Another characteristic features of The Burren are closed roughly circular depressions with no surface outlets for water (called polje). Around 100 of these exist, mostly present in the eastern Burren. The most notable are the valleys of Kilcorney, Poulawilan, Caherconnel and Carran, generally stretching from northeast to southwest. The largest is the Carran depression, more than two miles long, up to a mile wide and over 200 feet deep. This is where the pre-glacial rivers of the area first eroded away the upper layers and started to dissolve the exposed limestone. Some of the smaller ones were created when caves underneath collapsed (one example of this is the Glen of Clab).