I collected these samples on Peddocks Island in Boston Harbor.  We were told by the professor running the field trip from Northeastern University that the fossil clams were dated as being 440 thousand years old. The most surprising aspects of the  shells is that they aren't entirely calcium carbonate. Most of the CaCO3 has been replaced by silica! The shells are very thick. No complete shells were found in the cliff, only fragments. The magnified image shown under the multiple shells is of the shell at the bottom of the multiple samples.

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