Brent

Key facts

Country

Canada

Diameter

3.8 km

Age

about 450 million years (estimates from about 400 to 455 million)

Location

46.08° N, 78.48° W

Brent is a simple bowl-shaped crater about 3.8 km across in Algonquin Provincial Park, Ontario, Canada. It was one of the first ancient impact craters in Canada to be studied in detail, and much of what scientists know about how simple craters fill comes from here.

Discovery. The circular hollow was noticed on aerial photographs in 1951. Scientists from Canada's Dominion Observatory, then running a programme to search for impact craters on the Canadian Shield, visited the site and later drilled a series of boreholes through the crater floor in the 1950s and 1960s.

What it hit. The impactor struck ancient gneiss of the Canadian Shield, more than a billion years old. The blast dug a bowl, which was partly refilled by a lens of broken rock called breccia that slid and fell back in. Beneath the centre, drilling found a layer of rock that had been melted by the impact.

How it survived. Soon after it formed, the crater lay under a shallow sea, and layers of Ordovician limestone and shale were deposited inside it. These soft sediments filled the bowl and protected it from erosion, and much of the fill remains today. The fossils in these layers help date the crater to about 450 million years ago, though the uncertainty is large.

Proof of impact. Drill cores contain quartz with planar deformation features and the impact melt rock, which can only form in a shock event.

Visiting. The crater lies along a road in the northern part of Algonquin Park, where a lookout tower and short trail explain the structure. It is hard to see from the ground because of the forest cover; the shape is clearest from the tower or the air.

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