 Keeping in mind that this is a cold climate, a method for transporting much coarser sediment is by ice rafting. Glaciers have the ability to transport very coarse sediment like cobbles or boulders. If glaciers entered the lake from the surrounding highlands, the ice would have broken off to form icebergs (a process known as "calving"). At some point, the iceberg melted and the stones dropped down into the lake bottom. In places, you can see evidence that the stones deformed the underlying soft layers as they landed. Thus, these are called "dropstones", and are excellent evidence that glaciers existed in Virginia!!! Remember, the Konnarock Formation is between 760 and 570 million years old, so we are most definitely not talking about talking about the more recent ice ages from the Pleistocene. |
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The dropstones are light colored, coarse grained, and feldspar-rich. Thus, they appear to be pieces of the Cranberry Gneiss basement rock seen earlier in stop 1. We will see the Cranberry in actual outcrop in stop 5.
