A large bit of sandstone or cemented sand conglomerate thats been eroded. Some parts are harder than others in the layers and when worn give it that bivalve look. Good piece for yard art
I noticed this was your very first post - so, Welcome Aboard MRUS! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forum: Select Your Area.... and selecting location information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country). You might also consider adding your state (or country) to your displayed profile (SETTINGS -> EDIT PROFILE) - members may have more success helping you (your location may help, etc.)...