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FRAGILE GIANTSA NATURAL HISTORY OF THE LOESS HILLS
By Cornelia F. Mutel
UNIVERSITY OF IOWA PRESSCopyright © 1989 University of Iowa
All right reserved.
"These Bluffs are a great Curiosity." So stated railroad entrepreneur John Insley Blair during his 1863 expedition to survey a route west to the Missouri River. Thirteen years earlier, missionary Thaddeus Culbertson had described the Loess Hills in his travel journal in this way: "The term Mountains in Miniature is the most expressive one to describe these Bluffs. They have all the irregularity in shape, and in valleys that mountains have, they have no rocks and rarely timber."
In 1832, the artist George Catlin spoke of the Loess Hills far more eloquently: "the prairie, whose enamelled plains that lie beneath me in distance soften into sweetness, like an essence; whose thousand thousand velvet-covered hills ... go tossing and leaping down with steep or graceful declivities to the river's edge, as if to grace its pictured shores ... this prairie, where Heaven sheds its purest light and lends its richest tints."
Modern-day visitors are rarely that effusive, but they too comment on the special characteristics of the Loess Hills. "They look like they don't belong here," say some; others state that they seem to have just stepped into the Appalachian Mountains. Like the earlier travelers, today's visitors seem to recognize that they have entered a land apart, far different from other landscapes they know. Suddenly, the roads cut and curve through near-vertical walls of silt. Quaint farmsteads, tucked into hollows, nestle among forests and brownish red grasslands. Steep slopes are bathed by the sun and combed by the winds, which never seem to cease. Most of today's visitors to the Loess Hills have probably grown up on farms or in cities, landscapes shaped for human use. But regardless of background or training, they sense that in the Loess Hills, nature is still in control.
The sense of specialness is well entrusted in the Loess Hills, for this landscape is one of North America's gems, possessing natural features rarely duplicated elsewhere on the planet. The most obvious of those features are the substance and shape of the hills. They consist of exceptionally large, homogeneous piles of fine-grained, cohesive quartz silt. Depths of loess deposits in general exceed 60 feet, and well drillers have reported depths of more than 200 feet. The word "loess" denotes the silt's origin. The tiny particles were light enough to be lifted into place by the wind.
Loess is not in itself a rarity. The parent material for many of the world's richest agricultural soils, loess deposits are associated with major river valleys throughout the world. Deep loess deposits characterize sections of the Rhine Valley in Germany, where the word "loess" was coined. Deep loess dominates parts of the southern Soviet Union. The Yellow River, one of China's major drainages, is named for the large quantities of loess that color the river as they wash into it from adjacent highlands. Notable North American loess deposits occur along the Missouri, lower Mississippi, and Platte rivers and in eastern Washington. Loess covers tens of thousands of square miles of the Midwest, with depths varying greatly from place to place.
All of Iowa, excluding the tongue-shaped section in the north-central part of the state that was most recently covered by glaciers, is coated with several feet or more of loess. Nearly 40 percent of the state's prime soils are loess-derived. A good-sized piece of Nebraska and nearly all of Missouri are loess-mantled. However, the Loess Hills are unique in North America, for only here is loess deep enough and extensive enough to create new landforms—to give form and substance to the land surface.
Today's corrugated, angular, intricate Loess Hills landscape is the result of running water's reshaping of the deep loess primarily during the past 12,000 years. Undisturbed deep, dry loess is highly cohesive, able to stand in near-vertical cliffs. Loess is also extremely prone to erosion. Water has produced the characteristic shapes that caught the eye of the earliest explorers. Steep-sided knobs, narrow undulating ridges with abundant side spurs, numerous interconnected drainages, gullies cutting deeply into the valleys, and small terraces called catsteps give an impression of a giant's massive but delicately sculptured carving (fig. 1).
Hidden within this deep loess is a rich collection of fossils and archaeological artifacts. In addition, the native inhabitants of the Loess Hills have to some degree been protected by the rugged loess topography. While lands on all sides have been converted to cropland, extensive areas of the Loess Hills have remained in prairie and woodland, communities that contain rich and unusual mixtures of native species. Some species are rare or endangered; many are at the fringes of their ranges of distribution.
The prairies command the greatest attention of biologists, for Loess Hills prairies contain many plants and animals typical of hot, dry areas far to the west. Yucca, tumbleweed, the Great Plains toad, cowboy's delight, and the plains pocket mouse (to name a few) reside in the Loess Hills because the well-drained loess and its steep, sun- and wind-exposed slopes create a desertlike local climate. The Hills' assemblage of western species is not found elsewhere in Iowa, Missouri, or eastern Nebraska.
