GEOMORPHOLOGY AND STRATIGRAPHY OF SLUMPS ON MONKS MOUND, U.S.A

Heavy precipitation in late winter and spring of 2005 reactivated a mass wasting complex on the west-facing slope of Monks Mound, the largest man-made earthwork in North America. Although limited in scope, geomorphic assessment and preliminary stratigraphic investigation of the western slope provide important information on mound stratigraphy and age, the origin of mound fill, mound architecture, and stratigraphic conditions leading to slope failure.

Active slump features include an extensive scalloped main scarp, transverse cracks, and a creeping toe that is approaching vertical. Evidence of earlier slumping episodes includes rotated slump blocks with relict transverse ridges exhibiting reversed slopes, all riding on the current slump block(s), weathered scarps, including one huge scarp that perhaps suggests an initial failure of nearly the entire west flank of the mound, and weathered transverse and radial cracks. Clearly, the west side of Monks Mound consists of multiple generations of hillslope failure, rather then having been intentionally “terraced” by the prehistoric inhabitants as previously suggested by archaeologists.

Cores, nine cm in diameter, indicate: 1. The mound consists of basket loads of a dominant sediment type, usually with minor inclusions of up to three other sediment types. Major sediment types include redeposited silt loam A horizon, redeposited Peoria Silt, upper point bar silt to sandy loam, pedogenically altered floodplain clay, and even unoxidized paleomeander fill. 2. The slump block is riding on multiple shear planes in a shear zone that occurs in a consistently present black clay bed that in one core was over four meters thick 3. There was considerable pre-mound site preparation, including at least local removal of the original floodplain surface soil. 4. There are up to several incipient buried soils indicating hiatuses in mound construction. 5. Other paleogeomorphic surfaces are indicated by reed inclusions and mats, and compacted sediment.

Radiocarbon ages suggest the bulk of Monks Mound was constructed between about 1050 and 700 B.P.