GEOLOGY 1994 Iowa Department of Natural Resources Larry J. Wilson, Director Contents

Flood of '93 Uncovers Devonian Sea Floor 2 Geological Survey Bureau 109 Trowbridge Hall Glacial-Age Floods 8 Iowa City, Iowa 52242-1319 (319) 335-1575 Ancient River Channels 12 Donald L. Koch State Geologist and Bureau Chief Minerals 74

COVER: Hogs and the Environment 20 Iowa's State Rock is the geode. Those having hollow, ciystal-lined interiors are prized by mineral collectors. The well-known "Keokuk geodes" Mapping for the Next Century 22 (this opened one showing clusters of sparkling quartz ciystals) can be found in stream beds and dug from shale outcrops along the Des Moines and Skunk rivers and their tributaries in southeast Iowa. Living w ith a River 28 See "Minerals" pp. 14-19.

Cover design by Patricia Lohmann Cover photo by Tim Kemmis

Jean C. Prior Editor Patricia J. Lohmann Publication Designer

Printed on Recycled Paper The FLOOD OF '93 lapped at the Cizy of Keokuk on Ju(y 7, 1993, as itjlooded UNCOVERS the municipal water "' pollution co11trol ~ DEVONIAN SEA FLOOR plant (foreground). ,g E .c ,u "'t:: Floodwaters from the ~ "' '------" (background) turn Interstate 80 into a THE ICE-AGE RECORD causeway connecting E. Arthur Bettis III hollows in the rock surface and remaining segments of extends upward for a few feet. The dry land in this July 12, While much attention is given to .d eposit contains a few small igneous 1993, aerial view norlh the bedrock geology and fossils seen pebbles (erratics), a clue that it accu­ ofDes Moines. along the floor of the gorge, there are mulated sometime after the first Sl"' ::, layered deposits revealed along the glaciers invaded Iowa and brought steep banks of the flood-cut channel igneous rocks from the north. The that provide a glimpse into the histo1y deposit contains no pollen; it is not of the Iowa River valley. Recognized organic-rich, even though it is dark­ by different colors and textures, these colored; and it is unlike any other deposits record major periods of Quaternary deposit in eastern Iowa. THE FLOOD accumulation over the last few hun­ The isolated "mound" in the center of dred-thousand years. the gorge is composed mostly of this The Flood of 1993 was the costli­ the U.S. Army Corps of Engineers' The oldest Quaternary deposit, material (photo p. 4, middle). Even est, most devastating flood in U.S. , an event which lasted located immediately above the bed­ the mound's existence presents a history according to the U.S. Geologi­ 28 days. The floodwaters ripped out a rock, is the most mysterious with question for many visitors. It was cal Survey. Floodwaters covered as road and campground, and scoured regard to age and origin. This dark protected from the full eroding force of many as 23 million acres of agricultural away as much as 15 feet of glacial-age brown, "greasy" sediment fills irregular the floodwaters by a pile of large rocks and urban lands in the Upper Midwest sediments to expose a remarkable for weeks. The unusual duration and expanse of 375-million-year old magnitude of this event was triggered (Devonian) fossiliferous bedrock. by a wet-weather pattern that had Thousands of visitors followed in the Cascadingjloodwaters persisted since early in the year, flood's wake. The broad horizontal gnaw away at the Lower followed by a series of intense rain­ surfaces of limestone have provided Cottonwood Campground storms in late June and July. Iowa the public with an opportunity to walk below the emergency found itself in the center of the across an ancient sea floor and to see spillway at the U.S. Army catastrophic flooding that resulted. a clearly visible picture of bygone life Co1ps q( Engineers· Among the effects of the 1993 that thrived in the tropical waters that Coralville Lake in Johnson ~ Counzy on July 7, 1993 once covered interior regions of North summer floods in Iowa was the C ;...;°" overflow of the emergency spillway at America.

