Boulder Rock (photo by Johanna Titus)
Sometimes, while out hiking, we come across some wonderful geological locations. These are places where a geologist gets a real bang for the buck. Well, that happened to us recently. We revisited one of our favorite locations with a group from the Mountain Top Historical Society on one of their summer hikes. We’ve never tired of hiking the circuit around South Mountain in Greene County, so it was fun to do it again with this group. South Mountain lies just south of the old Catskill Mountain House site. It used to tower above the hotel. We had a nice hike, led by Bob Gildersleeve who, literally, wrote the book about hiking up there (Black Dome Press). He has explored the vicinity thoroughly, and knows the area extremely well. And he has learned a lot of geology from reading our columns. So, not surprisingly, he included some good geological locations along the way.
Well, from the Mountain House site we headed up the Blue Trail and then took a left at the Yellow Trail. That brought us back downhill to the very edge of the Catskill Front. It’s a cliff with a wonderful view of the Hudson Valley. Just don’t look down; it’s quite a drop off that edge. But there are a lot of cliffs like this one, so what makes this one special? Its star attraction is a boulder. Not just any boulder, but the boulder of “Boulder Rock Ledge.” Any good geologist would quickly recognize it right away for what it is. This is a remnant of the Ice Age. This boulder is called a glacial erratic.
Long ago, perhaps about 15,000 years ago, there was a glacier right here. It was the Hudson Valley glacier and it filled the whole valley and more. If you visit this site, you need to look out into the valley and use your mind’s eye to fill it with ice. Then you need to look up and see at least hundreds, if not thousands of feet of glacier, rising above you.
The ice is not here anymore; it melted, and that gets us to the boulder of Boulder Rock Ledge. Large glaciers scrape their ways across landscapes. Frequently, they yank loose masses of rock and drag them away. That’s what happened here. Boulder Rock used to be somewhere else, somewhere north of this ledge. It was yanked out of that old location and the advancing glacier carried it here. Then that glacier melted. There must have been a day when the melting ice allowed just a little of that rock to emerge from the glacier. Soon the ice was gone altogether, and the boulder was left behind. It’s been in exactly this spot ever since.
But, we were talking about “bang for the buck;” there has to be more. And there is. The surface that the boulder lies on is rather flat. That’s another thing that a self-respecting geologist will pick up on very quickly. This surface was ground to its present day smoothness by the grinding of the glacier’s ice as it lumbered across this site. Glaciers act like enormously thick and heavy sheets of sandpaper and they typically have large amounts of sand at their bottoms. That does the grinding.
Our group walked about 50 yards south, and found more evidence for this. There is a surface down there where the bedrock has been ground unusually smooth, almost a polish. That location also displays scratches cut into the surface by cobbles that were being dragged along by the glacier (see lower left of our second photo). We looked at that surface and could imagine the bottom of the glacier with its burden of sand, gravel, and cobbles being dragged by.
As luck would have it, the horizon of rock that was polished by the advancing ice, had a number of cobbles buried within it. Hundreds of millions of years before the Ice Age, this bedrock had been a mass of cobble-rich sand. That sediment hardened into rock and, much later, the glaciers went to work on it. As they passed by, not only did they grind this surface, but they ground into and through many of those buried cobbles. In so doing, they sliced right through many of them. These cobbles seem to have had their tops shaved off. See several of these scattered across our second photo. It is quite something to see this for the first time.
And, yet there is still more. At the same location where we saw the effects of glacial polishing, we saw something named “split rock.” See our third photo. Split rock seems to be exactly what its name implies. It looks like it could be fit back into the ledge it seems to have broken free from. How did this happen? The answer is that, as our glacier advanced across this site, its ice formed a bond with the underlying rock. As the ice moved forward, it yanked rock out of the ground and dragged it forward. But, in this case, forward was a very short distance. Split rock was broken free and then dragged only a few feet before the ice melted and transport ceased. Like Boulder Rock, Split Rock has been sitting where it is ever since the Ice Age ended.
We promised a lot of bang for the buck at this location and we think we have delivered. There is a lot of ice age geology up there and all of it is within a very small radius. There is still time this autumn to get out and do something scenic. This hike would be a good choice.
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