backwall

The steep slope at the back of a glacial cirque, which is subject to intense physical weathering, is called the backwall or sometimes the headwall. Because alpine glaciers typically form on the north side of a ridge, and therefore outside the reach of direct sunlight, the steep backwall casts an especially long shadow on the cirque floor. This keeps snow on the floor longer; the wet ground then erodes and deepens the cirque.

Jeffery Renard Allen