Stresses responsible for ice flow within glaciers are analyzed and classified as follows: (1) extrusion flow, (2) obstructed extrusion flow, (3) obstructed gravity flow, and (4) gravity flow. Though these types form a gradational series, each is characteristic of different conditions of glacier configuration and underlying topography.
With these concepts of flow in mind it is argued that:
Rates of flow within an ice sheet are at a maximum under and downstream from the névé line, which is generally near the periphery. Upstream from the névé line rates of flow diminish to zero under the areas of dispersal, which generally coincide with the areas of maximum accumulation on the ice-sheet surface.
Areas of accumulation on extensive ice sheets are generally closer to the ice sheet margins than to the centers, and movement and erosion take place only under the marginal and near-marginal parts of an ice sheet. Erosion is considered to be limited by the depth of weathered and loosely jointed mantle rock available for plucking and removal by ice. When surfaces of tightly jointed rock are reached plucking is inhibited (therefore no further tools are available for abrasion), and the ice cover prevents weathering. Thus erosion is practically stopped, and the ice sheet, as compared to subaerial erosive agents, becomes relatively protective of the land surface.
The configuration of the Greenland ice sheet is most likely due to subsurface topographic control.
Ice sheets generally originate by growth and expansion of piedmont, glaciers formed at the base of glaciated highland areas. Probably the Laurentide ice sheet thus came into being by expansion of glaciers originating in the highlands of Labrador and Baffin Island. Growth toward the south and west was favored by moisture-bearing cyclonic winds blowing from those directions.
Differences between shrinking ice sheets in the Antarctic and Greenland result from differences in the cause of shrinkage. In the Antarctic shrinkage results from reduced precipitation, particularly at high altitudes. Consequently low-altitude piedmonts are relatively favored, while the high-altitude ice sheet of the plateau approaches stagnation. In Greenland increased ablation, particularly at low altitudes, causes shrinkage. Consequently low-altitude piedmonts have generally disappeared, while the high-altitude ice sheet remains relatively well nourished.