Limestone is stronger and more resistant, but its cousin, dolostone, is the strongest. In the Niagara Escarpment, a layer of Silurian dolostone lies over layers of limestone, shale and sandstone. The diagram at right shows the process of escarpment formation. While soft layers quickly erode, the stronger upper level resists, and protects the layer below it. Erosion undercuts the upper layer, which occasionally breaks off because of its weight.
Water greatly accelerates the process of erosion.
This can be seen clearly at Niagara Falls, where the Niagara River continually cuts the escarpment. The river cuts about one meter per year. Ten thousand years ago, when the last ice age finished, the Niagara Falls was 11 kilometers closer to Lake Ontario. Now, between Queenston and the falls, the river has high cliff walls on both sides. Ice Age Geologists call the last 1.6 million years the Pleistocene era.