Glacial Boulders in Iowa
by Raymond R. Anderson and Jean Cutler Prior
Photo by Pat Lohmann.
"Peculiar," "irregular," and "uncommon," are words used to describe one class of Iowa rocks -- glacial boulders or "erratics." Geologists define erratics as stones or boulders that have been carried from their place of origin by a glacier and then left stranded by melting ice on bedrock of a different composition. In Iowa, glacial erratics are commonly observed where glacial deposits occur at the land surface, primarily in the north-central and northeastern parts of the state. In western and southern Iowa, erratics generally lie buried beneath wind-deposited silts (loess) that cover the glacial materials. In these areas, erratics generally are restricted to valleys, where streams have eroded through the loess and into the underlying glacial deposits.
The erratics seen in north-central Iowa are the most recent to arrive in the state. They are found on the Des Moines Lobe, the region last covered by glacial ice 14,000 years ago. The ice sheet entered Iowa from Minnesota and moved southward between what is now Mason City and Spencer, advancing as far as the capital city of Des Moines. This ice melted away about 12,500 years ago. Northeastern Iowa also has a significant concentration of boulders across the landscape, and the greatest number of exceptionally large erratics. This region, known as the Iowan Surface, was once much like southern Iowa, with loess deposits mantling steeply rolling terrain composed of glacial materials deposited in Iowa over 500,000 years ago. About 20,000 years ago, extremely cold climatic conditions led to erosional beveling of this area and removal of much of the finer-grained glacial materials, thus concentrating the larger pebbles and boulders at the land surface.
When these areas of the state were settled, farmers were forced to clear fields of the rock obstacles in order to plow and cultivate. Many of the erratics were used to build fences and foundations, while others were just piled along fence rows or into unused field corners where they are seen today. Clearing farm fields of glacial erratics is a necessary and frequent chore wherever glacial deposits are cultivated. Over time, seasonal freezes and thaws work these rocks upward from below the plow zone to the land surface. Smaller glacial erratics can be hauled out of the fields; larger ones are frequently blasted apart by dynamite and the pieces hauled away; while some of the largest are just left in place and avoided. At the municipal park in Nora Springs (Floyd County), an adjoining city street actually narrows to accommodate an erratic protruding into the right-of-way. Glacial erratics in Iowa are not difficult to identify. The vast majority are igneous or metamorphic rocks, rather than the usual sedimentary rocks of sandstone, limestone, dolomite, and shale that constitute the bedrock under most of Iowa. If you pick up a granite rock, composed of interlocking crystals of pink feldspar and glassy quartz, you can be sure it is not native and that it came from outside the state, most likely carried by glacial ice.
Most glacial erratics appear worn and rounded, and sometimes include beveled or faceted surfaces. During the course of their journey, the rocks were jostled against other erratics or scraped against the underlying bedrock, rounding off corners and planing smooth surfaces, eventually producing their characteristic appearance. Glacial transport also caused some boulders to fracture, producing fresh angular edges. Rocks carried by rivers also undergo abrasion and become rounded in the process. In fact, most of the igneous and metamorphic rocks in Iowa's river valleys were originally transported into the general area by glaciers, then eroded from the glacial deposits and moved some additional distance by a river.
Transportation by glacial ice, however, produces some other features unique to this mode of travel. The most easily observed of these tell-tale signs are glacial striations, a series of parallel lines or fine grooves gouged across the beveled faces of erratics or inscribed on the underlying bedrock surface.
Photo by Holmes Semken.
These glacial furrows are produced when an erratic, frozen firmly in the slowly moving glacial ice, grinds against another erratic or against the bedrock surface over which the glacier is moving. Glacial grooves and striations can be used to identify the direction of ice movement.
The composition of glacial erratics can often lead to identification of their point of origin and thereby provide some specific information about the direction of ice movement. At times, a string of erratics of similar composition can be observed across a broad region. These are referred to as "boulder trains," and are defined as a series of erratics that have come from the same bedrock source, usually with some special characteristic that makes it easy to recognize their common origin. Boulder trains appear as long lines or fans of erratics extending outward from their source in the direction of ice flow. Erratics from a given area are nearly always more numerous near their source and diminish in number with distance from the source.
