MAKING GLASLYN

Part 1 Snow falling on Snowdon collects in a small depression, formed by erosion, a weakness in the rock or during an earlier Ice Age.

Part 2 The snow compacts, freezes during cold periods and melts in warmer times. In the rock face above, repeated freeze-thaw expansion and contraction breaks off fragments of rock which tumble into the basin, steepening the rock face. Part 3 The rocks are rolled around the depression by the ice and water, eroding it to form a deeper basin which, when the Ice Age passes, is filled with rain and meltwater.