Simplified map of Iowa
Bedrock formations of Iowa

The geography of Iowa includes the study of bedrock, landforms, rivers, geology, paleontology and urbanisation of the U.S. state of Iowa. The state covers an area of 56,272.81 sq mi (145,746 km2).

Bedrock features

Iowa's bedrock geology generally increases in age from west to east. In northwest Iowa Cretaceous bedrock is ca. 74 million years old, in eastern Iowa Cambrian bedrock dates to ca. 500 million years ago.[1]

Meteor impact structures

Manson impact structure

Seventy-four million years ago, a large asteroid crashed into what is now southeast Pocahontas county creating the Manson crater. Nearly 22 miles in diameter,[2] it would have killed most animals within 650 miles, roughly an area from modern Denver to Detroit. This was originally thought to have been one of the causes of the dinosaur extinction, but recalculation of the impact's age indicates it occurred some 12 million years before the mass extinction. Although glaciation has erased all surface evidence of the impact, the bedrock associated with this impact is unique in Iowa.[3]

Decorah crater

U.S. Geological Survey aerial resistivity map of the Decorah, Iowa area, showing the Decorah crater.

A much older meteorite strike created the Decorah crater during the Middle Ordovician Period, 470 million years ago. The crater is estimated to be 3.5 miles (5.6 km) in diameter, covered by Winneshiek Shale.[4][5][6] There is no surface evidence of the impact, as the Winneshiek Shale is more than 50 feet below the bottom of the Upper Iowa River. The impact, equivalent to 1,000 megatons of TNT,[5] did not appear to penetrate the Earth's mantle, but it did push down the underlying Ordovician and Cambrian bedrock several hundred feet.[7] It may be one of several Middle Ordovician meteors that fell roughly simultaneously 469 million years ago, part of a proposed Ordovician meteor event.

Midcontinent rift

Iowa magnetic anomaly map showing the Midcontinent Rift curving from the north center to the southwest part of the state.[8]

Buried deeply within Iowa's bedrock, the Midcontinent Rift System can be seen clearly in magnetic anomaly maps of Iowa. This is a billion-year-old tectonic plate scar that extends from Kansas through Lake Superior. This rift is not seismically active.[9]

Seismic activity

No major active fault lines exist in Iowa, and Iowa is one of the most seismically stable states in the U.S. With the exception of the 1968 Illinois earthquake which caused the water tower at Lineville to leak,[10] no injuries or significant damage has ever been caused by earthquakes in Iowa. Occasional small earthquakes occur near Fremont County in the far southwest, and the Sioux City area can occasionally be shaken by nearby tremblors. Large earthquakes associated with the New Madrid Fault of far southern Illinois and Missouri can occasionally be felt in eastern Iowa.[11][12] Recent earthquakes centered in Oklahoma have also been felt in Iowa, but have caused no damage.[13]

Fossil fuels

Iowa coal mine, 1936.

Historically, Iowa was a significant coal producer, particularly the Des Moines River valley from Coalville south. Much of the greater City of Des Moines area was mined. Boone and What Cheer were important in the late 19th century. In the 20th century, the most important mines were farther south around Albia, Centerville, Lucas and Oskaloosa. Iowa coal tends to be too high in sulfur for modern applications, and the last commercial mine closed in 1994.[14]

Iowa has very limited natural gas and oil production.[15]

Sioux quartzite

Although Iowa's bedrock is generally younger in western parts of the state, one exception is a small part of far northwest Iowa where Precambrian Sioux quartzite bedrock is found in northwest Lyon County. Sioux quartzite is a very hard rock of ruddy pink color used extensively in the region for road and railroad beds.[16] Sioux quartzite has been dated to have been laid between 1.64 and 1.76 billion years ago.[17]

Geode half

Geodes

Geodes are found in southeast Iowa and are the official state rock. They consist of grey to pink cobbles within limestone that when cut or smashed open reveal a hollow crystal-filled interior. Geodes are common around Geode State Park in Henry County.[18]

Landforms and topography

Topography of Iowa, with counties and major streams
Landforms of Iowa, based on Prior (1991) and Calvin (1904), with major rivers and streams

Despite popular perception, Iowa is generally not flat; most of the state consists of rolling hills. Prior[19] divides Iowa into eight landforms based on glaciation, soils, topography, and river drainage:

Paleozoic plateau

Also known as the Driftless Area, this region of scenic, high relief landscapes includes such features as resistant, bluff-forming bedrock outcrops, deep V-shaped valleys, caves, springs, and sinkholes. Glacial deposits and loess are thin or absent over most of the region.

