Well, I had intended to take my students on a field trip tomorrow to the Mt. Rogers area, and I was going to do a post on the rocks there. Unfortunately, bad weather in that area has forced me to cancel the trip. Fortunately, I’ve been to the area before, so I can still post a picture of what we would have seen. The round rock in the photo above is called a dropstone. Dropstones are rocks that were imbedded in glaciers. When a glacier reaches the ocean, it breaks up into icebergs, which float away, still carrying any embedded rocks. Eventually the iceberg melts, and the embedded rocks drop to the seafloor. Since most seafloor sediments are fine-grained muds, large dropstones are easy to spot.
The dropstones found in the Mt. Rogers area were deposited in the late Proterozoic Eon, between 760 and 570 million years ago. It’s thought that, at that time, Virginia was pretty close to the equator.
It turns out that late Proterozoic sediments from all over the world show evidence of glacial activity. Andy Moore gave me this rock, from his backyard in Indiana. It is a tillite (another type of glacial rock), from the late Proterozoic of Canada (ironically, it was carried to Indiana by glaciers during the most recent Ice Age, around 15,000 years ago.)
The wide distribution of late Proterozoic glacial deposits led to the development of the “Snowball Earth” hypothesis. The basic theory is that the late Proterozoic experienced a runaway ice age, in which thick ice sheets covered the entire Earth. (A derivative hypothesis, “Slushball Earth”, holds that the equatorial oceans may have had semi-seasonal sea ice, like today’s Arctic Ocean.)
Snowball or Slushball Earth has had an impact on paleontology as well. One characteristic of Proterozoic sediments is that they contain almost no fossils. During the next time period, the Cambrian, there is a dramatic increase in the number of fossils, both the organisms themselves and their traces; this event is referred to as the “Cambrian Explosion”. Almost all the major phyla of animals first appear in the fossil record at this time. It may be that organisms were evolving in relatively isolated deep-sea environments (which don’t preserve in the rock record), and with the end of Snowball conditions they were able to rapidly move into shallow-water environments for the first time.