Dolostone
This block of dolostone, a magnesium-rich variety of limestone, was found buried in boulder-studded clay about 10 metres below the surface during the excavation of the Yonge Street subway in August of 1972. Outcroppings of dolostone are found generally in southern Quebec and probably this stone, weighing seven tonnes, was transported by glacial ice along the St. Lawrence Valley some 13,000 years ago.
Plaque via Alan L. Brown's site Toronto Plaques. Full page here.