Des Moines lobe

Barringer Slough, a remnant of the extensive prairie wetlands that once covered the Des Moines Lobe

Often called the Prairie Pothole Region, the Des Moines Lobe was glaciated up until 12,000 years ago during the Wisconsin glaciation. The area is marked by rolling terrain and ridges.[20] Historically, this area was peppered with small interconnected swamps, most of which were drained for farmland. The Iowa Great Lakes occur along the western edge of the Des Moines lobe.

Southern Iowa drift plain

Typical view of the Southern Iowa Drift Plain

The southern Iowa drift plain covers most of the southern half of Iowa. This is probably the most familiar landscape to travelers, since most of Interstate 80 in Iowa runs through the SIDP. The classic Iowa landscape, consisting of rolling hills of Wisconsin-age loess on Illinoian (or earlier) till. The SIDP is some of the most productive agricultural land in the world.

Mississippi alluvial plain

Mississippi Valley Wetland near Ft. Madison, Iowa

Generally level areas of stream terraces, paleochannels, backwater sloughs, and oxbow lakes are found within the broad Mississippi River valley.

Loess hills

The Loess Hills consist of very thick deposits of loess in far western Iowa deposited during the Wisconsin and Illinoian periods. Highly eroded, leaving stark, beautiful "golden hills".

Loess Hills east of Mondamin, Iowa, showing the transition with the Missouri alluvial plain.

Iowan surface

Northeast Iowa is covered with eroded Pre-Illinoian till with moderate loess formation, frequently in the form of paha ridges, muted relief except for steep rolling hills near river valleys, and deeper valleys. These picturesque hills are depicted in many of the landscapes of Grant Wood.

Northwest Iowa plains

Like the Iowan Surface, the Northwest Iowa Plains are rolling hills consisting of eroded soils developed since pre-Wisconsinan glaciation, but with significant amounts of loess.

Missouri alluvial plain

Perhaps the only truly flat region of Iowa, the Missouri Alluvial Plain contains areas of terraces, sloughs, and oxbows. Its valley trench is not as deep as the Mississippi River system, and the Missouri River is contained in a much narrower channel. In Iowa, the eastern border of the Missouri Plains is the Loess Hills, forming steep rounded bluffs.

Climate

Köppen climate types of Iowa, using 1991–2020 climate normals.

Iowa has a humid continental climate throughout the state (Köppen climate classification Dfa) with extremes of both heat and cold. The average annual temperature at Des Moines is 50 °F (10 °C); for some locations in the north, such as Mason City, the figure is about 45 °F (7 °C), while Keokuk, on the Mississippi River, averages 52 °F (11 °C).[21] Snowfall is common, with Des Moines getting about 26 days of snowfall a year, and other places, such as Shenandoah getting about 11 days of snowfall in a year.[22]

Spring ushers in the beginning of the severe weather season. Iowa averages about 50 days of thunderstorm activity per year.[23] The 30-year annual average of tornadoes in Iowa is 47.[24] In 2008, twelve people were killed by tornadoes in Iowa, making it the deadliest year since 1968 and also the second most tornadoes in a year with 105, matching the total from 2001.[25]

Iowa summers are known for heat and humidity, with daytime temperatures sometimes near 90 °F (32 °C) and occasionally exceeding 100 °F (38 °C). Average winters in the state have been known to drop well below freezing, even dropping below −18 °F (−28 °C). Iowa's all-time hottest temperature of 118 °F (48 °C) was recorded at Keokuk on July 20, 1934, during a nationwide heat wave;[26] the all-time lowest temperature of −47 °F (−44 °C) was recorded in Washta on January 12, 1912.[27]

Iowa has a relatively smooth gradient of varying precipitation across the state, with areas in the southeast of the state receiving an average of over 38 inches (97 cm) of rain annually, and the northwest of the state receiving less than 28 inches (71 cm).[28] The pattern of precipitation across Iowa is seasonal, with more rain falling in the summer months. Virtually statewide, the driest month is January or February, and the wettest month is June, owing to frequent showers and thunderstorms, some of which produce hail, damaging winds and/or tornadoes. In Des Moines, roughly in the center of the state, over two-thirds of the 34.72 inches (88.2 cm) of rain falls from April through September, and about half the average annual precipitation falls from May through August, peaking in June.[29]

