And yet the craton stirs. There is no part of the face of the earth that vertically and laterally does not move. The bedding planes in Midwestern rock, which appear to be absolutely level, do in fact dip. They will descend across a great many miles and then rise, arching over the far rim of a vast and shallow bowl, and then subtly dip again to form a similar bowl: the Findlay Arch, the Michigan Basin, the Kankakee Arch, the Illinois Basin. Anita called the arches “basement highs.” She said Hudson Bay is a continental basin, slowly filling up. The basins of the Midwest are filled to the brim with level ground. They are co-working space leeuwarden products of the creaking motions of the craton, in response perhaps to plays of force from deep within the mantle-a process that, in the general phrase, is “not well understood.” They represent a degree of tectonic activity about as lively as the setting in of rigor mortis. This has not always been the regional story. There are roots of long-gone mountains deep in the rock of the stable craton, but it has not had an orogeny in a thousand million years. “What has the Midwest been doing since then? It’s been sitting around doing nothing,” Anita said. “It has just sat here hohumming.” Shallow seas may have quietly arrived and departed, and coal beds formed in the ground, but in all that time there has been
no occurrence that can begin to rival in scope or total change the advent from the north of walls of marching ice. The ice was Antarctic in breadth. The traceable co-working space eindhoven episodes of recent continental glaciation have each placed about as much ice over North America as is upon Antarctica now. In Wisconsinan time, which lasted about seventy-five thousand years and ended ten thousand years ago, three-fifths of all the ice in the world was on North America, another fifth covered much of Europe, and the rest was scattered.