The wild, untrammeled character of this landscape, its relatively large remaining prairies, and its unusual and rare species have attracted national attention in recent years. The Loess Hills have become the focus of scientific research, education, and conservation. Such efforts are not misplaced, any more than efforts to preserve Rembrandt's paintings or Bach's music would be misplaced. The Loess Hills can feed our spiritual hunger and our intellectual curiosity as richly as the highest products of human civilization would feed them. We are now coming to realize that maintenance of biological diversity and healthy natural systems, such as those in the Loess Hills, is necessary for the perpetuation of human life as well. Wisely used and managed, the Loess Hills can continue to satisfy the needs of both humans and native species indefinitely.
SCOPE AND PURPOSE OF THIS BOOK
This book is intended to be a resource for the many types of people attracted to the Loess Hills, from the reader with no special training in natural history to the educator and research scientist. Readers with an interest in one specific topic may read appropriate sections. The book may also serve as a field reference.
Three car tours and a listing of Loess Hills public use areas and educational resources lead the casual visitor to points of interest and promote a general understanding of the region's natural features. Details of natural features are explained in chapters 2 through 6. Those chapters are useful as a field reference, study guide, and armchair exploration of the region. Because the book summarizes the majority of biological, geological, and archaeological research recently completed in the Loess Hills, it also is a useful scientific reference and educational tool and can serve as a text for students in environmental and ecological fields of study. Last, the book can provide guidance and useful background information to land-use managers, land-use planners, and private landowners involved in the Loess Hills.
This book is a comprehensive natural history. A broad spectrum of information is covered, including geology, climate, native plant and animal species, and communities formed by those species. Human cultures of the last 12,000 years and their environmental effects have also been considered. Instead of looking at features individually, as separate entities, the book attempts to integrate information and explain the interrelationships of all elements—that is, to discuss the ecological systems, or ecosystems, of the Loess Hills.
Chapters are arranged chronologically, beginning with the earliest times, long before the Loess Hills were formed, and proceeding through management issues that will determine the future of the landscape and its native species. Sidebars throughout the text give in-depth information on specific points.
Discussions concentrate on features of the Loess Hills as defined in following paragraphs and outlined in map 1. The book can also be used to understand a larger surrounding area, where prairies, woodlands, and loess deposits share some similarities with the Loess Hills. In addition, numerous references are made to sites and characteristics of the Missouri River floodplain, adjacent to the Loess Hills on their western side.
Throughout the book, comparisons have been made with the bluffs in Nebraska, which border the Missouri River valley on its western side. Loess has accumulated on both sides of the Missouri River valley. However, riverside bluffs in Nebraska and Kansas are not included in the formal definition of the Loess Hills because loess deposits west of the river are thinner and less extensive than in Iowa and because they do not constitute a distinctive regional topography. As a result, the rugged, angular topography typical of the deep loess can be found only sporadically. In addition, native communities differ significantly from those in Iowa and Missouri.
DEFINING THE BOUNDARIES OF THE LOESS HILLS
The Loess Hills, as defined in this book, stretch from the town of Westfield in central Plymouth County, Iowa, to just south of the Squaw Creek National Wildlife Refuge in central Holt County, Missouri (map 1). Between those two points, the Loess Hills form a north-south band of varying width approximately 200 miles long. Deep loess landforms are best developed in a strip between 3 and 10 miles wide along the edge of the Missouri River valley. In general, the band of deep loess landforms is greatest to the north, in Harrison, Monona, Woodbury, and Plymouth counties, Iowa. It narrows to the south, in places becoming extremely thin. For much of their extent in Missouri, the Loess Hills are reduced to a sliver that barely extends east of the westernmost bluffs.
The western boundary of the Loess Hills is clearly defined and easily recognized (fig. 2). Here the Hills border the Missouri River alluvial plain, a flat lowland shaped in the past by the river's shifting waters. The boundary between the level plain and rugged Hills is razor-sharp. Elsewhere the boundary of the Loess Hills is less obvious, and its definition is somewhat arbitrary. For example, to the east, the depth and dominance of loess gradually decrease, and the Loess Hills merge with a more rolling landscape, an eroded glacial plain. Loess mantles the plain but is not deep enough to mask or reshape the basic form of the land.
Loess Hills boundaries correspond approximately with a loess depth of 64 feet or greater, as mapped by R. V. Ruhe (1969). The landform region in Iowa is that defined by J. C. Prior (1976), which was mapped primarily on the basis of geological traits. In the field and on large-scale topographic maps, she identified those high-relief areas where loess was known to be deep and where loess obviously controlled the landscape. More specifically, she mapped the Loess Hills as a region of steep, finely dissected terrain with numerous interrelated drainages, shown by irregular, closely spaced contour lines, frequently indicating isolated knobs of loess. She also considered ecological characteristics and maps of soil types, since soils forming in deep loess and on steep slopes are distinctive.