Climate data

Monthly normal high and low temperatures for various Iowa cities (°F)[30]
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Davenport[31] 30/13 36/19 48/29 61/41 72/52 81/63 85/68 83/66 76/57 65/45 48/32 35/20
Des Moines[32] 31/14 36/19 49/30 62/41 72/52 82/62 86/67 84/65 76/55 63/43 48/31 34/18
Keokuk[33] 34/17 39/21 50/30 63/42 73/52 83/62 87/67 85/65 78/56 66/44 51/33 33/21
Mason City[34] 24/6 29/12 41/23 57/35 69/46 79/57 82/61 80/58 73/49 60/37 43/25 28/11
Sioux City[35] 31/10 35/15 47/26 62/37 73/49 82/59 86/63 83/63 76/51 63/38 46/25 32/13
Climate data for Des Moines International Airport, Iowa (1991–2020 normals,[lower-alpha 1] extremes 1878–present[lower-alpha 2])
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Record high °F (°C) 67
(19)		 78
(26)		 91
(33)		 93
(34)		 105
(41)		 103
(39)		 110
(43)		 110
(43)		 101
(38)		 95
(35)		 82
(28)		 74
(23)		 110
(43)
Mean maximum °F (°C) 53.4
(11.9)		 58.7
(14.8)		 74.6
(23.7)		 83.9
(28.8)		 88.9
(31.6)		 93.1
(33.9)		 96.2
(35.7)		 94.4
(34.7)		 91.3
(32.9)		 83.3
(28.5)		 70.4
(21.3)		 57.8
(14.3)		 97.4
(36.3)
Mean daily maximum °F (°C) 30.9
(−0.6)		 35.7
(2.1)		 49.2
(9.6)		 62.0
(16.7)		 72.4
(22.4)		 81.9
(27.7)		 85.6
(29.8)		 83.6
(28.7)		 76.9
(24.9)		 63.4
(17.4)		 48.3
(9.1)		 35.9
(2.2)		 60.5
(15.8)
Daily mean °F (°C) 22.3
(−5.4)		 26.9
(−2.8)		 39.4
(4.1)		 51.3
(10.7)		 62.4
(16.9)		 72.2
(22.3)		 76.0
(24.4)		 73.9
(23.3)		 66.2
(19.0)		 53.2
(11.8)		 39.3
(4.1)		 27.7
(−2.4)		 50.9
(10.5)
Mean daily minimum °F (°C) 13.8
(−10.1)		 18.0
(−7.8)		 29.6
(−1.3)		 40.6
(4.8)		 52.3
(11.3)		 62.4
(16.9)		 66.4
(19.1)		 64.2
(17.9)		 55.4
(13.0)		 42.9
(6.1)		 30.2
(−1.0)		 19.5
(−6.9)		 41.3
(5.2)
Mean minimum °F (°C) −7.8
(−22.1)		 −2.7
(−19.3)		 9.2
(−12.7)		 24.9
(−3.9)		 37.6
(3.1)		 50.2
(10.1)		 56.9
(13.8)		 54.8
(12.7)		 40.4
(4.7)		 26.8
(−2.9)		 12.6
(−10.8)		 −1.2
(−18.4)		 −11.4
(−24.1)
Record low °F (°C) −30
(−34)		 −26
(−32)		 −22
(−30)		 9
(−13)		 26
(−3)		 37
(3)		 47
(8)		 40
(4)		 26
(−3)		 7
(−14)		 −10
(−23)		 −22
(−30)		 −30
(−34)
Average precipitation inches (mm) 1.08
(27)		 1.34
(34)		 2.17
(55)		 4.02
(102)		 5.24
(133)		 5.26
(134)		 3.82
(97)		 4.17
(106)		 3.18
(81)		 2.78
(71)		 1.91
(49)		 1.58
(40)		 36.55
(928)
Average snowfall inches (cm) 9.4
(24)		 10.2
(26)		 4.4
(11)		 1.2
(3.0)		 0.2
(0.51)		 0.0
(0.0)		 0.0
(0.0)		 0.0
(0.0)		 0.0
(0.0)		 0.5
(1.3)		 2.7
(6.9)		 7.9
(20)		 36.5
(93)
Average precipitation days (≥ 0.01 in) 8.2 8.4 9.5 11.5 12.7 11.7 9.5 9.4 8.2 8.6 7.7 7.8 113.2
Average snowy days (≥ 0.1 in) 6.9 6.3 3.1 1.0 0.1 0.0 0.0 0.0 0.0 0.5 1.8 5.6 25.3
Average relative humidity (%) 71.0 71.3 67.9 63.2 63.0 64.8 67.7 70.0 70.9 66.5 71.0 74.6 68.5
Mean monthly sunshine hours 157.7 163.3 206.0 222.2 276.0 312.1 337.8 297.9 239.8 210.0 138.5 129.2 2,690.4
Percent possible sunshine 53 55 56 56 61 69 73 70 64 61 47 45 60
Average ultraviolet index 1 2 4 6 8 9 9 8 6 4 2 1 5
Source 1: NOAA (relative humidity and sun 1961−1990)[36][37][38]
Source 2: Weather Atlas (UV)[39]

