This is fundamentally different from stops 1 and 3. At stop 1, the large cobbles touched, and all the grains were quite well rounded. There was very little mud in this rock. This is evidence of water washing through the sediment, as in a stream. At stop 3, the large grains were "matrix supported" like here. However, the matrix was layered with the varves that were described in stop 2. Thus, stop 2 represented dropstones in lake deposits.

A couple of things need to be said about our lake. This lake was fairly large - the Konnarock Formation can be traced from the Tennessee border 45 km to the northeast through much of Grayson County with a total thickness of 1,100 meters of rock!!! The general trend is that the bottom of the formation appears to be dominated by the lake sediment itself - the layered muddy/silty rocks seen in stops 2 and 3. Higher in the formation the sandstones and diamictites take over, indicating that the trend was for the lake to fill in through glacial deposition, debris flows, or from stream channels entering the lake. Ultimately, the glaciers themselves advanced over the lake. How large were the glaciers? It is difficult to know for certain, but all of the cobbles seen here are derived from rocks found in the Mount Rogers area. Most of the cobbles are of the Cranberry Gneiss (light colored feldspar-rich rocks), and also of the maroon rhyolite. The Cranberry cobbles dominate, so the glaciers quite likely flowed off a granitic basement highland. Since all the cobbles are from local rock, the glaciers were probably small - otherwise known as "alpine" glaciers. How widespread was the glaciation? It is hard to give a definitive answer. Old glacial deposits, being land deposits, are inherently difficult to preserve. However, late Proterozoic glacial deposits from elsewhere in North America are known, so this might have been a time of significant cold climate, but these are the only glacial deposits known in the southeast from this time period.

There is a controversy about the contact that we just crossed: in places the Konnarock-Unicoi contact is knife-sharp, as if there was no break in the depositional sequence. However, the big environmental change indicates that some significant time may have elapsed between the two formations.

What about the geologic time scale? The Konnarock is from the Proterozoic Eon, part of the Precambrian, while the Unicoi is Cambrian (a period in the Paleozoic Era) which is in the Phanerozoic Eon. We have just crossed an Eon boundary!!!

Another big part of the geologic story is recorded here: the ocean these sands were deposited in was not the Atlantic Ocean of today, but the precursor ocean called the "Iapetus Ocean". This was the ocean that eventually closed when North America became part of the supercontinent of Pangea. The Iapetus Ocean survived for another 300 million years or so. Along the way, it became host to the reefs that formed the limestones of the Great Valley - but that is another field trip!

One more note about these rocks. The fossil Rusophycus has been found here. These are trace fossils made by trilobites. They apparently liked to scoop out the sand and create resting places that are heart-shaped. Don't try to find them, though. According to Ed Simpson, the Unicoi Formation expert around here, they are exceedingly rare (of course, how did he find them???). Just food for thought...