Paha (landform)
Encyclopedia
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 paha around which the town of Mount Vernon, Iowa
developed.
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). 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 sheets. During the cold phases of the Pleistocene, a 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 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 that is also in northwestern 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 an 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). The nearby regions of the Paleozoic Plateau & the Driftless Area as a unit, have their own version of similar landforms that are now known as "rock paha" which lay on extensive flat surfaces. 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 upon which lay isolated oblong ridges of rock (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). Thus, the isolated Iowan till paha are equated as unlithified paleopermafrost inselbergs while the rock paha would be equated simply as inselbergs (Iannicelli, 2010).
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 2 miles (3.2 km) long paha.
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.
Hill
A hill is a landform that extends above the surrounding terrain. Hills often have a distinct summit, although in areas with scarp/dip topography a hill may refer to a particular section of flat terrain without a massive summit A hill is a landform that extends above the surrounding terrain. Hills...
or ridge
Ridge
A ridge is a geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. Ridges are usually termed hills or mountains as well, depending on size. There are several main types of ridges:...
, typically formed of sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
and capped with loess
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
. The word paha means hill in Dakota Sioux
Dakota language
Dakota is a Siouan language spoken by the Dakota people of the Sioux tribes. Dakota is closely related to and mutually intelligible with the Lakota language.-Dialects:...
. The most well known Paha is the paha around which the town of Mount Vernon, Iowa
Mount Vernon, Iowa
Mount Vernon is a city in Linn County, Iowa, United States, adjacent to the city of Lisbon. The city's population was 3,390 when the 2000 census figures were released, but that number was later revised to 3,808 because the Census Bureau had incorrectly reported that 418 residents of a Cornell...
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."Later, after it came to be understood that loess
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
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
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
conditions that dominated glacial till
Till
thumb|right|Closeup of glacial till. Note that the larger grains in the till are completely surrounded by the matrix of finer material , and this characteristic, known as matrix support, is diagnostic of till....
plains of the Iowan surface during the last ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
. 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). 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 sheets. During the cold phases of the Pleistocene, a 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 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 that is also in northwestern 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 an 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). The nearby regions of the Paleozoic Plateau & the Driftless Area as a unit, have their own version of similar landforms that are now known as "rock paha" which lay on extensive flat surfaces. 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 upon which lay isolated oblong ridges of rock (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). Thus, the isolated Iowan till paha are equated as unlithified paleopermafrost inselbergs while the rock paha would be equated simply as inselbergs (Iannicelli, 2010).
Distribution
There is a well-defined band of pahas between Mount Vernon, IowaMount Vernon, Iowa
Mount Vernon is a city in Linn County, Iowa, United States, adjacent to the city of Lisbon. The city's population was 3,390 when the 2000 census figures were released, but that number was later revised to 3,808 because the Census Bureau had incorrectly reported that 418 residents of a Cornell...
and Martelle
Martelle, Iowa
Martelle is a city in Jones County, Iowa, United States. The population was 280 at the 2000 census. It is part of the Cedar Rapids Metropolitan Statistical Area.-Geography:Martelle is located at ....
, crossed by Iowa Highway 1. The large majority of pahas are in Benton
Benton County, Iowa
-2010 census:The 2010 census recorded a population of 26,076 in the county, with a population density of . There were 11,095 housing units, of which 10,302 were occupied.-2000 census:...
, Linn
Linn County, Iowa
-2010 census:The 2010 census recorded a population of 211,226 in the county, with a population density of . There were 92,251 housing units, of which 86,134 were occupied.-2000 census:...
, Johnson
Johnson County, Iowa
-2010 census:The 2010 census recorded a population of 130,882 in the county, with a population density of . There were 55,967 housing units, of which 52,715 were occupied.-2000 census:...
and Jones
Jones County, Iowa
-2010 census:The 2010 census recorded a population of 20,638 in the county, with a population density of . There were 8,911 housing units, of which 8,151 were occupied.-2000 census:...
counties in Iowa. These are on the Iowan surface in north-east Iowa
Iowa
Iowa is a state located in the Midwestern United States, an area often referred to as the "American Heartland". It derives its name from the Ioway people, one of the many American Indian tribes that occupied the state at the time of European exploration. Iowa was a part of the French colony of New...
.
Casey's Paha State Preserve in Hickory Hills County Park, Tama County, Iowa
Tama County, Iowa
-2010 census:The 2010 census recorded a population of 17,767 in the county, with a population density of . There were 7,766 housing units, of which 6,947 were occupied.-2000 census:...
preserves the south-east end of a 2 miles (3.2 km) long paha.
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
Palouse
The Palouse is a region of the northwestern United States, encompassing parts of southeastern Washington, north central Idaho and, in some definitions, extending south into northeast Oregon. It is a major agricultural area, primarily producing wheat and legumes...
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.