2 3 Left: Looking back toward the spillway, steep bank.s along A .flood-ravaged road, once the/load-scoured channel, traveled by uisitors to Coral­ reueal layers qfglacia l-age ville Lake, ends abmpt~y i11 a deposits. Broad e:xposures of series qf bedrock ledges that fossiliferous limestone form take visitors on an unex­ the floor. Below: A stream­ pected journey into Iowa's lined mound ofglacial -age materials is all that remains geologic past. of deposits that 011 ce couererl the limestone.

THE FOSSILS that lodged just upstream during Brian J. Witzke the overflow event. A thin gravel rests on the The fossil shells and skel- unknown deposit, indicating etons of sea-dwelling animals are seen Most were fragmented into pieces by that erosion preceded accumula­ in abundance in the broad expanses of scavengers and bottom currents before tion of overlying materials. Two limestone -- the closer one looks, the fossilization, and their stem segments later episodes of stream activity more o ne sees. These fossils are the are abundant. Howeve r, some excep­ are recorded above the gravel. remains of a multitude of creatures that tional specimens at the site were The older unit, reddish-brown inhabited a warm tropical seaway buried intact, displaying their stem, silts and clays, formed sometime covering Iowa about 375 million years head, and arms as they would have during the warm (interglacial) ago, during the Devonian period. appeared in life (photos, p. 6). period between about 30,000 Most of the fossils seen are from ani­ Fossil corals are the namesake for and 200,000 years ago. These mals that inhabited the sea bottom and the City of Coralville, and it is certainly silts and clays are cut out farther filtered small food particles from the fitting that spectacular accumulatio ns down along the gorge exposures and preserved stream deposits. water. Brachiopod (clam-like) shells of fossil corals are so we ll displayed at are replaced by a still younger stream This chapter of the region's are among the most conspicuous, and the spillway. Some beds show densely deposit that accumulated during the geologic history will not remain in a great variety of species can be seen. packed concentrations of hemispheri­ coldest part of the last glacial period, view as long as the more resistant Some limestone beds are crowded cal and disc-shaped colonial corals, between 22,000 and 16,000 years ago. limestone. Slumping of the softer with their shells. Crinoids are also while horn-shaped solitary corals Tan wind-blown silt (loess) that materials will eventually cover the present. Known as "sea lilies," and abound on other surfaces. Bryozoans accumulated between 21,000 and diagnostic contacts, and vegetation w ill plant-like in their form, crinoids are ("moss animals") are smaller colonial 12,000 years ago, when the Iowa River further obscure the slopes -- perennial actually animals related to starfish. organisms that are seen as small twig- carried glacial meltwater from north­ problems for those who study Iowa's central Iowa, mantles both of these Ice-Age record.

The beds ofDevonian limestone show evidence offaulting The break along displaced layers of rock, seen here as A flood-rafted slab of limestone a step, is marked by "slickensides, " displays interesting patterns caused polished, grooved surfaces caused by by the past flow ofgroundwater the friction ofrock movement in the along numerous verticalfractu.res geologic past. present in the bedrock.

Non-credited photos on pp. 4-5 by Ray Anderson

4 5 Exceptional specimens of crinoid fossils L11/11er Ostedgd11rd t:1 IDE VISITORS were found. b-11et:1r old visiti11g witlt 7bough plant-like Bill J. Bunker in form, crinoids lter qir! 3co11t troo1, ct:11/t are actually Exposures of Devonian limestone get over /tow old tlte fos ­ animals related in eastern Iowa are usually limited to sils are. to starfish. Th ey vertical quarry faces, roadcuts, and lived attached to stream cutbanks . ·n1e new spillway ?t's weird · site St:llfS. the sea floor by e},.rposures provide a rare opportunity long flexible YOH sit 011 tltese roctfs stems. Their to see broad ho rizontal surfaces, many t1t!d tlt/11! titer re older tltt111 arms, radiating containing spectacularly abundant ou twar"d from the marine fossils. Although the exposures 1111117t1re11ts -- or eve11 111tf head, moved with are of great interest to Iowa geologists, grt1t!d1711re11ts. • the currents and public inte rest in the rocks, fossils, and Copyright I 993. USA TODAY Reprinted w11h p

