PRESERVATION OF HOLOCENE ALLUVIAL SEQUENCES BENEATH HEAVILY DEVELOPED CITYSCAPES IN IOWA: IMPLICATIONS FOR THE ARCHAEOLOGICAL POTENTIAL OF "DISTURBED" URBAN LAND

Geoarchaeological investigations in Iowa demonstrate that when cities are built on large-river valley floors, much of the original valley fill and even portions of the original surficial geomorphology are preserved. The present-day cityscape is, geologically speaking, a mosaic of deposits and surfaces created by anthropogenic construction and demolition processes. Cutting and filling creates level surfaces for streets and buildings. Excavations for foundations, basements, and footings. Below this historic-period anthropogenic veneer, however, surprisingly intact sequences spanning the postglacial valley history are preserved, including morphologically intact profiles of the original, pre-urban surface soils. Buried historic and prehistoric archaeological sites are more commonly preserved than surface appearances or SSURGO mapping may suggest.

The historic, anthropogenic veneer may mask the original surface geomorphology, but elements of it can be identified through the study of early historic maps and photographs, as well as high resolution DEMs generated from lidar or civil surveys (e.g., point elevations obtain by public works departments on manhole covers). In Iowa, such research has enabled, for example, the relocation of the incised valley of a Des Moines River tributary and Early-Middle Holocene levee systems of the Cedar River in downtown Cedar Rapids. Geoarchaeological coring and trenching, supplemented by geotechnical logs, are a basis for stratigraphic analysis and archaeological potential.

“It’s all disturbed” is too often used by agencies and archaeoalogists to justify ruling out the need for archaeological investigations in urban areas. Geoarchaeologists must forcefully convey to all stakeholders in Section 106 compliance that this is often, and perhaps usually, not the case. SSURGO map units such as "Urban Land" and "Orthents" refer to a surface veneer may bury older, Holocene valley fills, in the same way that terrace veneers of historic alluvium often mantle "presettlement" surfaces.