Water

Iowa annual rainfall in inches
Mississippi River alluvial plain from SIDP bluffs north of Kingston, Iowa

As in most of the U.S., surface water in Iowa is never safe to drink untreated, contamination by agricultural runoff including nitrates, herbicides, pesticides, and animal waste is common, though the Clean Water Act has helped. Municipal water supplies are typically heavily chlorinated, this chlorine, combined with high nitrate levels, often give municipal water a strong smell, and the limestone bedrock in much of the state causes hard water.[40] Some communities, such as Iowa City resort to additional carbon filtration and lime softening coagulation-sedimentation to make the water more palatable.[41] Water treatment can be surprisingly effective; Des Moines' advanced filtration system has led to water quality ranked among the nation's best.[42]

Jordan Aquifer

The Jordan Aquifer is the largest source of groundwater, extending from northeast Iowa to south central Iowa, and is ultimately the source of much of Iowa's agricultural and industrial water. In addition to pollution threats, the aquifer is threatened by overuse in well-source irrigation, ethanol production, and the diminishment of resupply caused by extensive field tilling. The aquifer has dropped by as much as 300 feet since the 19th century, resulting in dry wells, the disappearance of natural surface springs, and the diminishment of water quality.[43][44]

Soils

The NRCS divides Iowa into 23 soil regions. In general, soils of southern, eastern, and western Iowa are loess-derived, while soils of northern and central Iowa are till-derived. Most level areas of Iowa have soils highly suitable for agriculture, making Iowa one of the most productive farming regions of the world.[45]

Radon

Radon levels in U.S., showing Iowa with some of the highest levels.

Like most Upper Midwest and Plains states, radon is a common problem in Iowa, especially in areas with clay-rich soils.[46] Radon is the highest in the southern and the western parts of Iowa.

Paleontology

Devonian Fossil Gorge

Floods in 1993 washed away all the soil and unconsolidated bedrock along the spillway of the Coralville Lake Dam in Johnson County, exposing a rich collection of Devonian-age fossils. This area has been transformed into a visitors' center, where hikers can tour the bedrock. Unfortunately, looters have illegally removed many of the better fossils.[47] The June 2008 floods expanded the fossil bed floor, and removed some of the weathered overburden.[48]

Dinosaurs

Jurassic and Cretaceous bedrock in western Iowa have potential to contain dinosaur remains, and in nearby parts of Nebraska Hadrosaurid (“duck-billed”) ornithopod dinosaur remains have been recovered in Cretaceous bedrock similar to that of Iowa. The deep loess that covers much of western Iowa typically conceals the bedrock, limiting opportunities to finding dinosaurs to mining and quarrying operations.[49]

Paleofauna

Archaeological and paleontological sites in Iowa have produced an extensive collection of Pleistocene and Holocene animals; these have been used to reconstruct past environmental conditions in the Midwest.[50] Some of the earliest paleoclimatic reconstructions of the midcontinent were made from Iowa collections, such as the Cherokee Sewer Site.[51]

A Pleistocene giant sloth is under excavation along West Tarkio Creek near Shenandoah, Iowa. Three individuals of Megalonyx jeffersonii, or Jefferson's Ground Sloth, have been identified so far, including one adult and two juveniles of different ages.[52][53]

Notable Iowa geologists

Samuel Calvin (1840–1911) was Iowa's first systematic geologist, who helped to make the first bedrock and landform maps of Iowa, as well as lead geological research throughout the state.[54] Calvin Hall at the University of Iowa is named for him. Clair Cameron Patterson (1922–1995) developed the lead-lead dating and calculated an age for the Earth of 4.55 billion years; a figure far more accurate than those that existed at the time and one that has remained unchanged for over 50 years. Charles Rollin Keyes was also an early Iowa geologist who helped map the soils and bedrock of southeast Iowa; he, with Calvin, was a founder of the Iowa Geological Survey.

