Hash plate from the Mahantango formation of Pennsylvania

On a recent stop at one of my favorite Mahantango formation collecting spots, I came across this hash plate. It's presumably a record of a storm event which is why there is a diversity of fossil species and why they are jumbled together.

The stippled bar shape in this photo is likely a segment from the exoskeleton of the trilobite Dipleura. It is surrounded by impressions of a Spirifirid and some Chonetid brachiopods.

Here is the impression of a crinoid column segment. The five fold symmetry gives it away. I'm not sure what species it belonged to but it is nicely decorated. It appears that it was resting under a Chonetid type brachiopod shell but was exposed when I split the rock.

This is a fairly good impression of a Chonetid type brachiopod. By the size and shape I would lean towards Devonochoonetes coronatus but without the spines that line margin of the hinge line, I can't say for sure.

Lastly there is the impression of a pelecypod with some very fine, concentric growth lines.  I would venture a guess that this is Nucula sp. similar to Nucula bellistriata like this one from the Panther Mountain formation of New York.

The Mahantango formation is Devonian in age (Givetian stage) and the portion I was collecting from is roughly equivalent to the Panther Mountain formation of New York. The exposure was a borrow pit near Seven Stars, PA in Juniata County.

Random hash plates from the Silica Shale

The megafauna of the Silica shale gets all the attention when I'm looking for fossils in the field. When I'm at a site I've been consciously trying to pick up pieces of the bedrock to bring back a study. One reason is to better understand the paleo-environment that the animals lived in and another is to search for smaller members of the community. These plates are often referred to as "Hash" plates in an effort to describe the large quantity of shelly debris that is often present. Presented for your viewing pleasure are three small matrix pieces. While the pictures are not of the best quality, see if you can pick out the following fossils:

Tentaculites
Hyolites
Phacops rana
Chonetes
Ostracods
Crinoid stem segments
Sulcoretipora
Bryozoans

Mahantango Hash Plates

In places the Mahantango is fairly limited in fossil content and what is found is restricted to thin layers or lenses that feature a large quantity of shells. These are often referred to as "Hash" layers as the fossils are generally overlapping one another and picking one shell out of the crowd can be difficult. Usually these type of occurances form as lag deposits due to currents or maybe a storm washes a bunch of shells into a lower lying area. Whatever the case they can make for interesting looking assemblages. Here are a few that I've recently found at different locations within the Mahantango.

I found this piece on a pile but could not locate more. Front side has lots of Chonetid type brachiopods.


Back side has a Brachiopod/Pelecypod mix


This piece fascinated me when I saw this side. It's a current deposited pile of Chonetid type shells. Cross section view:


Head on view:


Another angle:



A mix of Spirifer and Chionetid shells with some slight distortion due to geologic pressures.


A mix of small Pelecypods. Some of the shells have a silvery coating on them... Hematite perhaps?




At one locality there is a layer of this limestone Coquina of broken shells. Likely formed along a surf zone along the coast. Evidence of temporary shallowing?