Hays County, Texas

[denise.ereka]

This may be a wild goose hunt, but my teens brought back two rocks (private land, permitted collection) near Wimberley, Texas in Hays County. I'm hoping to have some rough idea of if they are fossils and what general family they may have fallen into. 

 

I apologize for the lack of a ruler as I am not at home. The large disk shaped possible fossil is about an inch and a quarter across. The larger flat one is about an inch and a half. 

 

If anyone has recommended resources for better understanding the geology and fossil history of this area, I would be grateful for any direction.

[Creek - Don]

I found lots of fossils in North Central Texas and lots of bivalves Protocardia texana in  Bell County, but your photo  doesn't look like bivalves at all.  Looks like a chunk of limestone rock. 

Bottom photo is from near Killeen, Texas, Stillhouse lake. Some of the rocks resembles your photo.  Perhaps @grandpa or @JohnJ can help.  His down in Austin area. 

Your formation is Glen Rose formation. 

 

 

The Glen Rose Formation is a shallow marine to shoreline geological formation from the lower Cretaceous period exposed over a large area from South Central to North Central Texas.

[Fossildude19]

These actually look like internal molds of bivalves.

[denise.ereka]

1 minute ago, Fossildude19 said:

These actually look like internal molds of bivalves.

that would also be very cool - they didn't like mineral to me but I also know that limestone can be oh so very weird! 

[denise.ereka]

12 hours ago, Creek - Don said:

I found lots of fossils in North Central Texas and lots of bivalves Protocardia texana in  Bell County, but your photo  doesn't look like bivalves at all.  Looks like a chunk of limestone rock. 

Bottom photo is from near Killeen, Texas, Stillhouse lake. Some of the rocks resembles your photo.  Perhaps @grandpa or @JohnJ can help.  His down in Austin area. 

Your formation is Glen Rose formation. 

 

 

The Glen Rose Formation is a shallow marine to shoreline geological formation from the lower Cretaceous period exposed over a large area from South Central to North Central Texas.

 

thank you for the help in placement - and I agree that limestone can be very funky. It has fooled me before.

[Creek - Don]

Most bivalve fossils from central Texas are like these.  Usually they are complete. 

 

[Fossildude19]

@erose @JohnJ  @Uncle Siphuncle  @BobWill  @DPS Ammonite

[BobWill]

It would be odd to have the same symmetrical shape in two places on the same rock so I am inclined to agree with internal mold of bivalve. Since molds are commonly composed of the same material as the matrix, the whole thing is likely to be limestone. Being internal they would just show the reverse of the inside of the shell which would not have the growth lines you can see on some of the examples Don posted. Some clams are almost round-shaped like that without the pointy bit seen near the hinge on others.

[denise.ereka]

Thank you all so much - I appreciate the information and kindness. 

[erose]

Yep, fossils for sure, and probably bivalves (clams). There are layers in the Glen Rose Formation that are packed with them, and other things too.  These appear to be from one of the more "weathered" beds, maybe eve part of one of the rudist reef layers. They have that "look." 

 

Your teen s should keep their eyes open for other types of fossils like snails, sea urchins or corals.