Ile Rouleau (50° 41′ N, 73 °53′ W) in Lake Mistassini has a circular shape, 1 km in diameter, with a glacial tail pointing southwest. The island is formed of argillaceous and micritic dolomite of the Mistassini Group and shows extensive tectonic disturbances, in contrast to the very slight regional deformation. The distortion of the bedding appears to reflect both solid rotation and internal deformation of tectonic blocks. Dikes and irregular masses of polymictic breccia with a matrix of finely comminuted rock particles cut the beds. Shatter cones are conspicuous all around the island. A few silty quartz grains in some breccia fragments show multiple sets of planar features. The size of the shatter cones is limited by the thickness of the beds in which they occur. Their average apical angle is 94°, and the bedding to cone axis angle appears to be between 45°and 70°. The cones point upward and towards the center of the island. These features suggest that the Ile Rouleau structure is an astrobleme (scar left by the impact of a large cosmic body). The age of the structure is unknown within wide limits; the Mistassini group in which it occurs is upper Aphebian (1.8 b.y.) and it is covered by Pleistocene Glacial deposits. The small size and good preservation suggest that the structure is fairly recent.