|Athro, Limited Gubik Field Trip Section 20 Karmuk|
You are standing about 3/4 of the way up the cliff (dark blue arrow), looking in the direction of the green arrow. This is near the base of your trench (light blue arrow).
Here is a little lens of shells in a sand layer. The sediments here are interbedded grey green silty clays and light brown fine sands. The shells are mostly well preserved, including both gastropods and bivalves.
Most of the bivalves at this horizon have a very unusual mode of preservation. They are butterflied bivalves. That is, both valves are present and attached to each other, but rather than being closed, as they would have been during the bivalve's life, the two valves are spread wide open. This is what will happen to a dead clam that you pick up from the beach if you bring it home with you. As the clam starts to decay, the muscles that hold the shell closed start to fail, and the elastic ligament in the hinge of the clam forces the valves apart. If you left this dead clam on the beach, wave action would soon have pulled the valves apart. A clam that dies in the sediment and gets fossilized there could be preserved with both valves held together in life position. Dead clams exposed to wave action and decay processes, soon get disarticulated into loose single valves (and then broken into fragments by more pounding by the waves). There is only a narrow window of death, exposure on the surface, and burial that will allow clams to be preserved like this in life position.
You find a small piece of wood here. You can collect the wood and attempt to carbon 14 date it by pressing the button below. You can also collect a sample of these shells for amino-acid geochronology.