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Iowa's valley landscapes were scape. Meltwater floods developed in formed through the action of flowing three different ways: normal seasonal water. Iowans experienced this melting of glacial ice; episodes of geologic process firsthand during the -extreme melting triggered by unusually soggy summer of 1993 when they warm or rainy periods; and sudden witnessed some of the worst flooding bursts of tempo rary glacial lakes. on record. Although the Flood of '93 These events introduced pulses of was unparalleled in its impact on meltwater that established the major Iowa's people and economy, its valleys in the upper part of the geologic and hydrologic impacts pale present-day Des Moines, Boone, Iowa, in comparison to those associat~d with Little Sioux, Big Sioux, Raccoon, glacial meltwater flooding that oc­ Skunk, and basins. curred between 13,500 and 12,000 Outside the margin of the Des Moines years ago in valleys draining the Des Lobe (note map, p. 20), where pre­ Moines Lobe of north-central Iowa. existing valleys weren't covered by the The Des Moines Lobe glacier glacier, meltwater floods eroded valley advanced rapidly into Iowa about walls, deepened some valleys, and 15,000 years ago during a period of filled others with sand and gravel. climatic warming. The initial advance What is the physical evidence of of the glacier was followed by ice glacial meltwater fl oods in Iowa's Between 20,000 and 9,500 yea1-s ago, the Mississippi River valley served as a conduit for stagnation, then by at least three more valleys? Affected river valleys display glacial meltwater, ou twash sediment, and giant floods from the sudden drainage of large rapid advances and longer stagnation several features that can be traced glacial lakes in the headwater areas. The valley segment shoum in this April 1984 color­ phases. While the Des Moines Lobe back to the ir heritage of glacial floods. infrared photo is northeast of Burlington, Iowa, with Oquawka, Illinois in lower center. was in Iowa, voluminous amounts of These include multiple terrace levels meltwater were released from on top (former floodplains) capped with sand of, within, and under the glacier into and coarse gravel deposits, many short glacial flooding, much like the Devo­ respective reservoirs in 1993. outwash streams that flowed from the steep tributaries and few long branch­ nian Fossil Gorge at Coralville Lake Multiple terrace levels in Iowa's ice margins. In many instances, large ing ones, and relatively deep gorges and the Saylorville Gorge in the valleys record many episodes of meltwater floods filled the valleys and with many sandy terrace levels. Such emergency spillway of Saylorville Dam powerful glacial-flood-induced had dramatic impacts on the land- features formed rapidly during major formed during overflow of their downcutting that carved deep valleys.

