Longbranch Monocline (Discarded)

Location

Northern Saline County

References

Cady et al. 1939

Description

Cady et al. (1939) used subsurface mapping of the Herrin Coal Member (Pennsylvanian) to define the Longbranch Monocline. Their map shows that the coal dips northward at a maximum rate of about 100 feet per mile (about 1°) near the north edge of Saline County. A structure map by Hopkins (1968) on the slightly older Springfield Coal Member shows a similar pattern, but places it in better regional perspective. The Longbranch Monocline is seen here as merely an area in which the regional northward tilt of the coal, toward the center of the Fairfield Basin, is slightly steeper than the average. On maps of the Beech Creek ("Barlow") Limestone (ISGS open files), much more irregularity is apparent and neither a linear trend nor a steeper than normal dip is shown. The Longbranch Monocline does not appear to be a valid structural feature, and reference to it should be discontinued.