This doctrine states that changes in the earth's surface occurring in past geologic time are like the same causes producing changes on the earth's surface now. This sounds like a very simple thought, but one of its implications is that if rivers erode valleys, as we see they do very slowly now, then a gorge such as the Grand Canyon must have been eroded over a period of many millions of years. Back in Sir Charles's time, the Earth, on Biblical grounds, was considered to be only a few thousand years old, so his Doctrine of Uniformitarianism was nothing less than a revolutionary, heretical thought at that time.
The Doctrine of Catastrophism was the challenging hypothesis on how landforms were created. Proponents of the Doctrine of Catastrophism, for instance, felt that the Mississippi River channel was carved in a sudden massive flood. They couldn't conceive of it being cut slowly, bit by bit, over millions of years, a process which rested quite comfortably with Sir Charles's Doctrine of Uniformitarianism.
Despite the accuracy of Sir Charles's Uniformitarianist views, what he saw at Natchez must have disturbed him greatly, for he knew of no geological process going on at that time that could explain how Natchez's silt-covered bluffs might be formed. It is certain that he was struck by what he saw, for he wrote the following about it:
From this brief statement we can see that Sir Charles was already making certain mental connections that later would help explain where loess comes from.
It would be a few years before others would point out that our loess-based landscape occurs many places besides the Mississippi River's western floodplain border and Germany's Rhine Valley. In the United States, loess landscapes occur here and there along some of the Mississippi's major tributaries, such as the Illinois and Missouri Rivers, plus there is a much smaller, independent deposit on the Columbia Plateau in Oregon, Washington, and Idaho. In Germany, loess appears along the Rhone and Danube Rivers, and farther afield there are massive deposits in Ukraine, the Siberian steppe region, the area east of the Caspian Sea in Kazakhstan and Turkmenistan, and there is even some in northern Africa. Perhaps the most extensive accumulation anywhere in the world is in northeastern China.
Our own remarkable loess landscape may indeed be exotic looking, then, but it is by no means unique in a worldwide context. On the other hand, when we consider southern North America, our Loess Hills are indeed one-of-a-kind. Only in our Loess Hills does loess profoundly affect the ecology of plants and animals native to southern North America.
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