8 9 In Boone County for example, the Des channel, are remnants of a terrace built This meltwater lake Moines River valley was cut about 220 by meltwater floods and marked by and its bordering feet deep during floods of the late the dimpled patterns of sand dunes coniferous trees are glacial period, forming the picturesque and, west of the channel, sweeping situated on an ice­ gorge characte ristic of this part of the braided patterns marking somewhat cored moraine of valley. Another of Iowa's scenic younger flow paths. (Note barge in the Klutlan glacier in the Yukon valleys, the deep and narrow Little middle of channel for scale.) These Territo,y, in a scene Sioux River valley between Gillett features formed as glacial meltwater resembling the Grove and Peterson, formed when the and lake-burst floods forced the river stagnant margins of Des Moines Lobe ice front blocked the to different levels and positions during the Des Moines Lobe eastern flow of rivers such as the glacial retreat in , Wisconsin, in no,th-central Ocheyedan, forming Glacial Lake and Illinois (see also, Mississippi Valley Iowa 13,500 to Spencer which then overflowed and satellite image and geologic map on 12,000 years ago. drained, swiftly cutting a new course to pp. 24-25, and 26-27). the west. The layering of sand and gravel in The general absence of long, terrace deposits within rowa's valleys branching tributaries in many glacial­ speaks to the great variations in flood-influenced valleys indicates that discharge during sediment accumula­ the valleys formed quickly, with little tion. Attempts to estimate the flow time for development of an extensive conditions that accompanied deposi­ tributary network. The aerial photo­ tion of the sand and gravel are based graph on page 9, of a portion of the on the layering and grain size of the Further information about glacial­ In contrast to most modern floods, broad Mississippi Valley, shows numer­ deposits. Estimates of ancient hydro­ age floods is found in comparing the whose impacts are often damaging, the ous steep, short, tributary valleys (dark­ logic conditions indicate that many thickness and properties of sand and impacts of Iowa's glacial floods have green branching patterns) along the glacial flood events far exceeded the gravel in river valleys that were close been valuable to us. Most sand and upland margins in Illinois (on the largest floods ever measured in the to the former ice front "vith deposits gravel aggregate, one of Iowa's largest right). Also seen, east of the river recorded history of these streams. more distant from the glacier. Sand mineral industries, is obtained from and gravel in valleys near the glacial deposits laid clown during glacial sources tends to be relatively thin -- on floods. The size, composition, and the o rder or 10 to 20 reel, relatively thickness of these deposits, which are coarse, and deposited in channels that important to the economics of produc­ River valleys that shifted rapidly across the late-glacial tion, varies along valleys primarily drained the Des Moines floodplain (see photo, left). On the because of variations in flow condi­ Lobe ice front in Iowa other hand, in valleys distant from the tions during glacial meltwater floods. may have looked similar to this glacier front, such as the Mississippi in These deposits are also important floodplain ofa modern Iowa, the late-glacial sand and gravel sources of groundwater for wells, outwash stream flowing is much thicker -- over 150 feet in serving many of Iowa's urban centers from the margins of the portions of the Mississippi valley which were built along river valleys. Klutlan glacier in the shown on p. 9, finer grained, and Finally, the scenic valleys that were Yukon Territ01y of deposited in braided channels that carved by these great floods are an Canada. looked somewhat like the Platte River integral part of Iowa's landscape channel in Nebraska today. diversity. •:•

10 11 The scenic and recreational Iowa, Missouri, Nebraska, Kansas and attractions along Iowa's rivers are Oklahoma. These sandstones typically unique to the valleys in which they form elongate, ribbon-shaped sand flow. Geologically, these valleys are bodies that were deposited in sinuous the result of flowing water eroding soft channels of ancient rivers and deltas. glacial-age materials or older, more The sandstone deposits are usually resistant bedrock materials. Along oriented northeast to southwest and steeper sides of the valleys, one are usually less than one mile wide. frequently sees thin soils mantling Their thickness may exceed 100 feet. prominent rock bluffs. These bluffs These ancient river channels mean­ often are composed of limestones and dered across Iowa as part of a vast dolostones, which are the biological equatorial ecosystem of lowland rivers and chemical products of tropical and coastal deltas. They drained the marine seaways that covered vast interior of North America from the portions of North America between 65 Appalachian Mountains in the east to and 550 million years ago. At other the edge of a large seaway to the localities, however, river clowncutting southwest. During this time the has exposed massive vertical bluffs of coastal margin migrated back and forth sandstone, sometimes forming steep­ between Oklahoma and Illinois, and walled box canyons. the river and delta systems deposited Scenic examples of such bluffs and clay, mud, peat and channel sands on canyon-forming sandstones are found the land mass between the mountains along several stretches of the Des and the sea. In the course of geologic Moines valley, the Iowa and Raccoon time, these sediments were compacted river valleys, and a few smaller stream and cemented into the familiar sand­ valleys in eastern and southern Iowa. stone, shale, and coal beds seen today. Many of these picturesque sandstones Iowa's modern rivers, which had have been included in Iowa's state their origins during the relatively parks, preserves, or county conserva­ recent glacial history of the continent, tion lands because of their exceptional have removed the covering of glacial scenic and botanical qualities. Good deposits and exposed the ancient examples include Wildcat Den State channel sandstones to view. These Park in Muscatine County (photo, left); rock formations are valuable to Iowans Dolliver, Ledges, and Elk Rock state as local sources of groundwater for parks in Webster, Boone, and Marion wells, and also as cool, moist, forested counties; Woodman Hollow State retreats in our state and county parks. Preserve in Webster County; and Cedar During historic settlement of Iowa, Bluffs Natural Area in Mahaska County. many of these channel sandstone The majority of these bluff-forming deposits were quarried for building sandstones are 300-million-year old and foundation stone. Perhaps the (Pennsylvanian-age) deposits that are best known examples of this use are part of the Cherokee Group, a major the sandstone buildings of the Amana rock sequence throughout much of Colonies. •:•

