Loess and Lyell

You can skip this as it is really my ramblings about geology.
Loess and Lyell, not an old couple, not a bank, but a coming together in Natchez, which I had not forseen, but which I found fascinating.
Back when I did geography as an A Level and again when I did Earth Science at college the name of Charles Lyell was inextricably linked to the development of modern geology and geomorphology. He had made the now obvious, but then almost heretic, cornerstone observation that the present is the key to the past. The geological processes we see happening now were the ones that shaped the earth millions, even billions, of years ago. Charles Lyell was a geologist in the early 19th Century. He could afford to travel the world looking at rocks. His observations and theories he set out in a series of books called Principles of Geology and later Elements of Geology. He was very influential amongst natural scientists of the day. He gave Darwin the first book of his Principles of Geology just as he was setting sail in The Beagle. This book influenced Darwin and helped crystallise his ideas on evolution (though paradoxically Lyell did not accept until much later that man was the product of the same principles that he defined for geology).
Although I had looked in detail at his work for A Level, I had of course completely forgotten all of it, until we drove down the Natchez Trace Parkway and started to see information about Loess, which is wind blown material, a product of glacial erosion.
In America there are large deposits of Loess which form the surface rocks right down the central states. In Mississippi State the deposits form a ridge of higher land to the east of the Mississippi river. The loess is soft, almost like clay, yet has the property of being able to form almost vertical cliffs. This shows itself in the topography by the appearance of deep gullies and ravines (up to 75 feet, not Grand canyon proportions) cut into the steep line of hills which forms the east bank of the Mississippi. Vicksburg is built on these hills as is Natchez. In Vicksburg this unique material enabled Pemberton to build very strong defences round the town against the Grant’s invading Union forces. However, the soft nature of the ground also enabled Grant to build his siege trenches very quickly and very close to the Confederate army. It even allowed for digging of tunnels under the defences to plant mines to blow up the defences.
While in Natchez that I realised that Charles Lyell had been there in 1842. \It as here that he saw the link between the Loess deposits and the river, as he had seen the same thing on the Rhine in Germany. This further confirmed to him his Theory of Uniformatism (Present key to the past). The Loess, which is wind blown, rather than water borne, seems to have been blown across the plains until it reaches the river, where for some reason, possibly higher humidity, it drops and builds into a clay like deposit over 120 feet thick. This means that it forms the first higher ground as you move from the west to the east across the Mississippi region. It was an important factor in the siteing of Natchez, as south of Natchez the deposits fade to the East, so Natchez is the first high ground on the Mississippi. It was equally important at Vicksburg, as it produced the high ground which enabled Vicksburg to dominate the Mississippi River.
Further de javu occurred when we visited the Grand Indian Village, as Lyell had described visiting the village in one of his books, and there we were standing in the same spot that this famous English Scientist had stood 167 years before.

Loess and Lyell