Last week we visited a ledge just north of the North-South Lake Campground. We discovered that it had origins dating back to a Devonian age river channel. We witnessed flood events in that ancient stream which are preserved in today’s strata. This week we will be looking at the very same strata from a different angle and seeing something entirely different. Take a look at our illustration. You will see that there is an overhanging ledge of sandstone beds at the top, the same as last week. But look below; there you will observe more sandstones. These must somehow be softer than the overhanging beds. They have thus found a way to erode more quickly and retreat farther into the hill. Now look at the very bottom of our photo. You will soon spot a heap of loose flat slabs. They used to be up above – up there as part of the overhanging strata. We can guess that, from time to time, each of them, one by one, has fallen from that “ceiling.” Over time they have piled up right where we see them. Altogether that’s a nice image of some rather commonplace stratified rock. But we looked and began to ponder more about what we were looking at.
We looked at this and thought about that something very different. And that “something” was the top of a typical waterfall. Specifically, we were thinking about something called the capstone of a waterfall. That’s Ge101, introductory geology. It’s a course that almost always talks about the formation of a waterfall. The teachers speak about a sturdy, tough horizon of rock called the capstone. Below it there is always softer, easier to erode rock. Well, there’s actually nothing “soft” about any of those lower sandstones but they are subject to relatively faster rates of erosion. Horizons of the soft stuff are constantly experiencing all the physical and chemical processes of weathering. Those horizons, over time, have been retreating just a little quicker than the overlying capstone strata. That leaves a growing “ceiling,” relatively resistant to all that weathering, hanging up there above. Take a look at our photo and you will see all this. You see, those slabs up there were all only softly cemented to each other. That means that there had been a constant stress imposed by gravity. Every once in a while, a slab of capstone dropped. Then, over long periods of time, more and more blocks of capstone rock dropped, and the waterfall finds itself retreating. We geologists call this waterfall retreat. So, we have discovered all of the elements of a typical waterfall in this typical Catskills ledge; the only thing that is missing is the water! Well, take a look at our second illustration, a print from 1873 by Winslow Homer. Theres the water. Now you can see how Kaaterskill Falls and our ledge are the products of these same processes.
All this conjures up into question an issue that let’s call “ledge retreat.” We are asking – do ledges retreat in a manner similar to waterfalls? On the face of it – yes, they seem to. But, without the help of flowing water, hillslope retreat is a much slower process.
Contact the authors at randjtitus@prodigy.net. Join their facebook page “The Catskill Geologist.” Read their blogs at “thecatskillgeologist.com.”
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