13 MINERAL~ J Jean Cutler P rior "Dog-tooth spar" is the name giuen Minerals are the building blocks of to sharpzy pointed the Earth's rocks. They have a specific c1ystals qfwhile calci te as seen on che mical composition and a characteris­ this massi/le piece tic crystal form. The Iowa minerals ofgray limestone Pointed c,ystals of white calcite and translucent ye/loll' shown here display an appealing range from Mahaska cubes offluorit e line the edges of this gray limestone of color and shape. collected near Postville in Allamakee County. Calcite Cou n~y. A /so (calcium carbonate, or lime) is the p11mary mineral in Ma ny people are introduced to the prominent are brass-colored limestone, while./luorite is rare. Such crystal growths field of geology through the fun of are found along open spaces (vugs orfracture traces) searching for and collecting minerals. masses of p y rite within the ,·ock. Beautiful varieties can be found in Iowa's c,ystals, known sedimentary rock strata, outcropping in as 1ools gold." road cuts, quaJTies, strip mines, and along stream banks or valley sides. Striking crystals make up many of the coarse­ g ra ined igneous and metamorphic cobbles and boulders that lie in pastures and farm fields where they were left by melting glaciers. Gravel pits along Iowa's This wind-polished and vall eys and the gravel bars within river lichen-covered rock qf channels are also good places to fi nd a Sioux Quartzite is wide assortment of mineral specimens. Geodes haue drab. ro1111ded from Lyon Co11n~y in In addition to their crystalline beauty, the northwest corner qf exteriors witb a information about a mineral's geologic hard 011/er layer Iowa. Quartzite is age and origins can be obtained from composed qf com­ and partial~v its chemical isotopes and from .its asso­ pacted quartz grains hollow inten·ors solidly cemented ciation w ith other minerals. Mineral re­ lined with in­ together with silic a , sources play a significant role in our daily ward(y projecting giving the rock a glassy lives, and Iowa's mineral ind ustries are mineral c,ystals. appearance and a very valuable to the state's econo my. This large geode, hard sw.face. Its containing pink and gray quartz crystals. was collected near resistance to weather­ KeokukJi·om the Warsaw Sbale, a rock formation that outcrops ing ma kes it useful as along stream beds in lowa 's southeastern counties. highway and railroad aggregate. \

14 15 This pyramid crystal q( translucent calcite is from Mahaska Coun~y. Calcite is the principal mineral in limestone, chalk, and marble. It occurs in a varie~y ofcolors and bubbles vigorous~y when a drop q( dilute hydrochloric acid is applied.

Galena has a distinct metallic-gray luster and a cube-shaped crystal form. It is very heavy and is the principal ore of lead . This mass of crystals is from Barile is an Dubuque County, where lead ores were mined for unusually heavy ouer 300 years from veins in the dolomite bedrock. mineral. This sample from Fayette County is composed ofrounded masses of The tall, slender c1ystal of gypsum, radiating crystals. a varie(y known as sele11ite, is from Barile is used Appanoose County. It has a so.ft, primarily as an additive in drilling easi~y scratched surface. A related s11lfate mineral, also formed by muds and paints. evaporation from seawate1; is called a nhy drite (lower, banded rock). Gypsum is mined in Webster and Des Moines counties/or wallboard I production. Heavy nuggets I of the mineral I copper , a good I conductor of I heat and I Limo11ite is a electrici~y, are distinctively found on rare yellowish brown occasions in ore of iro11 . It Coal is a combustible rock, rich in carbo n I Iowa's glacial and.formed by compaction of(ossil plant I takes many deposits. Ibis remains similar to peat. Thin veins in this 1 forms, including 67-pounder, ' the cellular piece are filled with pyrite, an abundant tarnished with st111cture seen in ore of s ulfur. Coal was mined from seams greenish oxides, in the Pennsylvanian-age rocks of south­ this sample from probabry ori­ central Iowa, with peak production the historic Iron ginated in the during the early 1900s. l Hill area near Lake Superior ..l Waukon in area of Allamakee Michiga n 's Coun~y. Upper 17 16 Peninsula. 1 Fragments of Feldspar is a widespread milleral petrified wood especialfcy co111mo11 i11 igneous rocks can be picked up such as granite. This blocky.fragment from glacial-age ofcrystalline .feldspar was .found in gravels along Iowa's gravel deposits along the Cedar River rivers. This water­ in southeastern Linn County. It worn piece.from the probablv weathered out ofa granite Cedar Ri11er in Linn boulder carried into Iowa by a Cou n~y shows that glacier. silica . in the .form of chalcedony or opal, completely replaced the original ;\'on ncdite

