CWMS

Cwms represent one of geology's most dramatic landforms—deep, steep-sided valleys carved by glacial action in mountainous terrain. These bowl-shaped depressions, known scientifically as cirques, tell the story of ice ages past when massive glaciers ground their way through solid rock, leaving behind amphitheater-like hollows that now hold mountain lakes, support unique ecosystems, and provide some of the world's most spectacular alpine scenery.

The formation of cwms follows a predictable glacial process. Snow accumulates in mountain hollows, compacting over time into firn and eventually glacial ice. As the glacier grows, it begins to move downhill, but not before carving deeper into the mountain through a process called plucking and abrasion. The glacier freezes to bedrock, then pulls away chunks of stone as it moves. Simultaneously, rock and debris frozen into the glacier's base act like sandpaper, grinding away the underlying surface.

Over thousands of years, this glacial erosion creates the characteristic cwm shape: a semi-circular hollow with steep headwall cliffs rising on three sides and a lower threshold where the glacier flowed out toward the valley. When the ice finally melts, it often leaves behind a tarn—a mountain lake trapped within the cwm by the rocky threshold. These tarns become jewel-like features in alpine landscapes, reflecting sky and peaks in pristine mountain waters.