A glacier carves the whole valley into a U-shape. The upper end of a river is a small stream in a small swale that merges with the drainage divide. Glaciers are not as easily influenced by rock hardness as are rivers. They tend to bulldoze ahead, straightening out twisty river valleys. Glaciers move very slowly, but can carry large amounts of rock debris.
Individual blocks can be huge compared to the largest rocks moved by running water.
When glaciers melt, they drop their loads as masses of unsorted debris, called till, in piles of various shapes called moraines. Commonly, at the terminus, or snout, of a glacier , till forms a horseshoe-shaped moraine, with the open side facing upvalley. This end moraine forms as the glacier brings debris downvalley to the melting snout. The debris piles up as if delivered from a conveyor belt, albeit a slow one. Similarly, as a glacier moves downvalley, it also deposits till along its margins, forming sinuous ridges that parallel the glacier's flow.