Date of Award

1950

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

Abstract

This study presents a general picture of loess in the Lower Mississippi Valley. Loess is recognized as unstratified, calcareous, slightly plastic, porous loam with an average grain size distribution between 0.05 and 0.01mm. diameter. Materials meeting this definition but lacking in calcium carbonate are termed "leached loess" when in association with loess and "brown loam" when not in association with loess.

Loess has been found to occur chiefly as pseudo-anticlinal caps on hills. It is mapped as occurring within an area averaging 10 to 15 miles in width extending from southern Illinois southward along the eastern walls of the Lower Mississippi River Alluvial Valley to near Bayou Sara in Louisiana. Deposits are found also on Crowley’s Ridge and Sicily Island. The loess is thickest at the bluffs facing the Mississippi River flood plain and thins progressively away from these bluffs. The rate of thinning is estimated at 0.3 feet per mile. There is no correlation between thickness of loess and age of terraces on which it occurs. These loess capped hills have a northeast-southwest and northwest-southeast orientation and alluvial fans of loess intermixed with terrace materials extend out onto the flood-plain. Eastward the loess merges with brown loam.

Chemical analyses of loess indicate a great uniformity of composition except for calcium carbonate which is highly variable. Loess is predominantly composed of Si02 (56.79 to 66.43%) together with Al2O3 (7.33 to 11.16%), CaO (7.74 to 11.72%), MgO (3.11 to 5.09%), and Fe203 (2.99 to 3.70%). Mechanical analyses show a great uniformity of grain size distributions over large areas. Leached loess has essentially the same grain size distribution as does loess but brown loam may be slightly finer grained. The range of variation of grain size distributions in loess is much less than in modern floodplain sediments.

Carbonates in loess of the Lower Mississippi Valley are almost entirely secondary, chiefly in the form of precipitates around grass roots. Nodular concretions and tree root fillings are also abundant. These carbonates add to the strength of the loess when it is in a dry state but furnish very little strength when it is wet. This contrast is such that a slope of equilibrium on an exposed surface is nearly vertical.

Terrestrial pulmonates are plentiful in loess. These are of species still living in the area.

Evidences of colluviation and faulting of loess are plentiful.

Pages

142

DOI

10.31390/gradschool_disstheses.7953

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