Fossil Weathering and Degradation

[FossilNerd]

I’m going through my finds from my last hunting trip and came across these brachiopod valves that I picked up. I grabbed them, not because they were the best preserved finds of the day, but because they are the worst. To clarify, I believe the initial preservation wasn’t too bad, but it’s the weathering and subsequent degradation of the fossil that caught my interest. 
 

Most valves that I find in the area (Upper Ordovician road cut) are in pieces. I presumed it was because the valves weathered out of the matrix and THEN were washed around by rain run off, trampled, exposed to the elements, etc. before breaking into pieces. However, these valves are cracked while still being attached to the underlying matrix. They remind me of mud that has dried in the sun. It seems to show that some of the valves at least cracked and split BEFORE coming completely out of the matrix. Then they weather out in pieces. I find it interesting to see examples of them in the middle of the process.

 

The matrix looks like dirt in the picture, but it is hard as rock and the valve fragments are stuck fast. I think this is what is known as mudstone(?). I’m sure the fragments would come off with a little dental pick work, but they are not so loose as to brush off or come off by picking with the finger nail.

 

 

 

 

 

[Notidanodon]

freeze and thaw or just general exanding and contracting by any means ie getting rained on then drying out generally causes cracks but i dont really know (: