Geologic Activity

Loess Erosion
Loess Soil Erosion

NPS Photo

"Geologic Activity" at first blush seems an oxymoron. Isn't geology basically just "rocks", and aren't rocks about the least active thing you can think of? The loess soil that predominates at Vicksburg NMP is derived from rocks, and if you'd have been at the park's Fort Hill section in 2004 when part of the hill suffered a massive "slump", rendering one of the park's roads impassable, "active" would likely have been one adjective to come to mind!

The Vicksburg area is somewhat unique in a geological sense, in that it is located in one of the few areas of the country where loess soil has formed massive bluffs. But what is "loess"? Loess is a type of soil that has its origins in the last ice age 20,000 years ago. At that time huge glaciers moved south out of the northern arctic. As these glaciers advanced southward, they churned up the underlying rock, moving it along and grinding it into smaller pieces. When temperatures started to rise, the glaciers stopped and began to retreat north again. As they melted they dropped their loads of mixed-up rock. Some of these rock remains were small enough to be caught up in the vast meltwater that was spawned by the shrinking glaciers. The meltwater formed into braided streams that seeked the path of least resistance to the sea. When the streams expended themselves and lost their energy, they dropped what gravel, sand, and mud they'd been carrying. In some places these deposits were huge. Over vast spans of time, prevailing winds lifted massive quantities of the finer and lighter material and brought it to rest in a narrow band stretching from present day Baton Rouge to Tennessee. Vicksburg sits atop a portion of the loess band that accumulated a particularly thick deposit of material just east of the Mississippi River.

The loess soil has an interesting feature in that while this fine-particled material is very erodable when subjected to any sort of disturbance, it is able to hold its form almost indefinitely if cut vertically at right angles. That is why there are some spectacular loess profiles exposed within the park and elsewhere. But this is also why in other areas erosion and disturbance have formed dramatic ridges and ravines throughout the landscape, and precipitated landslides such as the one that occurred recently at Fort Hill. When it comes to loess soil and its physical properties, "geologic activity" can be a very appropriate term indeed.

Last updated: April 14, 2015

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Vicksburg, MS 39183


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