Notes

  1. ↑ Mean monthly maxima and minima (i.e. the expected highest and lowest temperature readings at any point during the year or given month) calculated based on data at said location from 1991 to 2020.
  2. ↑ Official records for Des Moines kept August 1878 to August 1939 at downtown and at Des Moines Int'l since September 1939. For more information, see Threadex

References

  1. ↑ Prior: Geology of Iowa: Iowa's Earth History Shaped by Ice, Wind, Rivers, and Ancient Seas Archived 2009-04-16 at the Wayback Machine
  2. ↑ "Manson". Earth Impact Database. Planetary and Space Science Centre University of New Brunswick Fredericton. Retrieved 2022-10-04.
  3. ↑ Iowa's Manson Impact Structure Archived 2008-06-16 at the Wayback Machine
  4. ↑ Vastag, Brian (18 February 2013). "Crater found in Iowa points to asteroid break-up 470 million years ago". Washington Post. Retrieved 19 February 2013.
  5. 1 2 "Geological survey: Ancient meteorite crater sits below Decorah". Cedar Rapids Gazette. 5 March 2013. Retrieved 6 March 2013.
  6. ↑ US Geological Survey. "Iowa Meteorite Crater Confirmed". Retrieved 7 March 2013.
  7. ↑ Iowa Department of Natural Resources. "GEOLOGIC MAPPING FOR WATER QUALITY PROJECTS IN THE UPPER IOWA RIVER WATERSHED" (PDF). Technical Information Series No. 54, 2011. Retrieved 19 February 2013.
  8. ↑ Kucks, Robert P.; Hill, Patricia L. (2005). "Iowa magnetic and gravity maps and data". U.S. Geological Survey. Retrieved 2009-09-03.
  9. ↑ "Midcontinent Rift System In Iowa". Archived from the original on 2007-05-22. Retrieved 2008-06-09.
  10. ↑ Times-Republican Newspaper, Corydon, Iowa, 19 NOV 1968
  11. ↑ "Iowa Earthquake Information". Archived from the original on 2009-05-15. Retrieved 2017-08-28.
  12. ↑ "Iowa". Archived from the original on 2009-01-09. Retrieved 2017-08-28.
  13. ↑ Fickau, Ethan (3 September 2016). "What's the likelihood of feeling another earthquake in Iowa?". KCCI Des Moines (CBS). Retrieved 10 September 2016.
  14. ↑ "Iowa Coal Geology". Archived from the original on 2008-06-15. Retrieved 2008-06-09.
  15. ↑ "Oil, Gas, and Metallic Mineral Regulatory Information". Archived from the original on 2008-05-17. Retrieved 2008-06-09.
  16. ↑ "Sioux Quartzite geology". Archived from the original on 2008-07-23. Retrieved 2008-06-20.
  17. ↑ Wayne I. Anderson, Iowa's Geological Past: Three Billion Years Of Change, University Of Iowa Press, 1998, p.41 ISBN 978-0877456391
  18. ↑ IA DNR: State Parks, Geode State Park
  19. ↑ Prior, Jean C. (1991) Landforms of Iowa. University of Iowa Press, Iowa City. "Landforms". Archived from the original on 2009-03-02. Retrieved 2008-06-09.. See also Calvin, Samuel (1904) Outline Map of the Drift Sheets of Iowa. Iowa Publication Co., Davenport.
  20. ↑ "Landforms". Archived from the original on 2009-03-02. Retrieved 2008-06-09.
  21. ↑ "Climate Iowa: Temperature, climate graph, Climate table for Iowa - Climate-Data.org". en.climate-data.org. Archived from the original on April 24, 2019. Retrieved April 6, 2020.
  22. ↑ "Average Annual Snowfall Totals in Iowa – Current Results". Currentresults.com. Archived from the original on February 21, 2020. Retrieved April 6, 2020.
  23. ↑ US Thunderstorm distribution. src.noaa.gov. Retrieved February 13, 2008. Archived October 15, 2006, at the Wayback Machine
  24. ↑ "Des Moines, IA". noaa.gov. Archived from the original on August 21, 2014. Retrieved February 18, 2015.
  25. ↑ "2008 Iowa tornadoes deadliest since 1968". USA Today. January 2, 2009. Archived from the original on October 11, 2013. Retrieved January 2, 2009.
  26. ↑ Keokuk Comprehensive Plan 2018 (PDF) (Report). June 2018. Archived (PDF) from the original on February 20, 2021. Retrieved April 6, 2020.
