The Tully limestone stands out as a ridge of rock between valleys developed on the Moscow and Geneseo formations. What might account of this landscape pattern? That‘s the exact question. For those of you who aren‘t in 9th grade ES in NY, there‘s a thin layer of limestone between a layer of shale and a layer of black shale, with the black shale on top. Why would this be? THANK YOU!
Remember that shale and limestone form as depositionary features; that is, they form when sediment is layered on top of existing rocks. Shale is formed when finegrained sediments are layered. Limestone, in this case, forms from chemical depsoition of sediment. What is special is that limestone forms under water, and shale forms above sea level in streams and lakes. The best answer is that over the time period of the formations, the land went up and down. The land was up first, to form the Moscow formation. It then went underwater to form the Tully limestone. The land came back up to form the Geneseo formation. Black shale forms in anoxic (without oxygen) environments. As far as I can tell you, the shale was deposited in a stream carrying so many minerals that oxygen was not dissolved.
Shales weather in what are called a recessive pattern. They erode more readily than limestone or dolomite which weather very slowly. If the limestone has been tilted on edge , a valley would form on either side of the limestone ridge since the shales would weather much more rapidly around it. Likewise if two rivers erode the formations (even if gently dipping) at different positions of exposure of the different shales, the ridge will be left between them.
