Outwash Plains

The story of a glacier does not end where the ice stops. Beyond the snout of a mountain glacier, or past the outer edge of a vast continental ice sheet, the meltwater carries on the work. This water, loaded with rock debris, spreads out across the flat land at the glacier's foot, and the result is an outwash plain.

As the ice melts, streams of turbid water rush out from beneath it. These streams are heavily charged with sediment — everything from fine clay to coarse gravel. When they emerge onto the gentler slopes at the glacier's front, they lose velocity and drop their load. The coarser material settles first, forming broad, flat, fan-shaped deposits called alluvial fans. Where these fans from different meltwater channels merge and overlap, they coalesce into a single, continuous, gently sloping plain. That plain, built entirely from glacial meltwater deposits, is the outwash plain.

The material that makes up an outwash plain is a mixture of gravel, silt, sand, and clay. Because the deposits are laid down by running water rather than by the ice itself, they are described as glacio-fluvial deposits. The plain is not a random heap of debris; it is a sorted, stratified landscape, shaped by the sorting action of flowing water.

The book asks you to distinguish between a river alluvial plain and a glacial outwash plain. The difference lies in the source of the water and the nature of the sediment. A river alluvial plain is formed by a normal river depositing sediment along its course, typically in its lower reaches or where it floods. A glacial outwash plain, by contrast, is formed specifically by meltwater issuing from a glacier. The sediment in an outwash plain is freshly ground rock flour and debris scoured by the ice, whereas a river's alluvium has usually been transported and reworked over much longer distances. Both are water-laid plains, but the outwash plain owes its very existence to the glacier that feeds it.