A large-scale example of a boulder train that can be observed in Iowa is the distribution of glacial erratics composed of the distinctive Sioux Quartzite, a very hard, uniformly pink rock. Outcrops of this Precambrian-age, quartz-rich rock occur in the extreme northwest corner of Iowa and on across the border into southwest Minnesota, where they span an area from the town of New Ulm, westward to Mitchell, South Dakota. Since the glacial ice that moved through northwestern and north-central Iowa traveled generally southward, erratics of Sioux Quartzite are most common in the area west and southwest of Estherville (south of New Ulm, Minnesota) and are generally absent east of there. One of the largest examples of a Sioux Quartzite erratic is known as Pilot Rock and is seen about three miles south of Cherokee, perched above the eastern edge of the Little Sioux River valley -- doubtless an important landmark for early travelers through the area. Another example of a boulder train involves rare diamonds recovered from some Midwestern glacial deposits. Attempts to trace them back to a northern source area, however, have been unsuccessful.
Glacial erratics are known to have been transported great distances by the Pleistocene-age ice sheets that covered the midcontinent region. An erratic composed of solid native copper, and probably originating from the Lake Superior region along the Upper Peninsula of Michigan, was recovered from glacial deposits in southern Illinois, more than 600 miles from its source. Native copper erratics also have been recovered in Iowa, a journey of about 500 miles. One such copper erratic weighing 67 pounds is on display in the "Minerals of Iowa" exhibit in Trowbridge Hall on The University of Iowa campus in Iowa City (see photo, below).
This 67-pound nugget of native copper, tinged with greenish oxides, is one of the most distinctive glacial erratics found in Iowa and probably originated from the lake Superior area along the Upper Peninsula of Michigan. (Specimen is 15 inches long and 9 inches wide.)
Photo by Paul VanDorpe.
Glacial erratics range in size from pebbles to giant boulders. The greatest number of giant erratics are seen on the Iowan Surface of northeastern Iowa. They were described in a 1970 Iowa Academy of Science article by Drake University professors Richard Dirks and Carl Busch, who noted that 80 percent of the giant boulders had a similar composition, a light-colored, coarse-grained granite. They concluded from the boulders' composition and the direction of glacial striations on the underlying bedrock surface that these erratics probably originated in central and west-central Minnesota.
In another Iowa Academy of Science article in 1961, geologist Charles Gwynne of Iowa State University described the fate of a large Black Hawk County erratic near Waterloo. It originally measured 30 feet long, by 20 feet wide, by 27 feet high and was broken up in 1891; the pieces were used to construct the Boulder Church which housed the congregation of the First Presbyterian Church in Waterloo. This building was used as late as 1961 by the Salvation Army. The 1916 Annual Report of the Iowa Geological Survey described a boulder in Floyd County, about three miles west of Nashua, as the largest erratic remaining in Iowa. Its dimensions then were 50 feet long by 40 feet wide by 11.5 feet above the ground, with a nearby fragment measuring 17 feet by 7 feet by 1.5 feet apparently broken from the larger rock. In 1961, this same erratic was listed as being 40 feet by 30 feet by 12 feet. Other large erratics that can still be seen are St. Peter's Rock four miles southeast of Alta Vista in Chickasaw County, a granite specimen five miles west of Cedar Falls in Grundy County, and a granite boulder in Grammer Grove Park in Marshall County. Glacial erratics are an easily observed piece of Iowa's geological history. Each one has a story to tell about its original composition, its point of origin, its journey to Iowa, and its final resting place. Find some for yourself and see what they tell you.
A large, weathered and rounded boulder of granite in this circa 1900 photograph of a Mason City neighborhood is a monument to the massive glacier that brought it south over 500,000 years ago. The nearest bedrock source of this erratic is central Minnesota.
Photo courtesy of The University of Iowa Calvin Collection.
Adapted from Iowa Geology 1990, No. 15, Iowa Department of Natural Resources