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10 Cenozoic Erosion Climaxed by the Great Ice Age To Iowa the scientific world has learned to look for the standard American section ofthe deposits ofthe great ice age, for here the record of successive glacial invasions ofthe Pleistocene period are most complete and have been most satisfactorily deciphered. Samuel Calvin (1906) remarking on Iowa's glacial record The trail ofthe ice monster has been traced, his magnitude measured, his form and even his features figured forth, and all from the slime ofhis body alone, where even his characteristic tracks fail. W. J. McGee (1891) commenting on the Pleistocene history of northeastern Iowa In general the rocks transported by the Iowan ice were brought in large masses, ten, fifteen, or twenty feet in diameter. ... The largest ofthe many large boulders seen in this county is that known as Saint Peter.... Saint Peter is fully twenty feet in height and more than eighty feet in circumference. Samuel Calvin (1902) discussing the glacial deposits ofChickasaw County The Cenozoic Era is divided into two periods, the Tertiary and the Quaternary. The Tertiary Period represents about 65 million years, and the QuaternaryPeriod includes the last 1.65 million years. The Quaternary consists of the Pleistocene Epoch and the Holocene (Recent) Epoch. The Holocene represents the last 10,000 to 10,500 years, essentially the time since the continental glaciations in North America and Europe. Iowa was above sea level during the Tertiary and subjected to extensive weathering and erosion. States to the west of Iowa, however, have a well-preserved record of Tertiary rocks in the form of stream-laid sediments. These Tertiary sediments represent the deposits of stream systems that flowed eastward from sources in the Rocky Mountain area. The ancient stream deposits contain a diverse fauna of mammals and other land vertebrates. The Tertiary strata of the Badlands of South Dakota and Nebraska are particularly well known for their abundant andwell-preservedvertebrate faunas. Afew Tertiaryvertebrate remains have been recovered in southwestern Iowa from alluvial sediments. Among these are parts of rhinoceroses and three-toed horses in Pleistocene sand-andgravel deposits. The excellent preservation and relatively unabraded condition of these fossils suggesttheywere not transportedfar. Perhaps theywere derivedfrom nearby Tertiary strata that remain undetected in the subsurface ofwestern Iowa. So-calledsalt-and-peppersands are commoninwestern Iowa. Theyare similar in appearance and characteristics to deposits ofthe present-day Platte and Missouri rivers to the west of Iowa. The "salt" in the sands is common quartz, but the "pepper" is more distinctive. Many ofthe dark or pepper-colored grains are fragments ofvolcanic glass, indicative ofvolcanic source lands such as those in the Rockies. These telltale grains in western Iowa strongly suggest the presence ofan eastward-flowing stream system with headwaters in a volcanic source land. Such a stream system apparently existed in western Iowa prior to the establishment ofthe present course ofthe Missouri River. Most ofthe salt-and-pepper deposits are older than the start ofglacial deposition in western Iowa some 2.5 million years ago. The tooth ofan ancient Pliocene elephant, recovered from gravels in Plymouth County, helps date some ofthese western-derived alluvial sediments. Successive glaciations in western Iowa probably diverted the southeastward-flowing ancestral drainage system to the south, forming the present Missouri River system. During the long episode of erosion in Tertiary time, Iowa's Mesozoic rock record was largely removed by erosion, and streams carved their valleys into the Paleozoic bedrock ofthe state. This topography had some control on the path of the Late Pliocene and Pleistocene continental ice sheets that invaded Iowa. The ice sheets tended to follow low topographic trends. Glacial deposits and glacially related deposits constitute a significant part of Iowa's Cenozoic record, as this chapter's opening quotation from Samuel Calvin implies. However, the understanding ofthese deposits was not as complete as Calvin envisioned back in 1906. Since then, geologists have learned a great deal more about this intriguing record. The past three decades in particular have seen major revisions in Iowa's Ice Age history. The climate in Iowa during the Late Pliocene and Pleistocene was considerably different than it had been previously. Whereas Paleozoic and Mesozoic climates were warm and tropical or subtropical, conditions during the late Pliocene and Pleistocene were temperate and punctuated by glacial ages. The Pleistocene Epoch started about 1.65 million years ago. The placement of the end ofthe Pleistocene is somewhat arbitrary, but most North American geologists place the last 10,000 to 10,500 years in the Holocene...

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