The Grotto is a sea cave on the Georgian Bay shore of Bruce Peninsula National Park in Ontario, between the Marr Lake and Georgian Bay trails, and the water inside it is the reason people queue for a parking permit to see it.
The geology is the same story as Niagara Falls, a couple of hundred kilometres south along the same escarpment. A hard dolomite caprock sits over softer limestone. The dolomite resists erosion and the limestone does not, so wave action eats the rock out from underneath and leaves overhanging cliffs standing above it. Where the water has cut deeper, it has opened caves, and the Grotto is the famous one.
Water levels in the region have swung enormously since the last ice age, which is why the cutting sits where it does rather than at the present shoreline.
Look down and you can see the rest of the process. Great blocks of dolomite, undercut by the waves until they let go, lie tumbled below the surface of Georgian Bay, clearly visible through water that is startlingly clear and startlingly cold.
The overhanging cliffs along this shore are the main draw of the Cyprus Lake trails, and the Grotto is reached on foot from there over rough limestone. Getting down into it means a scramble, and the rock is slippery.
The park was established in 1987 to protect the rock formations and shoreline of the Niagara Escarpment, and it forms the core of UNESCO's Niagara Escarpment World Biosphere Reserve. Access to the Grotto is managed by Parks Canada in the busy season, so check before travelling. The climate along this coast is among the most temperate in Ontario.
Commons photos · Atlas Obscura · Protected Planet · GeoNames · OpenStreetMap
Other places on the map within a couple of hours.
