Historical:Pleasant Grove Soil
H. B. Willman and John C. Frye
The Pleasant Grove Soil is herein named for Pleasant Grove School, from its occurrence in the Pleasant Grove School Section (table 6), center SE Sec. 20, T. 3 N., R. 8 W., Madison County.
|Table 6 -- Stratigraphic Sections (partial)|
The following 21 stratigraphic sections describe exposures in Illinois and illustrate many of the aspects of Pleistocene stratigraphy. These sections contain the type localities for 21 rock-stratigraphic units, 4 soil-stratigraphic units, and 3 time-stratigraphic units and include paratypes for several other units. The sample numbers preceded by "P" are the numbers used in the Illinois State Geological Survey collections. Analytical data on many of these samples are on file at the Survey. The sections are arrange alphabetically by name.
At its type locality the Pleasant Grove Soil is developed in the McDonough Loess Member and overlain by the Meadow Loess Member of the Roxana Silt.
The soil at the type section presents an "A-C" profile, lacking a recognizable B-zone. A dark gray, organic-rich A-zone occurs above tan to gray-tan leached loess.
At the type section, 2 to 3 feet below the top of the soil the loess is weakly calcareous and contains a few badly etched fossil polygyrid shells. A few "pods" of seed hulls of Celtis occidentalis occur 1.5 to 2.5 feet below the surface of the buried soil, suggesting the action of burrowing rodents that lived on the soil surface.
Age and correlation
The contrast in degree of development between the Pleasant Grove and Chapin Soils is as great as that between the Chapin and Sangamon Soils. These contrasts in morphology suggest that, whereas the Chapin Soil required only a fraction of the time that was needed for the development of the Sangamon Soil, the Pleasant Grove Soil required only a fraction of the time required for the development of the Chapin Soil. At the type section a radiocarbon date of 35,200 ± 1,000 (W-729) was obtained on shells from the middle of the Meadow Loess Member above the Pleasant Grove Soil, and other radiocarbon dates as old as 37,000 B.P. have been obtained from the mid-part of the Meadow Member.
|Stratigraphic Code||Geo Unit Designation|