Mount Rogers

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.

Late Proterozoic Konnarock Formation sediments from Mt. Rogers, with no trace fossils.









Trace fossils (burrows) in Cambrian rocks. Left-Deadwood Formation from South Dakota. Almost everything you can see is a burrow. Right-Unicoi Formation from Virginia. The dark lines are burrows.

Radford University has an excellent virtual field trip for the Mt. Rogers area. Here is a site on the Snowball Earth hypothesis.

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11 Responses to Mount Rogers

  1. Grenda says:

    ” relatively isolated deep-sea environments (which don’t preserve in the rock record)” – why the lack of preservation?

  2. Alton Dooley says:

    Deep sea environments are, well, in the deep sea (and by deep, I mean maybe a mile below the surface). These areas are on the abyssal plain of the seafloor, which is composed of ocean crust that will eventually be destroyed by tectonic subduction. I believe the oldest ocean crust anywhere in the world is only Jurassic in age, maybe 180 million years old (someone correct me if I’m wrong, I’m doing this from memory.) Any abyssal plain deposits older than that have been subducted, an the ones that haven’t been are still at the bottom of the ocean and mostly inaccessible.

    The rich marine fossil deposits that are older, like the Ordovician material I was collecting in Indiana a few days ago, were deposited during high sea level periods when low-lying parts of continents were covered by the ocean (called eperic seas). Most fossiliferous sediments were deposited in water no more than a few hundred feet deep. In certain tectonic settings you can uplift deeper water deposits, but this is fairly unusual, and they’re often metamorphosed.

    The discovery that the ocean floor was young was a mystery until the development of plate tectonics in the 1960s, which predicted a young seafloor.

    As an aside, a teacher told me yesterday that the earth science textbook her district uses (which was published in 2002, I think) still says that the oldest rocks on Earth are in the deep ocean–an idea that was disproven at least 50 years ago.

  3. Alton Dooley says:

    That would be an epeiric sea, not eperic.

  4. Doug says:

    Ooh, critter burrows. Smithsonian had a really cool burrow. It was from a beaver, not some little invertebrate, though.

  5. Alton Dooley says:

    The burrow you’re referring to is probably Daemonelix. They’re found in the Miocene of central North America. They’re huge spiral structures (nicknamed Devil’s Corkscrews), up to 6 feet deep. Very impressive looking.

    Daemonelix is unusual among trace fossils in that we know exactly what animal made them; the extinct terrestrial beaver Paleocastor. Paleocastor skeletons have been found in the bottoms of the burrows; the Smithsonian specimen has the skeleton still in the burrow. If I recall correctly, it was only around the turn of the century that Daemonelix was recognized as a rodent burrow.

    Besides the one on exhibit at the Smithsonian, I’ve seen them on display at the Nebraska State Museum and the American Museum.

    Oddly, some years ago I saw a burrow at Chesapeake Beach in Maryland, in the marine Calvert Formation (also Miocene), that had a somewhat similar spiral structure, but it was much smaller than typical Daemonelix (only around 1 foot). Don’t know what made that one!

  6. Doug says:

    Yeah, it still had the beaver in the bottom. That’s how we know who made it! Early theories were they were either casts of tree roots or lighting strikes. On the subject of holes in the ground, at Agate Springs, Nebraksa, paleontologists found the remains of a bear dog in it’s den. Did you see that at themuseum? And almost a year ago scientists found the first burrowing dinosaur.

  7. Alton Dooley says:

    Hadn’t heard about the bear dog den; they didn’t have anything about it in Lincoln. It’s possible Agate Fossil Beds NM could have it. I’ve never visited there.

  8. Grenda says:

    I had thought that tectonic action might be the reason why but was not sure. I didn’t realize that a mile was considered deep ocean. I more or less think Marianas Trench. I think I had better do some extra research….

  9. Alton Dooley says:

    Average depth of the modern ocean is about 2 miles, but 1 mile is deeper than anything you would get on top of the continents. The photis zone (the depth to which photosynthesis can occur) is only about 100 meters.

    The bear dog is very cool. I didn’t see it on exhibit in the museum. The museum’s research collections are in a different building, and I didn’t get over there last week. I did visit the UNL collections last summer, but I was looking at horses, tapirs, and dromomerycids, not carnivores.

  10. Alton Dooley says:

    Umm- PHOTIC zone. Seems I need a bigger keyboard (or smaller fingers.)

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