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A Paha is a hill or ridge, typically formed of sand and capped with loess. The word paha means hill in Dakota Sioux. The most well known Paha is the hill around which the town of Mount Vernon, Iowa developed.
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-molded till."
Later, after it came to be understood that loess 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."
Explaining Pahas as a kind of dune does not explain 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.
The origin of paha can be explained using a relatively new concept involving snowmelt erosion (Iannicelli, 2000; Iannicelli, 2003; Iannicelli 2010). There are two categories of unlithified paha: Illinois-type paha and Iowan paha. The geomorphic shapes of both categories are basically the same, that being all paha are either oval, oblong or elongated. However, the regional geomorphology of both categories of paha differ greatly: the Illinois-type paha occur in great numbers while the paha-ridges are contiguous and bunched together; the Iowan paha occur in less numbers and are usually at a distance from one another. The region of the Iowan Erosion Surface (upon which the paha lay) and contiguous regions northeast of it (Paleozoic Plateau & the Driftless Area) laid in relative proximity to the edge of the nearby ancient continental ice sheet. During the cold phases of the Pleistocene, a harsh & peculiar snowy world existed in all three of these regions. This is because an anticyclonic wind system hovered over the ice sheets while it blew snow-bearing winds from off the glacial ice sheet towards the southwest. This caused extra accumulated snow amounts within the previously-mentioned regions. All of the paha are oriented NW-SE which would be transverse to the snow-bearing winds that blew to the southwest. If we apply the concept of transverse snow dunes and their erosive snowmelt carving out parallel linear depressions and sub-parallel (slightly anastomising) depressions, we can manifest the early stages of initial paha formation (Iannicelli, 2003). This early form of the paha topography is seen in the extreme northwestern part of Illinois because of the existence of relict Illinois-type loess paha (Iannicelli, 2003). These type paha are oblong ridges that are separated from one another by linear depressions. Illinois-type paha cannot be loess dunes because true loess is deposited from dust settling out from atmospheric suspension and then onto the ground which contrasts from that of dunes because dunes only originate from saltation transport. Thus, newly formed loess deposits originally expresses itself as a blanket of fine-grain material. The general position of Illinois-type paha in northwestern Illinois, lays south of the Driftless Area of Illinois. Wind strength generated from the anticyclonic system was minimal here, so that it drifted the snow into the shape of transverse snow dunes. Erosion here was only vertical and it only carved down into the loess and not into the till that lay beneath the loess. As any observer looks towards the northwest into Iowa, the topography changes so that an extensive flat tract of till surrounds loess-capped till paha, causing the paha to be isolated from one another. This planed expanse of land that surrounds the paha is known as the Iowan Erosion Surface (I.E.S.). Other investigators have identified the I.E.S. as being formed and flattened by running water (Ruhe et al., 1968) or sheetwash (Hallberg et al., 1978). Hallberg et al. (1978) identified the flat I.E.S. as "non-classical" pediments. However, when we apply the concept of snow-bearing winds dumping an extraordinary amount of snow onto the I.E.S., a better-modified interpretation of the I.E.S. is that it was leveled by cryopedimentation through widespread, blanket-like snowmelt erosion which would account for its planar form (Iannicelli, 2010). Hence, Iowan paha embryonically started off in similarity to Illinois-type paha but then cryopedimentation isolated them (essentially unlithified inselbergs) to give them their own distinctive geomorphic flavor. The nearby regions of the Paleozoic Plateau & the Driftless Area as a unit, have their own version of paha-forms, that being these are lithified paha which are now known as "rock paha" (oriented, oblong ridges of rock) (Iannicelli, 2010). They too also lay on extensive flat surfaces and the same concept that was applied to the I.E.S. is applied to them because they have correlating planed surfaces known as the Lancaster & Dodgeville Erosion Surfaces (Iannicelli, 2010). The upland planed surface of the Driftless Area & Paleozoic Plateau regions is specifically explained as being created by cryoplanation that was sourced from snowmelt (Iannicelli, 2010) that originated from snow blown off the continental ice sheet. Thus, the isolated Iowan till paha are equated as unlithified paleopermafrost inselbergs while the rock paha would be equated simply as inselbergs in the non-classical sense (Iannicelli, 2010).
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
- G. Hallberg, T.E. Fenton, G.A. Miller, A.J. Luteneggar (1978), The Iowan Erosion Surface, an important lesson and some new wrinkles
- In, R.R. Anderson (Ed.), "Geology of east-central Iowa", 42nd Annual Tri-State Geological Field Conference, pp. 2.2 - 2.94 (specifically pp. 18 & 20)
- M. Iannicelli (2000), Snow dune erosion and landforms, Northeastern Geology and Environmental Sciences, Vol. 22, No. 4, pp.324 - 335
- M. Iannicelli (2003), Devon Island's oriented landforms as an analog to Illinois-type paha, Polar Geography, Vol. 27, No. 4, pp. 339 - 350
- M. Iannicelli (2010), Evolution of the Driftless area and contiguous regions of midwestern USA through Pleistocene periglacial processes, The Open Geology Journal, Vol. 4, pp. 35 - 54
- R.V. Ruhe, W.P. Dietz, T.E. Fenton, G.F. Hall (1968), Iowan drift problem, northeastern Iowa, Iowa Geological Survey, Report of Investigations, Vol. 7, 40 pp
- 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.