Ten thousand years ago, the receding glacier left deep outwash consisting of sand and gravel deposits that form the banks of the Seekonk River and the bluffs above it. On eroded trails and banks the yellow sand and small stones that underlie the topsoil are visible. Over time, decaying plant material formed the layer of topsoil that sustains the woods and undergrowth of Blackstone Park.
Cumberlandite, a mineral unique to Cumberland, RI, was carried from Cumberland by the glacier and deposited in the park. A number of pieces of this black rock flecked with light crystals have been found and continue to turn up. Cumberlandite is weakly magnetic and unusually heavy. For the true lover of geologic detail, Alonzo W. Quinn, in Rhode Island Geology for the Non-Geologist, writes that it “is a porphyritic rock, composed of larger crystals of plagioclase feldspar in a finer-grained groundmass of magnetite, ilmenite, and olivine”. Cumberlandite may also be found along the riverbank in Swan Point Cemetery.