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Paha are landforms composed of prominent hills that are oriented from northwest to southeast, and typically have large deposits of loess on them. They were developed during the period of mass wasting that developed the Iowan surface, and are considered erosional remnants and are often at interstream divides. Paha generally rise above the surrounding landscape more than 20 feet. The word paha means hill in Dakota Sioux. A well known Paha is the hill on which the town of Mount Vernon, Iowa developed.
An early theory of the origin of the paha hills of Lowa described them as being "composed in part of water-laid sand and silt and in part of ice-molded till".
Later, after it came to be understood that loess soil was wind deposited silt, pahas came to be interpreted as a kind of sand dune. "Their persistent southeasterly trend suggests deposition of the loess by prevailing northwesterly winds, possibly anticyclonic winds blowing from the retreating ice sheet."
The modern explanation is that the shape of Pahas is the result of the permafrost conditions that dominated glacial till plains of the Iowan surface during the last ice age. Permafrost effects controlled both the way this surface eroded and the way loess accumulated on this surface.
There is a well-defined band of pahas between Mount Vernon, Iowa and Martelle, crossed by Iowa Highway 1. The large majority of pahas are in Benton, Linn, Johnson and Jones counties in Iowa. These are on the Iowan surface in north-east Iowa.
Paha ridges have also been identified on the Kansan surface, generally not far from Iowa, and in western Illinois and eastern Europe. Similar ridge forms occur in the arid upwind parts of the Palouse region of Washington. Outside of the Midwest, several of the above-cited authors use the term greda to refer to features that are indistinguishable from Paha ridges.
- Paha Ridge Landform Features of Iowa, Iowa Geological Survey, 2006. Accessed 2008-08-12.
- Landforms of Iowa by Jean C. Prior, University of Iowa Press, Iowa City, 1991
- Iowa Geological Survey Annual Report, 1898, Samuel Calvin, 1899, pages 395-3969. Available from Google Books
- Iowa Geologial Survey Annual Report, 1915, George F. Kay, pages 150-152. Available from Google Books
- Landform Regions of Iowa Iowa Geologic Survey, accessed 2008-08-12
- Iowa Geological Survey Annual Report, 1915, pages 150-151.
- Joseph A. Mason, et al. A new model of topographic effects on the distribution of loess, Geomorphology 28, 3-4, July 1999, Pages 223-236
- David R. Gaylord, Geomorphic Development of a Late Quaternary Paired Eolian Sequence, Columbia Plateau, Washington, Geological Society of America 2002 Annual Meeting, Denver.