Within the hilly uplands and flat terraces along the eastern edge of the Mississippi Alluvial Valley, roadcuts, excavations, stream bluffs, and other exposures reveal vertical banks of massive silt, called "loess." Sometimes fossil shells can be found within the lower, unweathered parts of these exposures. To the north, within Mississippi where the loess is much thicker, these fossil shells are very abundant especially within deep roadcuts.
The major fossils found within loess consist of the shells of various terrestrial and freshwater snails and other mollusks. These shells are those of land snails (pulmonate gastropods), freshwater snails (gastropods), and freshwater clams (pelecypods). The delicate shells of the land snails are the most abundant fossils found within the loess. Typically, they are well preserved and unbroken. Smaller numbers of freshwater snails and clams occur within isolated lenses within the loess (Krinitzsky and Turnbull 1967, Saucier 1995).
The loess forms a blanket of relatively homogeneous, seemingly massive, well-sorted, silt that covers the Tertiary uplands and Pleistocene terraces bordering both sides of the Mississippi Alluvial Valley. A belt of loess 30 to 60 miles in width covering the uplands and terraces extends along the east side of the Mississippi Alluvial Valley from the Cairo, Illinois area to just south of Baton Rouge, Louisiana. Where exposed in either roadcuts, stream banks, gullies, excavations, or other exposures, the loess consists of unconsolidated, massive, tan- to brown-colored loess and forms steep slopes and vertical cliffs. Within Mississippi the loess is as much as 75 feet (23 meters) thick. On both the east and west sides of the Mississippi Alluvial Valley within Louisiana, the loess is at most just over 30 feet (9 meters) thick. The loess is thickest adjacent to the valley wall and thins exponentially away from the valley walls (Krinitzsky and Turnbull 1967, Miller et al. 1984, Mossa and Autin 1989, Saucier 1995).
Loess consists of well-sorted wind-blown silt eroded from the floodplain of the Pleistocene Mississippi Rivers. When large continental glaciers covered the Midwestern United States, melting along the southern edge of this ice sheet created huge volumes of meltwater that flooded down the Mississippi, Missouri, and Ohio Rivers. During the spring and summer, this meltwater carried large quantities of glacial sediment downstream with it. This sediment included large amounts silt-size glacial sediment, called "rock-flour," that was created by the glaciers as they ground over bedrock. During the spring and summer, the meltwater spread the glacial sediment, including large volumes of rock flour, over the Pleistocene floodplain of the Mississippi River (Krinitzsky and Turnbull 1967, Saucier 1995).
During the fall and winter, melting of the ice sheet largely ceased. The meltwater flowing down the Mississippi, Missouri, and Ohio Rivers largely, if not entirely dried up. As a result, large areas of previously flooded Mississippi River Valley became dry and largely unvegetated floodplain. At the same time, strong winds blew across the dry floodplain and eroded large quantities of silt-size rock flour from it. The rock flour became large dust storms that rolled out of the Mississippi Alluvial Valley and into the adjacent countryside. Eventually, the silt-size dust settled out and accumulated as a blanket of well-sorted silt covering the uplands and terraces adjacent to the Mississippi River Valley. After thousands of years of constant accumulation, the accumulation of silt created loess deposits that are tens of feet (several meters) thick (Krinitzsky and Turnbull 1967, Saucier 1995).
If the source of the loess was the glacial sediments within the Pleistocene floodplain of the Mississippi River, the loess deposits should be most extensive and thickest downwind from the Mississippi River Valley. Furthermore, the mean grain size of the loess should decrease away from the Mississippi River Valley as the heaviest particles would settle first after being blown off of the floodplain. Studies of the loess deposits in Louisiana confirms that both the thickness of the loess and its mean grain-size do decrease away from the Mississippi River Alluvial Valley as wind-blown silt should (Krinitzsky and Turnbull 1967, Miller et al. 1985).
The fossil shells found within the loess show that uplands and terraces adjacent to the Mississippi River Alluvial Valley were covered by deciduous forest. The abundant of land snails show that the silt comprising the loess settled on dry, well-drained land. Within Louisiana and Mississippi, types of snails found within the loess now occur associated with deciduous forests. This indicates that the landscape on which the loess accumulated was covered by forests dominated by deciduous trees. The small lenses of freshwater snails and clams found within the loess show that small ponds and lakes existed for brief periods of time within this landscape (Krinitzsky and Turnbull 1967, Saucier 1995).
Along both sides of the Mississippi Alluvial Valley, careful study of the loess deposits have shown that several different loess deposits often occur stacked on top of each other in many places. Each of these loess deposits represents a separate period of glaciation when glacial meltwater floods covered the Mississippi River floodplain with rock flour. Then strong, fall and winter winds eroded this silt from dry floodplains and deposited it on the adjacent hilly uplands and terraces as a blanket of loess. The youngest of the loess deposits, designated as the "Peoria Loess," accumulated between 10,000 to 22,000 years ago. Soils developed within the top of each loess deposit represent the tens of thousands of years of time when loess ceased to accumulate, e.g. the last 10,000 years (Krinitzsky and Turnbull 1967, Saucier 1995).
(NOTE: For more information about loess, there is the Illinois State Museum exhibit on loess .)
Krinitzsky, E. L., and Turnbull, W. J., 1967, Loess deposits of Mississippi. Geological Society of America Special Paper no. 94. Geological Society of America, Boulder, Colorado, 64 pp.
Miller, B. J., Lewis, G. C., Alford, J. J., and Day, W. J., 1984, Loesses in Louisiana and at Vicksburg, Mississippi. Guidebook for the Friends of the Pleistocene Field Trip, April 12, 13, and 14, 1984. LSU Agricultural Center, Louisiana State University, Baton Rouge, Louisiana, 126 pp.
Mossa, J. and Autin, W. J., 1989, Quaternary Geomorphology and Stratigraphy of the Florida Parishes, Southeast Louisiana. Louisiana Geological Survey Guidebook Series no. 5. Louisiana Geological Survey, Baton Rouge, Louisiana, 98 pp.
Saucier, R. T., 1994, Geomorphology and Quaternary Geologic History of the Lower Mississippi Valley. U.S. Army Waterways Experimental Station, Vicksburg, Mississippi, 346 pp.
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