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I'm visiting family in San Antonio, Texas, and have been doing a little fossil hunting as well, including in a nearby creek. Based on a local geologic map, I believe this creek mostly exposes the Edwards Group (Albian age), though there appear to be some younger (Cenomanian age) rocks from the Del Rio Formation mixed in as well, based on an Ilymatogyra arietina shell that I found a couple of days ago. This creek isn't very fossiliferous. In about 3 hours of searching, I've only found a single I. arietina and a few small shell impressions in rocks. However, today, I also found the rock below and am wondering if it's something more. I am moderately confident that the bit in the lower right is a shell impression. What I'm unsure of is whether there's a bigger (worn) shell mold here too or just a bit of pareidolia. Any thoughts? Here's the outline of what I'm seeing -- the dimensions of what I've outlined are approximately 7cm wide x 6cm tall. Here's further detail of the part in the lower right that I'm more confident is part of a fossil shell/shell impression: The "shell mold" is raised relative to the surrounding rock, though is pretty heavily eroded if it is a fossil: Here as well is the I. arietina I found -- no doubt that this one's a fossil at least! And here's a wider shot of the creek: Thanks for looking!