If you are like us, then one of your favorite spots to visit is the Mountain House ledge. That’s where the famed Catskill Mountain House Hotel once stood. It stood along the edge of this cliff and towered above the Hudson Valley. Countless people have stood right there and, enthralled, gazed down at the view before them. We have done this, ourselves, many times; we never get tired of being there. But then we look left and right and see something else. Take a good look at our photo. In the foreground is the edge of a very straight cliff. Behind it lies another straight ledge. This second one has a compass direction which is approximately northeast to southwest. Then in the distance are three more. These ledges seem to define a series of 90-degree angles. There is a pattern here.
How many times have we said it: when Nature presents scientists with a pattern then she is challenging them to solve a problem. Let’s get to work on this one. There’s not a geologist anywhere who would not quickly recognize that these cliffs are a series of straight vertical fractures. These are called joints: geological joints. They formed in response to stresses generated when Africa collided with North America. That was about 325 million years ago. Take a look at our second photo. That shows two more joints, but these do not define cliffs. How is that? What turned some joints into cliffs and left the others alone?
Well, we have been around for quite some time so we quickly saw the answer. We looked north and in our mind’s eye we envisioned the great Hudson Valley glacier advancing to the south. It swelled up the slope below and rose all the way to this level. Now it was rubbing up against the Wall of Manitou, this the Catskill Front. The moving ice stuck to the bedrock and then yanked massive blocks of rock out and dragged them off to the south. Those blocks broke along the joints, mostly the northeast/southwest-oriented ones.
So, there was a lot going on right here late in the Ice Age. See our third photo, courtesy of the US Geological Survey. Do you see the dashed line? We are guessing that all that is bedrock that was eroded away by the passing ice, That’s a lot of missing Catskill Mountains! Now, see the southwestern trending Wall of Manitou. It’s ten-mile-long straight line and it was carved along those joints by the Hudson Valley glacier. All these are things we think you should know and understand the next time you visit the ledge.
Contact the authors at “randjtitus@prodigy.net.” Join their facebook page “The Catskill Geologist.” Read their blogs at “The Catskill Geologist.com.”
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