Paha (landform)

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.

An early theory of the origin of the paha hills of Iowa described them as "composed in part of water-laid sand and silt and in part of ice-molded till".

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." However, recent findings show that the NW-SE orientation of paha are transverse to anticyclonic snow-bearing winds that hovered over the continental ice sheet that blew towards the southwest in this region at that time.

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. This general concept has now been advanced because the permafrost conditions at that time in the Midwest were conducive to paleo-snowmelt flowing over the permafrost table (termed as "overland flow") that caused widespread sheetwash erosion.

The paleo-snow was also laid down in parallel dune fashion just like transverse sand dunes, so that transverse, NW-SE snow dunes formed in northeastern Iowa and in northwestern Illinois due to the previously-mentioned anticyclonic, snowbearing wind system hovering over the continental ice sheet which blew to the southwest, and hence, transversely to the snow dunes during the . The snow dunes carved out meltwater valleys in a NW-SE orientation so that NW-SE snow-melt interfluves alternated between the NW-SE valleys. These "snow-melt interfluves" are the same as the "interstream divides" that were mentioned in the above introduction section.

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