"Ripple Rock"
The ripple marks on the adjacent rock face were made some two billion years ago by waves in a shallow body of water. Buried by later deposition of silt and then subjected to great pressure, the rippled sand became sandstone. The formation was subsequently tilted 60 degrees from the horizontal by crumpling of the Earth's crust. This exceptional feature is of particular interest in that there are at least three distinct beds of sandstone, two of them with ripple-marks oriented almost at right angles to each other. This implies an abrupt change in the direction either of the shoreline of of the current.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.