Grains ofthe mineral glauconite can give sandstone a distinctive greenish color. Glauconite is.found in marine sedimentary rocks, and it 111dicates a slow rate of sediment accumulation. ]bis glauconitic Known to mineral co/lecto,:, sa11dstone outcrops in Allamakee as "rice agate, " these Cou n~y, along the polished stones of black and at Lansing. chert (/lint) consist ofa dense variezy of silica found in the sedimentmy rocks o.f Montgome,y Coun(y. These agates The "rice" pattern comes ( varieties ofdense but from numerous white shells translucent quartz, of.fossil /usu linids, a tiny chalcedony, and marine protozoan. opal) are .from Mississippi River gravel dep osits in Clayton Coun ty and have been tumbled to a high polish. They include the prized Lake Superior 18 agates, known for their.fine, alternating bands of rich 19 colors. Hogs In response, the Geological depths over 100 feet. Earlier investiga­ Survey Bureau is monitoring shallow tions have shown a greater potential and the Environm.ent groundwater quality at four manure­ for contaminants to move through storage lagoons at hog confinement fractures in glacial till than previously operations in d ifferent landform believed. Karst (sinkhole) areas in this Deborah J. Quade regio ns or geologic settings across regio n, having little glacial material Iowa. These four lagoons are con­ covering the bedrock, also are suscep­ Lynette S. Seigley structed of local earthen materials. If a tible to groundwater contamination. lagoon leaks, the sequence of underly­ The Southern Iowa Drift Plain is ing geologic deposits may either represented by a site in Unio n County. enhance or limit the movement of This region contains steeply rolling contaminants to shallow groundwater topography with moderate deposits of supplies. Limited funding for the loess mantling weathered and frac­ project and difficulty in finding coop­ tured glacial tills, similar to those of erating owners restricted sampling to the Iowan Surface. The majority of the sites shown (left). private water wells in this region rely The site in Hancock County is on shallow "seepage" wells (30 to 50 located on the Des Moines Lobe, a feet deep) that are completed in loess region blanketed by clay-rich glacial or at the interface between the loess till ranging from 50 to 150 feet thick. and glacial till, making them vulner­ The general absence of deep vertical able to contamination from surface fractures in this young, unweathered sources. material sho uld limit the movement of The Pottawattamie County site is contaminants to wells drilled into the located in the Loess Hills, alo ng a underlying bedrock. Other wells, small valley underlain by silty alluvial however, are finished in shallow, (floodplain) deposits derived from porous sand and gravel zones within loess. The region is characterized by the glacial clays. Significant sand and steep slopes and thick loess blanketing gravel deposits also occur beneath the olde r, weathered and fractured glacial knobs, ridges, and depressions in more tills. Many private water wells in this Land.form Regions of Iowa hummocky areas (moraines) of the region are less than 50 feet deep and and Distribution of Rog Confinement Lagoons region. These areas have greater utilize the seepage-well design seen in potential for leakage to shallow the Southern Iowa Drift Plain. • monitored sites a ll lagoons groundwater supplies. Monitoring wells at each earthen­ The Linn County site is located on lagoon site are sampled monthly for Iowa is the national leader in hog production, the Iowan Erosion Surface. This changes in water chemistry that will with 24 million marketed in 1993. This number has region consists of much o lder, weath­ indicate w hether animal waste is ered, and deeply fractured glacial tills leaking from the lagoon into shallow increased over 20% since 1970, while the number that are mantled by a variable thick­ groundwater. Results of this study will of farms raising hogs has decreased by 60%. The ness of loess (windblown silt). Depth indicate the effectiveness of earthen increased concentration of hogs per farm signals a to bedrock ranges from O to 400 feet. lagoons in different geologic and move to larger scale confinement operations. Result­ The majority of private water wells are hydrogeologic settings in Iowa. •:• ing concerns focus on social, economic, and envi­ drilled into fractured limestone to roDmental issues, especially odor and potential gro•.mdwater contamination. 