Paha are landforms composed of prominent hills that are oriented from northwest to southeast and typically have large loess deposits. They developed during the period of mass erosion 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 (6.1 m). The word paha means hill in Dakota Sioux. A well known Paha is the hill on which the town of Mount Vernon, Iowa developed.
After it came to be understood that loess soil was wind deposited silt, pahas came to be initially interpreted as a kind of sand dune. "Their persistent southeasterly trend hypothetically suggested deposition of the loess by prevailing northwesterly winds blowing south of the continental 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.. One recent hypothesis attempts to account for the paha as being remnants of interstream divides by attributing snowmelt-erosion as the agent caused by NW-SE, parallel snowdunes that were transverse to an anticyclonic wind system hovering over the continental ice sheet. This was contemporaneous with snowmelt-erosion caused by blankets of snow flattening out the terrain surrounding the paha, known as the Iowan Erosion Surface (I.E.S.).
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.
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- Black Hawk County, 2008-2012 Resource Enhancement and Protection Plan, July 31, 2007, p. 6
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