The map of Missouri's Loess Hills, also drawn by Prior for use in this book, was based on examination of much smaller scale topographic maps. In addition, it took into account the author's field identification of bedrock features, loess depth, and ecological characteristics (primarily prairie distribution). The southern terminus corresponds with the most southerly significant, characteristic dry prairies found on loess-dominated terrain that remain today. Isolated patches of loess-dominated terrain are found south of there, but they are not significant enough to be mapped at the scale utilized here.
The southern boundary of the Loess Hills is perhaps the most arbitrary, and it is more restrictive here than in other publications. In the far southern Loess Hills, the loess becomes increasingly discontinuous and the influence of bedrock increases. Rugged loess terrain can be observed south of St. Joseph, in Bluffwoods State Forest. Sporadic, isolated patches of fairly deep loess extent eastward along the Missouri River nearly to its confluence with the Mississippi River (Beveridge, 1980). R. H. Thorn and J. H. Wilson (1980) state that loess dominates the topography of the western section of their Glaciated Plains Natural Division of Missouri. Their boundaries include a large section of northwestern Missouri, extending east and south to where the original loess was only approximately 10 feet thick. They also point attention to the steep loess mounds along the Missouri River.
In the past, a southern boundary based on the location of dry loess bluff prairies may have differed because of the shrinking range of the prairies. B. F. Bush wrote in 1895 that such prairies extended to a few miles south of St. Joseph. Today small bluff prairies can still be seen in Riverbreaks State Forest, south of our southern terminus of the Loess Hills but 20 miles northwest of St. Joseph. Future studies taking all of those factors and additional fieldwork into account may redefine the boundaries of the southernmost Loess Hills.
Chapter TwoANCIENT LANDSCAPES AND COMMUNITIES
The history of the Loess Hills began when extraordinarily deep blankets of silt were first deposited in this region. Much can be told of how the silt was lifted by wind and then shaped by water; of the gigantic mammals that wandered the region's valleys and climbed its slopes; of the plant communities that migrated to and from the Hills, broad-leaved and needled trees from the north and east, grasses from the south and west. The Loess Hills' history is one of climate, inhabitants, and landscapes constantly responding to one another.
Long before the loess was blown skyward, a series of other landscapes existed in the region, landscapes far different from those of today. Their varied histories lie recorded within the fossils and rock layers that for the most part are today hidden beneath the Loess Hills, a mute testimony to slowly but constantly changing landscapes, climates, and communities. At least six times the region was smoothed by advancing glaciers, which as they receded left piles of loose debris to be carved by rivers and rain. Long before that, warm, carbonate-rich seas cyclically covered the region, leaving deposits that were compressed into bedrock over still more ancient solid rock.
Those early periods, preceding the Holocene (the last 10,000 years), are the focus of this chapter. (The standard geological terms used to outline the chronology are listed in table 1.)
EARLIEST TIMES: LAYING DOWN THE ANCIENT BEDROCK
If one were able to cut through the Loess Hills deep into the earth, the knife would slide easily through the unconsolidated earth nearly to the base of the hills and then would saw with difficulty through several horizontal layers of successively older solid rock. Eventually, the knife would dig deep enough to hit Precambrian bedrock one billion to two billion years old, a part of the same rock system that forms today's Rocky Mountains. However, although the younger layered rocks were later uplifted and then washed away in western mountainous states, they have remained as cover over much of the Midwest. Thus Precambrian rocks can be seen at ground surface in Iowa only in one place, the extreme northwestern corner of the state at Gitchee Manitou State Preserve. They also are evident aboveground farther to the north in central Minnesota, Wisconsin, and Canada.
Lying on top of that ancient rock are layers upon layers of rocks deposited at successively younger times, each under somewhat different conditions. Fossils embedded within the layers portray life from its earliest beginnings through the evolving complexity of marine and nonmarine life. If the lowest of the deeply buried layered rocks were exposed, it would tell of late Cambrian times approximately 500 million years ago, when a shallow inland sea periodically covered the Loess Hills region, leaving shoreline, inner shelf, and offshore deposits. Fossils in the sandstone and other Cambrian rocks would reveal that life then was limited to primitive forms: trilobites, algae, brachiopods, burrowing wormlike organisms.
Excerpted from FRAGILE GIANTS by Cornelia F. Mutel Copyright © 1989 by University of Iowa. Excerpted by permission of UNIVERSITY OF IOWA PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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