The Great Aletsch glacier has carved and polished on its underside together with finely grated rock the bedrock. Elongated, whale-shaped boulders emerged, called rock outcrops (rock humps). Where the Great Aletsch glacier flowed over rocky sills, numerous, tightly lying outcrops were sculpted. In this way rock outcrop fields originated like in the Üsseren Aletschji. Further up the valley the rock outcrops are more flat shaped than down the valley, where rock split-offs have occurred frequently. Boulders frozen in ice show often scratches (glacial striae) on the rock surface. These indicate the direction of the ice flow.