SLACK WATER TERRACES REMNANTS ALONG THE LOWER SPOON RIVER, IN FULTON COUNTY, ILLINOIS

The southward draining Spoon River, developed as an ice marginal stream between end moraines of the of the Monican and Jubileean substages of the Illinois Glacial Episode. The slack water terrace was deposited when the stage of the Spoon River was elevated by water backed up from the Illinois River during major melt water floods that resculptured the Middle Illinois River Valley. These floods occurred as large glacial lakes were catastrophically drained during late Wisconsin Glacial Episode deglaciation from Illinois. Remnants of a depositional terrace extend 37 kilometers up the Spoon River Valley. The general elevation of the slack water terrace is about the same general elevation as the highest erosional terrace remnants along the Middle Illinois River Valley. The terrace remnants slope gently toward the axis of the Spoon River valley but maintain a relatively constant elevation of 153 to 159 meters parallel the valley axis. Near its mouth, the Spoon River has down cut 15.25 meters through the terrace materials. The silt to sand and gravel that comprise the terrace is considerably more erodible than local bedrock. Because of the difference in erodibility some gullies and ravines have been excavated along the bedrock-terrace contact to define and enhance the topographic appearance of the terrace. Locally, some of the gullies and ravines are parallel the Spoon River to produce pseudo-yazoo streams.