Grand Magazine and Explosion Crater
You are near the blast crater created by the explosion of the Grand Magazine at the climax of the Battle of York fought on April 27, 1813. The magazine was a large capacity gunpowder and ammunition storage structure built into the shoreline bluff close to the water's edge. Though its actual design remains a mystery, contemporary magazines of stone and timber construction suggests some clues. The Grand Magazine stored about 300 barrels of black gunpowder and small arms ammunition.
The retreating British detonated the magazine to halt American attackers and to prevent military stores from falling into U.S. hands. The resulting explosion was responsible for 263 dead and wounded, mostly American. It produced a massive "balloon-shaped cloud" that was visible from the mouth of the Niagara River, about 45 kilometres (28 miles) away, across Lake Ontario. Shortly after, British Regular troops fled York east toward the safety of Kingston.
Plaque via Alan L. Brown's site Toronto Plaques. Full page here.