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Past Physical Environments


Topographic map of Cahokia region, 1935.

To understand how a past people utlized nearby natural resources, the archaeologist must have a detailed picture not of the present environment, but of the past environments around which people structured their activities. Laying drainage tiles and ditch digging have drained land once too wet to be farmed; artificial levees now control most flooding; locks, dams, and artificial levees have altered the height of river flow; roads, towns, and factories have spread out across the bottomland. Combined, the results of these activities are effectively blocking our view of the pre-settlement landscape. This is a situation archaeologists often face.

Nevertheless, careful examination of USGS topographic maps often provides the archaeologist and geologist with a general understanding of a site's natural landscape. Knowing the location of drier ground, slope, proximity to streams and other water bodies, for example, can be critical to designing both site survey and sampling methods. Modern topographic maps also provide information on how contemporary activities have modified the natural topography and drainage patterns. Also, an appreciation of the modern landscape revealed on topographic maps helps the geologists define landforms whose genesis can be tested by fieldwork (e.g., examination of soils, stratigraphic exposures, and sediment coring).

On the above map, for example, Horseshoe Lake shows up clearly as an abandoned river channel, but other abandoned channels are also visible. Note the area of swamp (stipled blue) arching from the southern tip of Horseshoe Lake up to the northeast part of the map. This meander scar, called Edelhardt Lake by geomorphologists, also represents an old channel that, through time, has filled with sediment. Analysis of sediment cores from these abandoned and filled sloughs and examination of historic records (e.g., Public Land Survey Maps of the early 1800s) indicates that this now marshy area contained standing water and was directly connected to Horseshoe Lake in the not too distant past. Another older channel-to-slough-to-marsh called the Spring Lake meander scar is located just south and west of Cahokia. Even older meander scars are located to the south and east of the Spring Lake meander scar.

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