As a result of the constant movement of the ice during the brief summer, when great fields are detached from the glaciers and are driven hither and thither under the impulse of the wind and the tides--impinging against one another, splitting in two from the violence of contact with other large fields, crushing up the thinner ice between them, having their edges shattered and piled up into pressure ridges--the surface of the polar sea during the winter may be one of almost unimaginable unevenness and roughness.
At least nine-tenths of the surface of the polar sea between Cape Columbia and the Pole is made up of these floes.

The other one-tenth, the ice between the floes, is formed by the direct freezing of the sea water each autumn and winter.

This ice never exceeds eight or ten feet in thickness.
The weather conditions of the fall determine to a great extent the character of the ice surface of the polar sea during the following winter.

If there have been continuous shoreward winds at the time when the increasing cold was gradually cementing the ice masses together, then the heavier ice will have been forced toward the shore; and the edges of the ice-fields farther out, where they come in contact, will have piled up into a series of pressure ridges, one beyond the other, which any one traveling northward from the land must go over, as one would go over a series of hills.
If, on the other hand, there has been little wind in the fall, when the surface of the polar sea was becoming cemented and frozen over, many of these great floes will have been separated from other floes of a like size and character, and there may be stretches of comparatively smooth, young, or new, ice between them.

If, after the winter has set in, there should still be violent winds, much of this thinner ice may be crushed up by the movement of the heavier floes; but if the winter remains calm, this smoother ice may continue until the general breaking up in the following summer.
But the pressure ridges above described are not the worst feature of the arctic ice.