21 rangles in southern Muscatine and maps derived from geologic maps and northern Louisa counties. Tools accompanying data bases. Features MAPPING unavailable in the earlier years of such as water wells and core holes traditional geologic mapping were may be selected for display based on applied. These include satellite images their depth, construction, capacity, or fOR THE to assist with mapping boundaries their use -- whatever is recorded in the between different geologic deposits, database about them. Similarly, NEXT CENTURY: and computer technology to convert streams, sinkholes, geologic contacts, maps to digital databases, which then or faults could be selected by a can be transferred in electronic form, database criterion. Lakes, sand dunes, Satellite images printed as colored maps, combined or bedrock units may also be selected and computer technology with other geographic information, based on attributes that describe them, and be easily updated. such as water quality, thickness, or add new dimensions Geologic maps (of Iowa) display permeability. The result is that maps the location and distribution of various may be constructed for one general to detailed geologic rock types, faults, and sediments purpose but can be converted quickly deposited by marine seaways, glaciers, into another, more specialized purpose mapping of Iowa wind, streams, and hillslope processes. if appropriate attributes are available in They also convey information about a database. the three-dimensional geometry of The completed Letts and Greg A. Ludvigson these deposits and their relative age Blanchard Island quadrangles encom­ E. Arthur Bettis III relationships. Such qualities make pass about 110 square miles of the Bernard E. Hoyer geologic maps valuable to scientists in understanding the Earth's composition Passage of the National Geologic and structure, GOAL: Informed decisions on future environ· Mapping Act of 1992 provided a processes and mental and natural resource issues. stimulus for detailed geologic mapping history. They also of the U.S. directed towards the are valuable to resolution of environmental problems. society, which lives This decade-long mapping program is and depends on geologic materials. Mississippi Vall ey and adjoining administered by the U.S. Geological People need answers to questions uplands south of Muscatine in eastern Survey and includes a "STATEMAP" such as the best place to locate a Iowa and adjacent western Illinois. component which offers financial landfill, the extent of groundwater The goal of the project was to map support for geologic mapping to state contamination problems, sources of geologic materials to a depth of five geological surveys through a competi­ road-building aggregate, and geologic meters (about 18 ft) at 1 :24,000 scale tive grant process. During the first hazards affecting subdivision develop­ (1 in = 2,000 ft) in order to provide year of the program in 1993-94, the ment. baseline geologic information for a Geological Survey Bureau (GSB) Geographic information systems host of environmental and resource mapped a segment of the Mississippi (GIS) combine computer mapping and issues. The mapped area includes River valley covered by the Blanchard assorted databases. This technology Muscatine Island, a portion of the Island and Letts 7.5 minute quad- can be used to prepare customized Mississippi Va lley under competitive

