This is an example of what i start with. The Matrix is very hard. Fine grain which is good for preservation. Some of these large concretions also have a crabs in the center which most likely was the catalyst for the concretion the Aturia are present in. The Aturia are also found in the siltstone around the concretions but are poorly preserved. The productive layer is only 1.5 feet thick. There are concretions above and below this layer but they are non productive below and there is a very productive 6 inch thick crab concretion layer above which i call the beach layer due to the characteristics of the crab concretions i find there which are riddled with piddock borings and some of which have fossilized barnacles on the outside of the concretion. Also in the beach layer there are fine gravels along with the occasional bone fragment and i also found a whale tooth in this layer. Over a 4 square mile area i have found the beach layer in five exposures with the Aturia layer just below. I can picture the rising of the sea floor due to volcanic activity on the olympic peninsula. Bluffs eroding crab concretions to the beach below and then a large event which covered the beaches below preserving a beach 32 million years old.
The Aturia bearing concretions are smooth and rounded though typicaly irregular shaped. They never rolled around on a beach and i have not found another beach layer lower in the formation. Tells me that uplifting possibly happened faster after the event that covered the beach. Today the Aturia layer is several hundred feet above sea level in the mountains of western Washington state. At which depth were the Aturia at when they were preserved is a mystery to me but information on Aturia Angustata within the Astoria formation suggest two hundred meters.
The rock is so hard and the concretions so large i split them with a sledge hammer first then go through them with a three pound sledge and a chisel. Once i have a workable piece i use a me9100 airscribe to expose the Aturia fully.
Some of these are works in progress :)