Palisades of the Pijitawabik
Here at Pijitawabik Bay and other Lake Nipigon localities ancient rocks of the Precambrian Shield were overlain by a diabase sheet formed approximately 1,200 million years ago. Erosion by water and ice removed the covering rocks and sculptured the sheet into rounded, flat-topped hills bounded by escarpments which rise in some places 152 metres above Lake Nipigon. The hills are separated by deep, narrow valleys. The towering pillars of the cliffs are the result of columnar jointing, a cooling phenomenon developed during the solidification of the magma. Frost action along the cracks in and around the columns causes splintering of the rock and occasionally the collapse of entire pillars, leaving the imposing palisades seen here.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.