The symbionts are marching. A temporary and small retreat of the northern icecap has created vast new tundra areas over the northern continents. For the first time in 5,000 years the rate of melting of the edge of the glaciers is exceeding their rate of southward movement. In effect, the edge of the icecap is melting back. Rocky debris, broken up by the weight of the ice and shoved along the ground by the southward movement, now lies in hummocks and thick beds of mixed clay and boulders. Here and there a long winding esker (a steep ridge of rubble marking the old course of a subglacial river) snakes across the plain. Huge lumps of abandoned ice embedded in the clay melt slowly, gradually becoming lakes.