Vicksburg: Home to a World-Class Geologic Story
So how did Vicksburg become a place of geologic significance? It all started 70 million years ago during the Cretaceous. At that time a structural trough, known as the Mississippi Embayment became increasingly active. This trough extended from the Gulf up through Illinois, with an axis that largely followed the Mississippi River. Throughout the late Cretaceous and into the Tertiary, this trough became filled with sediments with sources in the Appalachian Mountains due to erosion and transportation, and from the Gulf of Mexico during periods of higher sea-level. Over time, these sediments were compacted and cemented, forming the rock units we see today.
By the time the Oligocene came about, much of the inland sea that had formed in the embayment trough had filled with sediment. The shore of this inland finger of the Gulf was much closer to Vicksburg at this point, as the sea retreated south. It was during this time that rocks seen in outcrops in the park today were formed. These rock units make up the Vicksburg Group. The Vicksburg Group is important geologically because it records the last significant world-wide rise and fall of sea level.
The loess deposits of Western Mississippi originated to the north during a time when glacial activity was much higher than it is today. As the glaciers in what is now Canada moved, they would grind up the bedrock and other rock debris into a fine flour-like soil. As glaciers melted, this soil was washed down the Mississippi River and deposited on flood plains. Once dried, this powdery soil was picked up by wind before being deposited a second time up on the high eastern bluffs of the river.
Loess has several characteristic qualities that gave it a key role during the Vicksburg Campaign. These include its tendency to remain relatively stable when cut at 90 degree angles. It is also highly unstable when on a slope, which has led to many serious erosional issues both in and outside the park.
The geology of Vicksburg is dynamic and still changes the landscape today. Explore the links below to read more about the processes that shape the park and how those processes have impacted history.
Last updated: February 28, 2018