Grades 3–5 · Earth & Space
How can a ridge of stones show where a glacier once ended?
A glacier carries pieces of rock as well as ice. Some material falls onto its surface, while other pieces are picked up along its route. The moving ice transports a mixture that can include fine grains and large boulders.
In the diagramThe debris ridge remains after ice retreatsFormer margindebris accumulates at edgeLater marginridge beyond smaller glacierInvented positions; dating requires additional evidence.
Near the glacier’s end, melting releases carried material onto the ground. If the end stays in roughly the same place for a while, more debris can collect there. A ridge of this material is called a moraine.
If melting later removes ice faster than moving ice replaces it, the glacier’s end retreats up the valley. The ridge may remain beyond the new ice edge. It records a former margin rather than moving back with the ice.
Several ridges can preserve evidence of different past positions. Their materials and locations help geologists reconstruct glacier changes. A ridge alone does not provide an exact calendar date; dating requires additional evidence and suitable methods.
About this explanation
Starting knowledge: Know that a glacier is moving ice and that ice can carry loose rock.
Sources
- National Park Service — Terminal and Recessional Moraines