Paha (landform)

A Paha is a hill or ridge, typically formed of sand and capped with loess. [ [ftp://ftp.igsb.uiowa.edu/gis_library/ia_state/geologic/Landform/paha_ridges.html Paha Ridge Landform Features of Iowa] , Iowa Geological Survey, 2006. Accessed 2008-08-12.] The word "paha" means hill in Dakota Sioux. ["Landforms of Iowa" by Jean C. Prior, University of Iowa Press, Iowa City, 1991] . The most well known Paha is the paha around which the town of Mount Vernon, Iowa developed.

Origin

An early theory of the origin of the paha hills of Iowa described them as being "composed in part of water-laid sand and silt and in part of ice-moulded till." ["Iowa Geological Survey Annual Report, 1898," Samuel Calvin, 1899, pages 395-3969. Available from [http://books.google.com/books?id=NBEMAAAAYAAJ Google Books] ]

Later, after it came to be understood that loess was wind deposited silt, paha's 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." ["Iowa Geologial Survey Annual Report, 1915," George F. Kay, pages 150-152. Available from [http://books.google.com/books?id=pxYMAAAAYAAJ Google Books] ]

Explaining Pahas as a kind of dune does notexplain why they are not shaped like other dune forms. The modern explanation is that the shape of Pahas was 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. [ [http://www.igsb.uiowa.edu/Browse/landform.htm Landform Regions of Iowa] Iowa Geologic Survey, accessed 2008-08-12]

Distribution

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.

Casey's Paha State Preserve in Hickory Hills County Park, Tama County, Iowa preserves the south-east end of a two-mile long (3 Km) paha.

Paha ridges have also been identified on the Kansan surface, generally not far from Iowa, ["Iowa Geologial Survey Annual Report, 1915," pages 150-151.] and in western Illinois and eastern Europe. [Joseph A. Mason, et al. [http://dx.doi.org/10.1016/S0169-555X(98)00112-3 A new model of topographic effects on the distribution of loess] , "Geomorphology 28," 3-4, July 1999, Pages 223-236] Similar ridge forms occur in the arid upwind parts of the Palouse region of Washington. [David R. Gaylord, [http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_43442.htm Geomorphic Development of a Late Quaternary Paired Eolian Sequence, Columbia Plateau, Washington] , Geological Society of America 2002 Annual Meeting, Denver.] Outside of the Midwest, several of the above-cited authors use the term "greda" to refer to features that are indistinguishable from Paha ridges.

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