With its proximity to Old Mine Park, a place famous for its topaz and fluorite-rich hydrothermal veins and amphibolite-hosted scheelite and ferberite pseudomorphs thereof, the defunct plaza and the new Home Depot construction were a magnet for mineral collecting. The bedrock is mostly amphibolite, with some marble, mapped as the Silurian basal member of the Silurian-Devonian The Straits Schist, which are locally very well exposed. There was generally little overburden of Pleistocene age on the site.
However, during the Home Depot construction a large, underground storm water storage gallery was constructed by blasting a chamber into the bedrock. This was presumably done because of the limited land available for a surface retention basin, so it was placed under the parking lot by filling the space with nested, hollow concrete structures that could support the overlying parking (see Figure 1). The walls of the gallery exposed mostly amphibolite bedrock, but the southeast corner revealed a layer of overlying glacial till.