Purgatory Chasm, Rhode Island

August 12, 2013

While on a trip in New England we happened to visit Purgatory Chasm, which had incredibly interesting geological features. After doing a bit of research, I found out about boudinage and Purgatory Conglomerate. I wrote the following paragraph before researching the true geology of Purgatory Chasm.

Did you notice the intrusion of what I think to be quartz pictured above? It looks a whole lot like quartz, but quartz is a harder rock to erode, and there are other intrusions that have been weathered away, possible forming “shears”. So the rock could be quartz and there was another kind of rock that also formed dikes, or the rock could look like quartz but be a soft rock. One of my hypotheses was that a conglomerate of ocean tossed stones, because all had smoothed surfaces, was forced deeper into the earth. Once this happened, a finger, or dike, of magma pushed its way through the stones, because they are only held together by some sort of natural cement. Of course, this gives no explanation as to why the rocks were sheared, seeing as the magma would not likely cut all the rocks in such a smooth line, let alone the same line. Another idea would be an earthquake, but seeing as Purgatory Chasm itself ends abruptly, this suggest it wasn’t cracked. It almost looks like a chunk was just removed. But one wall, the left, made in reference as one faces the ocean, is smooth, while the right is rough and jagged. There are also far more shears on the left wall and its respective cliff-top. Below is what I have found about the real geology of Purgatory Chasm.

After skimming part of this article: http://www.planetary.brown.edu/mappers/Fieldtrip_2003/Purgatory.html, and not finding out why the stone, called “Purgatory Conglomerate,” shears the way it does, I found out that the conglomerate was formed by “extension,” or stretching, under or around a less yielding type of stone, in this case that was sandstone. Later on in the article I found that the shears are a result of “boudinage.” Boudinage is when a rock is formed by extension, as I mentioned above, but I did not know that this caused shears. By bending under high pressure, certain areas of this layer of rock were stressed higher than others. I believe I read the diagram, featured below, correctly when I inferred that this caused the rock layer to snap. This would also explain why the chasm ends abruptly, seeing as it makes a shift from Purgatory Conglomerate to sandstone. As the years went by, the ocean probably eroded further into the sandstone backing to the crack, causing the chasm to look as it is today.

Diagram of

Diagram of “boudinage”

For more information about boudinage visit the boudinage Wikipedia article: http://en.wikipedia.org/wiki/Boudinage

Also see: http://blogs.agu.org/mountainbeltway/2010/08/20/purgatory-conglomerate/