LANDSLIDE AND ROCKFALL HAZARDS IN THE ST. PETER SANDSTONE OUTCROP AREA

The outcrop area of St. Peter Sandstone includes portions of Callaway, Cape Girardeau, Franklin, Gasconade, Jefferson, Montgomery, Perry, St. Charles, St. Louis, Ste. Genevieve and Warren counties. It crops out in areas of the state that are experiencing increased development. While the bluffs formed by the St. Peter offer many scenic siting options, developers need to be aware of areas where geologic hazards may be present. The Hermann Quadrangle is used here to illustrate these hazards, but they exist in many areas of the outcrop.

Landslides are a geologic hazard that exists below St. Peter Bluffs. Several landslide deposits were noted during the 2002-2003 mapping season. These deposits can be recognized as areas covered by large boulders of St. Peter Sandstone well below the base of the St. Peter bluff. The causal mechanism appears to be groundwater sapping of the blue-green shale at the St. Peter - Cotter contact. Precipitation infiltrating into the highlands above the bluff percolates though the St. Peter until it reaches the lower permeability of the Cotter Dolomite where it migrates laterally discharging at the base of the St. Peter Sandstone. This discharge weathers and removes the shale, undermining the bluff, resulting in the rock shelters that occur in the area. Eventually the cantilevered section fails catastrophically. Timing of these events appears infrequent. None of the local people interviewed during the mapping season could recall a landslide in their lifetime or their forefathers. Due to the volume of material involved engineering a mitigation would seem impractical. Before developing a house site beneath the bluff it would be prudent to examine the bluff for enlarged fractures and undermining.

Rockfall hazards exist beneath many bluffs in the state. Any area where massive-bedded units outcrop high on a bluff can experience significant rockfall hazards. On the Hermann Quadrangle, a massive-bedded unnamed sandstone occurs in the undifferentiated Pennsylvanian section above the St. Peter. This unit can create ten-foot thick car-sized slope wash. Large boulders of this unit were occasionally found below St. Peter bluffs.