23 SATELLITE IMAGE: Taken on May 27, 1989, this color-infrared Landsat TM image of the Mississippi River valley shows the distribution of vegetation (red), bare soil (light blue-greens), and wet soil (dark blue· greens). Sandy, beHer drained materials (purple) and open water (black) also can be distinguished. These patterns help to differen­ tiate deposits representing thousands of years of floodplain history. Circular features are center-pivot irrigation plots. This image was used to confirm and adjust geologic contacts mapped on pp. 26-27.

pressures from agriculture and industiy for land and groundwater. A portion of the Upper Mississippi River naviga­ tion system and several wildlife refuges and game management areas also occur in this area and present a series of contrasting resource manage­ ment issues. Land degradation from soil erosion and headward advance of Mississippi tributa1y valleys, as well as landfill ~icing are important issues on the upland. Several sources of subsurface information, including water well on high-altitude air photos and satellite deposits in the Mississippi Valley, to-day activities in this area and can records, engineering boring records imageiy to formulate and draw the while the bordering uplands contain give planners a sound base of geologic obtained from public utilities and the map units. Thirty-two map units, each mainly two units of loess-mantled information from which to make Iowa Dept. of Transportation, monitor­ depicting a unique succession of glacial drift that are dissected by resource decisions. ing well records of the U.S. Geological geologic materials to a depth of five branching units of younger stream In July 1994, the GSB began work Survey, borings made by the GSB, and meters, were developed. deposits along drainage ways. The on its second STATEMAP project, with published soil surveys, were used to The pattern of map units (p. 26- map and related cross-sections (not the inauguration of a three-year construct the geologic maps. This data 27) shows many elliptical, smooth­ shown) provide a detailed view of the program to map the shallow geology was compared to landscape patterns edged units that characterize river geologic materials that most affect day- of Linn County. In compliance with

24 25 COMPUTERIZED IMPS: Geologic maps of the LeHs and Blanchard Island quadrangles (along the Mississippi Valley) are combined in this graphic and are the first detailed mapping of the shallow (upper 18 ft) geologic ma· terials in Iowa done under the STATEA\AP program. Two major groups of (Quaternary) deposits are present ·· those left by glacial ice and wind com· pose the uplands, and those left by streams occupy the valleys. The information is available in eledronic or paper form and can be updated as more subsurface information is acqui red.

national program directives, the GSB assembled a statewide geologic mapping advisory panel that consisted of individuals from government, academia, professional societies, engineering firms, and mineral produc­ ing firms. This panel, representing potential users of geologic information, selected Linn County as the area where surficial geologic mapping could be most usefully applied to recognized environmental problems. During the 1994-95 project year, the Cedar Rapids North and Marion 7.5 minute quad­ rangles are being mapped, and the addition to the federal award, financial compiled using GIS technology and information. GIS techniques provide bedrock geology of the entire county support for the mapping has been will be stored in DNR's Geographic an effective means to develop specific is being update... -\. Specific environ­ provided by the Linn County Engineer­ Information System Library, procedures information, tailored to specific needs, mental problems being addressed in ing, Planning and Zoning, Regional that will facilitate digital access for in a timely manner. The use of this these quadrangles include drainage Planning, and Solid Waste agencies, as many users and provide flexible use of technology enables geologists to and groundwater contamination, well as the cities of Cedar Rapids, the geologic mapping for various develop map information in ways that suburban expansion in areas of Hiawatha, and Marion, all of whom are applied purposes in the future. better assist society in understanding sinkholes (karst), and long-range plans anticipated end users of the map Today there is a recognized need and resolving its environmental and for the county sanitary landfill. In information. All maps are being for more rapid access to more detailed resource problems. •:•

26 27 Living WJtft t1 ~/ver .

Among Iowa's geologic deposits are sand~·tone beds recording the position ofancient river channel'> that flooded Iowa lowlands millions ofyears ago. Many large river valleys existing today are partial~yfilled with sands and gravels left by surging .floodwaters released by glacial melting thousand<; ofyears ago. The energy offlooding rivers is re.,ponsible for shaping Iowa's valleys, the feat1 ires of their bottom/ands, and the distribution oftheir soils. While rivers and their floodplains have been around for a long time, people are relatively new to the landscape and tend to be rather short-sighted when it comes to sharing the Earth 's surface with on-going geologic processes. Iowans live with numerous rivers, and most of the time it is a peaceful coexistence. The Hood

kt111 C11tler Prior Editor

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