The term 'ground moraine' was originally applied to till surfaces deposited under an active glacier; later workers extended this meaning to include till surfaces formed of ablation debris. Other variations in the application of this term, combined with the above mentioned alternative usages, have placed a limitation on the usefulness of this term as a label for gently rolling till surfaces.

A detailed photogeologic study of selected till surfaces mapped as ground moraine in central North America, Ontario and Alaska, indicates that one or the other of two main classes of surface features noted is predominant within a particular region: (1) streamlined surface features are distinctive on the surface studied in Ontario, are common in the Alaska study area and appear as several small patches of fluting on the surfaces studied in Alberta and Montana and (2), low hummocks, straight to circular ridges and depressions are common to the areas studied in central North America.

The till surfaces marked by streamlined forms are classified as 'ground moraine' and the more or less irregular till surfaces as 'ice-disintegration moraine.' Ice-disintegration moraine is considered to have formed primarily by ablation and by the squeezing-up of basal debris into cavities and crevasses in stagnant or near-stagnant ice. Some of the ice-disintegration features noted possess characteristics indicative of one or the other of these modes of origin under generally stagnant ice conditions, therefore four secondary and six tertiary sub-classes are included, involving morphological and genetic considerations.

These findings lead to the conclusion that stagnation was the mode of ice wastage over large parts of the continental interior, as has been suggested by other workers. Normal recession (more or less uniform retreat of the ice edge) within the areas studied appears to have been common only to the Ontario and Alaska areas.

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