In the northern part of Quebec, among some of the oldest rocks on the continent, a curious ring of water surrounds a remarkably circular island. This is the site of Manicouagan crater; with a diameter of more than 70 km across its prominent rim (the outer shore of the lake), it is one of the largest and best exposed craters on the planet. Manicouagan was formed by a meteor’s collision with Earth more than 200 million years ago. Many of the local rocks were melted by the heat and pressure of the impact, and their ages were "reset" to reflect the time of impact. As can be seen in the image, most of the region is made of Precambrian rocks (the reds, yellows and oranges), but much of the island is made up of Triassic rock (green).
As a result of underground fluid pressure pushing the rock in the crater upward during a bout of post-impact equilibration, Manicouagan has a plateau in the center, rather than the characteristic depression associated with craters. The remaining, ring-shaped depression filled with water, creating the lakes Manicouagan and Mouchalagane. These lakes have since been dammed to create the annular Manicouagan Reservoir around that central plateau, now an island, the Ile René-Levasseur.
source:
http://www.nationalatlas.gov/articles/geology/features/manicouagan.html
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