Glacial advance and retreat is determined by the balance between the accumulation of snow and the removal of ice by sublimation, melting, and calving (ablation). When the rate of ablation below the snowline equals the rate of accumulation above it, the glacier is stationary, as in View 1. During glacial retreat, View 2, the rate of ablation exceeds the rate of accumulation, and the position of the toe retreats toward the origin of the glacier. Glacial advance, View 3, occurs when the rate of accumulation exceeds the rate of ablation. For all views, pay attention to the motion of the stones. Note that in all cases, ice flows downhill.

Why do glaciers advance and retreat periodically during an ice age? In 1920, Mulutin Milankovitch showed that regular variations in the shape of Earth’s orbit and the orientation of its axis create variations of solar intensity at high latitudes: warm summers in which glaciers retreat, and cool summers when they advance. These climate cycles, called Milankovitch Cycles, are determined by three factors: orbital eccentricity, shown in View 1, changes in the tilt of Earth’s axis, View 2, and the precession of Earth’s axis, View 3