Missouri's Natural Divisions
Natural forces and events of the remote past have influenced Missouri's landscape in both stark and subtle ways. A naturalist muses over the effects of ancient forces just as an historian considers past human events in order to build an understanding of the present.
A student of Missouri's natural history might ask what north Missouri would be like if the great ice sheets had not crunched and scraped their way over that part of the state 500,000 years ago or what the Ozarks would be like if they had not been exposed to erosion for hundreds of millions of years.
The Natural Divisions of Missouri is a map which divides the state into major regions based on geologic history, soils, topography, plant and animal distribution, and other natural features. The map divides the state into six major regions and 19 subregions which are termed natural divisions and sections. The Missouri Natural Areas System represents forests, prairies, marshes, glades and other natural communities from the various natural divisions and sections.
Many people think of forested Ozark hills and hollows when they think of Missouri. This is indeed a typical landscape of the Ozark Natural Division. The division has an ancient geological history which included several periods of slow uplift accompanied by deep erosion by its streams. This erosion of a fairly level plateau has created a landscape of deep, winding hollows and steep cliffs with few prominent peaks. The erosion also exposed a variety of rocks-sandstone, limestone, dolomite, chert, granite and rhyolite.
The Ozarks have been an exposed land mass for over 250 million years, while surrounding regions were repeatedly covered by glaciers, seas or floods. Staying high and dry for so long permitted uninterrupted use of the Ozarks by plants and animals. The great age, together with diversity of rock types, soils and topography, created habitats for more species of animals and plants than in any other part of the state. Some species, such as the cave-dwelling grotto salamander, are found nowhere else in the world. Special features such as caves, sinkholes, springs and losing streams are associated with limestone geology and, again, with great geologic age. Thin Ozark soils often allow the bedrock to be exposed to form another typical Ozark natural community--the glade. Savannas, communities of thinly spaced trees with a well developed ground flora of native grasses and forbs, occurred throughout the Ozarks, especially where the westernprairies met the Ozark forests. Pine and deciduous woodlands made up most of the presettlement vegetation. The Ozark Natural Division is subdivided into six sections based on differences in soils, topography, bedrock and river drainage.
Most of the state north of the Missouri River was twice covered by great ice sheets. Two major glaciers leveled the landscape and pushed rocks and debris over the bedrock. The last Missouri glacier retreated perhaps 400,000 years ago, although other glaciers occupied parts of Iowa and Illinois as recently as 12,000 years ago. The topographic features of the Glaciated Plains Natural Division are therefore much younger than those of the Ozarks, and erosion has not had so long a time to sculpt the landscape.
Besides having a leveling effect, the glaciers deposited silts, sands, gravels and boulders, providing parent materials for soil development. The ice sometimes ground rock into a fine glacial flour which was picked up by the wind and deposited over the landscape as a substance called loess. Along the Missouri River, the loess often accumulated to thicknesses of ten to sixty feet or more. In the northwestern counties the wind deposited a thick layer of loess which formed a series of steep dunes or mounds along the east side of the Missouri River.
The gentle terrain and deep soils have made the Glaciated Plains Natural Division ideal for agriculture. Presettlement vegetation was deciduous forest, tallgrass prairie and oak savanna. Prairie has all but disappeared from the Glaciated Plains, but tiny remnants along some railroad tracks still testify to its former existence. The four sections of the Glaciated Plains Natural Division are based on differences in soils, river drainage and glacial history.
The Ozark Border Natural Division is a broad transitional zone where the Ozarks blend into other regions to the north and east. Ozark-like hills and hollows occur along the major streams, but the soils, often derived from loess, are often deeper and more productive than those of the Ozarks. The ranges of many plants and animals of the Glaciated Plains and the Ozarks overlap in the Ozark Border Natural Division. The division was mostly wooded in presettlement times, but prairies, glades, savannas and other natural communities also occurred. Springs, caves, sinkholes, cliffs and pinnacles are common natural features. The Ozark Border Natural Division is subdivided into two sections.
The Missouri and Mississippi rivers and their former floodplains and terraces are distinctive enough to form their own Big Rivers Natural Division. The lower portions of the Grand and Des Moines rivers also are included. The big rivers and their associated land areas have been altered greatly by channel modifications, levying, construction of locks and dams, and agriculture. Chutes, sloughs, islands, sand and mud bars, marshes, prairies and forests once covered this division. The soils are deep sediments that the rivers deposited during recent geologic times.
About ten species of fish are restricted in Missouri to the Big Rivers Natural Division and about 30 more have distributions centered in this area. In the fall, hundreds of bald eagles concentrate along the rivers to over winter. The Big Rivers Natural Division is subdivided into four sections.
The Mississippi River flows southward into the Mississippi Lowlands Natural Division, a region that less than 100 years ago was mostly swamps and lowland forests. The soils of the lowlands are formed from alluvial deposits hundreds of feet thick. There is a fairly sharp separation in the form of an escarpment between the Mississippi Lowlands and the Ozark and Ozark Border natural divisions to the north. The lowlands landscape dates back several hundred thousand years, when it formed the upper end of what is now the Gulf of Mexico. Much later, during the time of the glaciers, meltwater from the retreating ice formed the Ohio and Mississippi river systems. These rivers altered their courses several times and along with the Castor, St. Francis and Whitewater rivers, formed the ridges, basins and low terraces characteristic of the division. These rivers also deposited the thick sediments over the division. Crowleys Ridge, the division's most prominent topographic feature, forms its own section. During the early to mid-1900s, the Mississippi Lowlands Natural Division was drained, the timber cleared and the land converted to agriculture.
Many plants and animals of the coastal plains occur in the lowlands, and many of these species are restricted in Missouri to this region.
The Osage Plains Natural Division is Missouri's prairie region. About three-quarters of this division was covered with prairie before settlement, and most of our remaining prairie is here. Forest and savanna occupied most of what was not prairie. This unglaciated area is characterized by gently rolling hills and plains. The soils were developed mainly from the underlying bedrock instead of from material formed by the glaciers as in the Glaciated Plains. The Osage Plains Natural Division is generally lower in elevation than the Ozarks, has few caves, springs or sinkholes, and has less rugged terrain. Streams commonly have shallow valleys and broad floodplains withmany sloughs and marshes. Erosion has carved cliffs and shelter caves along some of the same streams. Most of the common plants and animals of the Osage Plains are also found in the Glaciated Plains. There is only one section in this natural division.
The natural divisions map provides a framework for thinking of Missouri's natural history. Of course, the exact boundaries of the divisions and sections are not clearly visible on the landscape, but dividing the state in this way does allow us to make certain generalizations about Missouri. The map also gives us some understanding of the influence of the land's history on the present landscape, plants and animals.
(NOTE: For a technical discussion of this subject see "The Natural Divisions of Missouri" by R.H. Thom and J.H. Wilson in Transactions of the Missouri Academy of Science, Vol. 14, 1980, pp. 9-23.)
