The Canadian Rockies may have rose up to approximately 70 m.y.a. Once mountain formation ceased, erosion carved the mountains into their present rugged shape. The erosion was first due to water, then was greatly accelerated by the Quaternary glaciation 2.5 million years ago. Glacial landforms dominate Banff's geomorphology, with examples of all classic glacial forms, including cirques, arêtes, hanging valleys, moraines, and U-shaped valleys. The pre-existing structure left over from mountain-building strongly guided glacial erosion: mountains in Banff include complex, irregular, anticlinal, synclinal, castellate, dogtooth, and sawback mountains.