Historical:Roby Silt Member
H. B. Willman and John C. Frye
The Roby Silt Member of the Glasford Formation was named the Roby Silt by Johnson (1964, p. 8) in the region of Sangamon and adjacent counties in central Illinois. It was named for Roby, Christian County.
The type section is the Roby Section (Johnson, 1964, p. 35), NW SE NE Sec. 14, T. 15 N., R. 3 W., Sangamon County.
The Roby Member occurs below the Radnor Till Member and above the Vandalia Till Member of the Glasford Formation. The member is recognized only where it is bounded by these two tills. Its position below the Radnor Till suggests that the Roby Member is stratigraphically equivalent to the silt in the upper part of the Toulon Member of central western Illinois, which is exposed in the Jubilee College Section (table 6).
|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.
Extent and thickness
The Roby Silt Member is locally as much as 13 feet thick. Its character and extent have been described by Johnson (1964).
The unit consists of silt, clay, and sand.
The Roby locally contains a fauna of fossil mollusks.
Age and correlation
The Roby Silt is in the earliest part of the Jubileean Substage of the Illinoian Stage. It is a pro-glacial deposit of the advancing glacier that deposited the Radnor Till, and it was subsequently overridden by that glacier.
JOHNSON, W. H., 1964, Stratigraphy and petrography of Illinoian and Kansan drift in central Illinois: Illinois State Geological Survey Circular 378, 38 p.
|Geo Unit Designation