Over the Thanksgiving holiday I had the chance to do some exploring of borrow pits and quarries north of Harrisburg. One of the sites I visited was a borrow pit near Roseburg, PA. I actually passed it while headed somewhere else but turned around to see if it had anything good.
Here is a picture of the pit facing north. You can see that there is a gentle slope to the rock and it actually dips to the southeast (right side of the picture).
Here are a couple of the more common fossils that I found
A large Rhipidomella penelope brachiopod
A flattened Bembexia gastropod
Some fossil "hash"
I did find some other fossils there that were new finds for me from PA and from the area that I will detail in subsequent posts.
I suspected that the rock was part of the Hamilton group and more specifically the Mahantango formation and I was right. However what surprised me was that when I looked at the geologic map most of the quarry is shown to be part of the Trimmers Rock formation which is upper Devonian and part of the Chemung group.
Here is a stratigraphic diagram of the Devonian rocks in Pennsylvania from the PA Geological Survey website HERE.
Here is a screen capture of the local geology of the pit from Google Earth. I am using the geologic map overlay available from the USGS website HERE.
The orange color represents the Mahantango formation and the green color is the Trimmers Rock formation. The main areas I was exploring were on the northeast side to the central part of the pit although most of the fossils seemed to be in the layers that were on the bottom of the pit. This map together with the dip of the rocks and my knowledge of local stratigraphy tells me that the layers in the bottom of the pit are most likely from the Mahantango formation while the top most layers are from the Trimmers Rock formation. That still doesn't tell me for sure where the fossils I found in the gravel piles that were pushed around came from but I'm fairly confident they are from the Mahantango.
Below is a wider geologic map view to put the regional geology into perspective. The layers I was exploring are on the outer edge of a large synclinorium (the Minersville Synclinorium) that has Mississippian rocks at it's center (actually Pennsylvanian rocks outcrop farther to the northeast but Mississippian is the youngest visible in this picture). A synclinorium is a large group of layered rocks that generally bend downward into a relaxed "U" shape but has smaller folds within it that may bend into individual tighter anticlines ("A" shapes) and synclines ("U" shapes). Not far to the north and east the synclinorium bends up into an anticlinorium (the Tuscorora Anticlinorium) which exposes older rocks from the Ordovician. The thumbtack shows the general location of the pit.
This same pattern is repeated all over the ridge and valley providence that the Appalachian mountains compose. With the Google maps and my knowledge of the stratigraphy I am better able to look for other potential sites and have a good idea what to expect. So to bring it all back together, the pit has fossils from the Mahantango and possibly the Trimmers Rock formations. Also while the rock layers are only gently dipping there may have been some geologic effects visible in the fossils (as you'll read in subsequent posts).