Why is the midwest flat?

Doc,
Why is the midwest so flat? We travel back and forth from coast to coast. From the west coast to about central Colorado and Wyoming, the landscape is dramatic- there are mountains, or cliffs of exposed bedrock. The east has the Appalachians. The real odd ball seems to be the Plains Statesand also way up into central Canada. Why is the landscape so flat? Is this due to something about the geology?
Henry Wolston,
Sacramento, CA

Dear Henry,

The geology in the Great Plains is, well, subtle. Much of it
is understood by looking at cores drilled deep into the Earth.

It is ALL about geology. Everything is, didn't you know? Heh. The eastern and western thirds of the continent are generally mountainous with lots of rock visible. The rocks that lie below the thick soil of the midwest states of the Great Plains have not been buckled upward by plate collisions to form mountains. Sure, bedrock is exposed in some places, but these are often few and far between. In the long stretch of time between 570 and 70 million years ago, the central part of the continent was covered by shallow salt water seas. Flat-lying sediment was deposited on the sea floor. Marine fossils are abundant, including ancient armored fish and huge, long-necked swimming reptiles called Plesiosaurs (look 'em up!). The marine sediment was compressed by the weight of the steadily accumulating sediment into rocks: mostly limestone, shale or sandstone. In some places, these rock layers are 10,000 feet thick. Over the past 70 million years, the rocks have been slowly and gradually uplifted above sea level, and the interior sea drained off the land.
The northern part of the Great Plains are covered with sediment from repeated glaciations coming south out of the Canadian interior. Rivers and streams flowed outward from the terminus of these glaciers, and deposited river sediment, mostly sand, for hundreds of miles further south, covering the ancient sea beds and the soils that had developed on top of them.
The geologic story of the Great Plains is more complex than I can go into on this page. A website hosted by North Dakota State University explains the geologic history in some detail, with figures.
http://library.ndsu.edu/exhibits/text/greatplains/text.html
The mountains that bound the Plains on east and west are another topic. We can get into that later.

Doctor Rock