LATE WISCONSINAN AND HOLOCENE LANDSCAPE EVOLUTION AND ALLUVIAL STRATIGRAPHY

E.A. Bettis III and Bernard E. Hoyer


Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 86-1, 1986, 319 p.

ABSTRACT


This is a report on the first detailed geomorphologic investigation of the Saylorville Lake area in the central Des Moines River Valley, Iowa. The purpose of this study is to identify, map, and date the landforms making up the valley landscape as well as the deposits below those landforms, in order to provide archaeologists and managers with a physical framework in which the cultural resources of the area can be evaluated.

Detailed stratigraphic studies were undertaken throughout the non-inundated portions of the Saylorville Lake area. Numerous radiocarbon and thermoluminescence dates were used to provide a chronologic framework for the alluvial stratigraphic units.

The highest and oldest fluvial deposits and alluvial landforms are late Wisconsinan-age benches and terraces. These developed as the late glacial Des Moines River downcut rapidly between 12,600 and 11,000 B.P. Five Holocene-age landform/sediment assemblages were identified and mapped throughout the area. These include: alluvial fans, colluvial slopes, the High Terrace (TH), the Intermediate Terrace (TI), and Low Terrace (TL). Alluvial fans, colluvial slopes and TH accumulated concurrently between about 10,500 and 4,000 years ago. TI accumulated between 4,000 and about 700 years ago, while TL accumulated after 700 years ago.

Limited investigations of the deposits preserved in tributary valleys suggest that the major period of tributary development was during the latest Wisconsinan and early through middle Holocene. Deposits preserved in the tributaries are time-equivalent to those in alluvial fans. Younger, late Holocene deposits are also present in the tributaries.

Maps of the landform/sediment assemblages were constructed in order to delineate the areas where deposits from the various culture periods could be preserved. Area and volume estimates of the landform/sediment assemblages provide the baseline information necessary to derive valid samples of the deposits for buried archaeological deposits. These maps and volume estimates also demonstrate that there are gaps in the remaining record. These need to be considered when reconstructing the culture history of the area.