Erratics are large boulders that have been transported by glaciers, and often deposited a considerable distance from their origin. For example, there are erratics in the county of Norfolk, in the United Kingdom, that originated in Norway.
By establishing the origins of the erratic, by comparing its lithology with possible originating areas, it is possible to determine the direction of ice flow that brought it to its present location. This helps glaciologists to monitor and plot past ice movements across large areas.
In the picture above, a very large boulder is being transported by an Alpine glacier. It is unlikely to be moved many kilometers from its original position because the glacier is small. In times of large scale glaciation such as the last ice age, with ice sheets covering hundreds of kilometers, it was possible for boulders to be transported over much greater distances.
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