The Brent Crater
First recognized in 1951 from aerial photographs, the crater is a circular depression about 3 km in diameter formed in Precambrian crystalline rocks. Geophysical and diamond drilling investigations show that the crater has a present depth of about 427 metres but is partly filled by sedimentary rocks with a thickness of 274 metres. The rocks beneath the crater floor are thoroughly fragmented over a depth of 610 metres. Like the similar New Quebec (Chubb) crater, the Brent crater is attributed to the high speed impact of a giant meteorite. It is calculated that the impact released energy equaling 250 megatons of TNT and occurred about 450 million years ago when this area was probably covered by a shallow sea.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.