Thick enough ice flows like a plastic, and that slow creep is what grinds a river’s V-shaped valley into a flat-floored U-shaped trough. As the climate cools, the equilibrium line (ELA) drops with it, accumulation beats melting, and the snout thickens and advances, bulldozing moraine ahead of it; as it warms, the snout retreats and leaves a terminal moraine ridge marking how far it once reached. That cold-warm rhythm repeats on Milankovitch cycles, the slow changes in Earth’s orbit and tilt.
Thick enough ice flows like a plastic, and that slow creep is what grinds a river’s V-shaped valley into a flat-floored U-shaped trough. As the climate cools, the equilibrium line (ELA) drops with it, accumulation beats melting, and the snout thickens and advances, bulldozing moraine ahead of it; as it warms, the snout retreats and leaves a terminal moraine ridge marking how far it once reached. That cold-warm rhythm repeats on Milankovitch cycles, the slow changes in Earth’s orbit and tilt.
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