This morning Jim Beard, Denny Casey and I accompanied Charles Craddock of Boxley Materials to Boxley’s quarry near Beckley, West Virginia. The VMNH collection is a little thin on Paleozoic plant fossils, so I was hoping to find some nice specimens to take back to Martinsville. Once we arrived in Beckley we were shown around by the quarry superintendent, Greg Jones.
The Beckley quarry cuts through Pennsylvanian rocks, I believe from the lower Pennsylvanian New River Formation (~318 million years old). These are a classic prograding delta sequence of interbedded sandstones and shales (in the photo above the sandstones are light-colored, the shales dark). The sandstones are the deposits in the channels running through the delta, and at the toe of the delta. The shales are overbank deposits; essentially, the low-lying marshes between the channels. Plants grew in these marshes and are preserved in the shales and in the tops of the sandstones.
Deltaic sediments tend to be coarser upstream, and finer downstream. That means if you stay in one spot over time as the delta grows (progrades) to seaward, the toe of the delta moves further away and the sediment in that spot will get more coarse over time. That means in a prograding delta sequence the sediments are inversely graded (they get coarser as you move up the section). That’s what we see at Beckley. Here’s a coarse-grained piece of sandstone from the uppermost unit:
Another interesting feature is this apparent channel cutting into the light-colored sandstone (below). This is unusual because the channel is filled with shale; usually you would expect the channel to be filled with sands. I suspect this was a channel that was cut off and became an oxbow lake. Once it was isolated as a lake, it filled up with muds instead of sands.
The shales are what we were really after, as that’s where most of the fossils are found. The sediments didn’t disappoint us, as they are absolutely packed with plant fossils. Here are some stem segments from Calamites, a giant horsetail:
We’ll probably be making additional trips to Beckley in the future, to collect additional samples of this excellent unit. Thanks to Boxley Materials for making us aware of the site and giving the museum access.