The layers of rock in the cliffs and waterfall were once mud, silt, and sand on the bottom of a sea that covered much of what is now New York State. Rivers eroded these sediments from the rocks of an ancient mountain range in eastern New York and New England and washed them into the sea nearly 400 million years ago. Thousands of feet of mud, silt, and sand accumulated and were slowly compressed under their own weight to become the stone seen today. In the waterfall, layers of siltstone form ledges over alternating layers of weaker shale, and water flowing over the ledges has the appearance of frothy buttermilk. Siltstone and shale make up most of the rock of Buttermilk Glen, and shale, composed of compressed clay mud, is weaker and more easily eroded than siltstone.