenly. Glaciers are a more likely explanation. Glaciers move material like rocks through a process called plucking, which is when rocks are frozen to the bottom of a glacier and then carried away, eventually falling off as the glacier encounters friction with other bedrock. The fact that there is no source of water in the chasm for a stream, and the boulders are large, not rounded, and scattered over a large area indicates that glacier erosion helped shape the chasm. While a glacier itself is too large to flow through the chasm, water coming off of the glacier and refreezing in the chasm could have allowed the rocks to be added to the base of the glacier and transported out of the chasm ("Purgatory Chasm State Reservation").