The statement;"finding clam shells in a rock indicates that the region was once a shallow marine environment" is true.
Rocks serve as reservoirs of fossil records. By examining fossil records in rock samples, we can have an idea of what species existed in an area and what the area originally looked like.
Clam shells are shallow dwellers, they are found in shallow marine environment. Hence, if clam shells are found in rocks, it was certainly a shallow marine environment.
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