  27. ↑ Munson, Kyle. "Site of Iowa's coldest temp shivers with rest of state". USA TODAY. Archived from the original on February 28, 2020. Retrieved April 6, 2020.
  28. ↑ Average Annual Precipitation Iowa, 1961–1990 (GIF File) Archived February 13, 2010, at the Wayback Machine—Christopher Daly, Jenny Weisburg
  29. ↑ "Average Weather for Des Moines, IA—Temperature and Precipitation, Weather.com, Retrieved Jan. 7, 2009". Weather.com. Archived from the original on December 3, 2010. Retrieved July 31, 2010.
  30. ↑ "Iowa Weather-Iowa Weather Forecast-Iowa Climate". ustravelweather.com. Archived from the original on January 31, 2011. Retrieved February 17, 2011.
  31. ↑ "Monthly Averages for Davenport, Iowa". Weather.com. Archived from the original on October 8, 2008. Retrieved November 1, 2008.
  32. ↑ "Average Weather for Des Moines, IA—Temperature and Precipitation". Weather.com. Archived from the original on October 24, 2012. Retrieved February 18, 2012.
  33. ↑ "Daily Averages for Keokuk, IA". weather.com. Archived from the original on October 24, 2012. Retrieved February 18, 2012.
  34. ↑ "Average Weather for Mason City, IA—Temperature and Precipitation". Weather.com. Archived from the original on October 24, 2012. Retrieved February 18, 2012.
  35. ↑ "Average Weather for Sioux City, IA—Temperature and Precipitation". Weather.com. Archived from the original on October 24, 2012. Retrieved February 18, 2012.
  36. ↑ "NowData – NOAA Online Weather Data". National Oceanic and Atmospheric Administration. Retrieved June 26, 2021.
  37. ↑ "Station: Des Moines INTP AP, IA". U.S. Climate Normals 2020: U.S. Monthly Climate Normals (1991–2020). National Oceanic and Atmospheric Administration. Retrieved June 26, 2021.
  38. ↑ "WMO Climate Normals for DES MOINES/MUNICIPAL, IA 1961–1990". National Oceanic and Atmospheric Administration. Retrieved June 26, 2021.
  39. ↑ "Des Moines, Iowa, USA - Monthly weather forecast and Climate data". Weather Atlas. Retrieved July 4, 2019.
  40. ↑ Iowa City Water Quality Archived May 17, 2008, at the Wayback Machine
  41. ↑ "Housing Authority | City of Iowa City".
  42. ↑ 'Drink up - from the city tap.' Des Moines Register 2 July 2008, http://www.desmoinesregister.com/apps/pbcs.dll/article?AID=/20080702/OPINION03/807020346/1110%5B%5D Missing on 2018-9-17.
  43. ↑ Love, Orlan (Dec 6, 2009). "Heavy use draining aquifer". Cedar Rapids Gazette. Archived from the original on December 9, 2009. Retrieved 20 December 2009.
  44. ↑ Tecklenburg, Jeff (Dec 18, 2009). "Don't take aquifer for granted". Cedar Rapids Gazette. Archived from the original on July 10, 2012. Retrieved 20 December 2009.
  45. ↑ Information About Soils | Iowa NRCS
  46. ↑ "Extension Store" (PDF).
  47. ↑ Flood of 1993 Archived June 16, 2008, at the Wayback Machine
  48. ↑ Iowa City Press Citizen, http://www.press-citizen.com/apps/pbcs.dll/article?AID=/20080709/NEWS01/807090324/1079
  49. ↑ Witzke, http://www.igsb.uiowa.edu/Browse/dinosaurs/age_of_dinosaurs_in_iowa.htm Archived 2008-05-11 at the Wayback Machine
  50. ↑ "Archived copy". Archived from the original on 2008-05-24. Retrieved 2008-06-09.{{cite web}}: CS1 maint: archived copy as title (link)
  51. ↑ Anderson and Semken (1980) The Cherokee Excavations: Holocene Ecology and Human Adaptations in Northwestern Iowa. Academic Press, New York.
  52. ↑ https://www.sciencedaily.com/releases/2006/06/060626123721.htm, http://slothcentral.com/ Archived 2008-10-08 at the Wayback Machine
  53. ↑ Semken and Brenzel (2007) One Sloth Becomes Three. Newsletter of the Iowa Archeological Society 57(1).
  54. ↑ Calvin obituary, The Journal of Geology, July 1911, pp. 385-391.
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