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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 district of the Iowan Erosion Surface (whereupon the paha lay) and adjacent areas northeast of it (Paleozoic Plateau & the Driftless Area) laid in relative nearness to the edge of the close-by aged mainland ice sheet. Amid the frosty periods of the Pleistocene, a brutal & unconventional cold world existed in every one of the three of these areas. This is on the grounds that an anticyclonic wind framework drifted over the ice sheets while it blew snow-bearing winds from off the frosty ice sheet towards the southwest. This created additional amassed snow sums inside the long ago said areas. The majority of the paha are arranged NW-SE which would be transverse to the snow-bearing winds that blew to the southwest. In the event that we apply the idea of transverse snow hills and their erosive snow melt cutting out parallel straight discouragements and sub-parallel (somewhat anastomosing) dejections, we can show the early phases of starting paha development (Iannicelli, 2003). This early manifestation of the paha geography is seen in the great northwestern piece of Illinois as a result of the presence of relict Illinois-sort loess paha (Iannicelli, 2003). These sort paha are elliptical edges that are divided from each other by straight melancholies. Illinois-sort paha can't be loess hills in light of the fact that genuine loess is saved from dust settling out from air suspension and afterward onto the ground which differentiates from that of rises on the grounds that ridges just start from saltation transport. Hence, recently framed loess stores initially communicates as a cover of fine-grain material which was later on, dismembered by the subparallel erosive snow ridges. The general position of Illinois-sort paha in northwestern Illinois, lays south of the Driftless Area of Illinois. Wind quality produced from the anticyclonic framework was insignificant here, with the goal that it floated the snow into the state of transverse snow rises. Disintegration here was just vertical and it just cut down into the loess and not into the till that lay underneath the loess. As any onlooker looks towards the northwest into Iowa, the geology changes so that a far reaching level tract of till encompasses loess-topped till paha, bringing on the paha to be detached from each other. This planed scope of land that encompasses 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 sheet-wash (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 snow-melt 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 snow-melt (Iannicelli, 2010) originating from infinite amounts of 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 mid-western 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.