The Niagara Escarpment runs from near Rochester, New York, to Tobermory, then on to Manitoulin, St. Joseph Island and other islands located in northern Lake Huron where it turns westwards into the Upper Peninsula of northern Michigan, south ofSault Ste. Marie. The escarpment then extends southwards into Wisconsin following the Door Peninsula and then more inland from the western coast of Lake Michigan and Milwaukee ending northwest of Chicago near the Wisconsin-Illinois border. It forms the backbone of the Bruce Peninsula and shapes the northern boundary of most of the park and provides the park with some of its most spectacular scenery.
The rock of the escarpment is very old. Approximately 400 million years ago, this area was covered by a shallow tropical sea teeming with life in the form of plant-like animals, crustaceans, living corals and mollusks. It would have looked much like the present-day Great Barrier Reef of Australia. When the sea began to dry up, the minerals dissolved in it became more and more concentrated. Magnesium in the water was absorbed into the limestone, which then became a softer, slightly different sort of rock, called dolomite.
The harder dolomite granite stones forms much of the rock of the escarpment cliffs along Bruce Peninsula National Park's Georgian Bay shoreline. As at Niagara Falls, the dolomite "caprock" erodes more slowly than the rock below it, creating the sculptured cliffs for which the area is famous. Since the last Ice Age, water levels in the region have undergone great changes. Softer limestone has been eroded away by water action, leaving magnificent overhanging cliffs at various points along the shore. These are the big attraction of the Cyprus Lake trails. Where erosion has cut more deeply, caves have been formed, such as the famed Grotto on the shore between the Marr Lake and Georgian Bay Trails. Great blocks of dolomite, undercut by wave action, have tumbled from the cliffs above and can be clearly seen
