1.   As the snow accumulates from that little boreal patch, growing inexorably year after year, gargantuan ice sheets begin to form.

2.   Icebergs break off from the ice sheets and float southwards.

3.   In the last one million years the ice sheets spread a layer of boulder clay across the lowlands.

4.   It is equally difficult to work out what will happen to the ice sheets at the poles.

5.   No doubt the ice sheet preserves specimens that would weather away more quickly in other regions.

6.   The cutter rolls briefly as it splits an ice sheet the size of a suburban back yard.

7.   The tremendous weight of the ice sheet loaded and depressed that part of the lithosphere.

8.   These differences were caused first by ice sheets and later by rivers.

9.   Thus the ice sheet would expand.

10.   Huge ice sheets were formed.