Sands of time: a small prairie that formed at the end of the Ice Age is an Iowa gem

Natural History, May, 2002 by Robert H. Mohlenbrock

I have taught several spring and autumn classes in the heart of Iowa, where the plants we studied inhabited wetlands, uplands, woods, and prairies, but when I first stepped onto the upper end of Marietta Sand Prairie, I knew I was in a unique place. This seventeen-acre state preserve, which lies seven miles northwest of Marshalltown, harbors a large and unusual selection of species.

The main attraction is a sand prairie, which slopes gently down from the top of a small hill. About 12,000 years ago, when the last of the great glaciers were melting, strong winds picked up sand from the nearby Iowa River valley and blew it across the countryside. Where the sand hit elevated areas such as this hill, it created deposits as much as thirty-six feet deep. The sand in the Marietta preserve overlies an ancient soil surface so hard and dense that water cannot percolate through it. Water trapped in the sand slowly drains downhill to a small creek at the preserve's eastern end, which is about fifty feet lower than the hilltop.

Once the sand was in place, sand-tolerant species colonized it to create the prairie we see today. My first visit was in early May, when the ground was covered with delicately beautiful sand love grass, punctuated by bright yellow clusters of hoary puccoon and golden alexander. When I next visited, in September, the dominant grass had become the equally lovely prairie sandreed, above which grew other yellow-flowering species.

The sand prairie occupies the western half of the preserve, where the habitat is very dry and hot. But as the land slopes downward to the east, the impact of the afternoon sun lessens and the pure sand begins to peter out, allowing the soil to retain more moisture. Exploring this zone, I passed through a wet prairie, into which intrudes a small strip of aspen woods. I then entered a shallow, two-acre basin of extremely spongy terrain. In this boglike area, where the soil is too saturated for trees and most woody plants, a wet meadow has developed. Finally, reaching the small creek at the eastern border of the preserve, I came to a riparian woods. Perhaps the most surprising plant here was the crested fern, a species rare in Iowa.

HABITATS

Sand prairie. In May, sand love grass dominates, and the conspicuous wildflowers are hoary puccoon and golden alexander. Less conspicuous species are woolly plantain, wild four-o'clock, yellow stargrass, and false toadflax. By May, prairie pussytoes, one of the earliest bloomers, is already covered with its fuzzy fruiting heads. In September, prairie sandreed grass dominates, and prominent flowers are sawtooth sunflower, western sunflower, prairie sunflower, grass-leaved goldenrod, yellow coneflower, round-headed bush clover, white sage, rattlesnake-master, smooth beard-tongue, and horsetail milkweed. Other grasses and sedges that appear during the summer: switch-grass, little blue stem, Indian grass, Schweinitz's flatsedge.

Moist prairie. The transition into the moist prairie is heralded by scattered scouring rushes. Common species include Virginia mountain mint, perfoliate boneset, and New England aster. Here and there are small clusters of the shrubby meadow willow. Other wildflowers: bottle gentian, flat-topped aster, blue vervain.

Aspen woods. Quaking aspen is the dominant tree; others are roughleaf dogwood and wild black cherry. Because the aspens are crowded together, little light penetrates them, thus limiting the growth of understory species. A small grass known as white grass, which is related to the much larger rice cut-grass, is abundant. Wildflowers: bluntleaf bedstraw, woolly blue violet, cinquefoil.

Wet meadow. Early spring brings the bright yellow flowers of marsh marigold--followed, as the seasons progress, by woolly sedge, false nettle, swamp aster, water horehound, and swamp goldenrod. Marsh fern and the vinelike arrow-leaved tearthumb (with its bristles on the stem) are also common. Other wetland wildflowers: tussock sedge, a pink St.-John's-wort, giant water dock, willow herb.

Riparian community. American elm, honey locust, and box elder line the small creek, whose waters ultimately flow into the Iowa River. Prominent shrubs are swamp dogwood and elderberry. Other plants: crested fern, white turtlehead, clearweed, several species of smartweeds.

For visitor information, contact:

Director
Marshall County Conservation Board
1302 East Olive Street
Marshalltown, Iowa 50158
(641) 754-6303
E-mail: mccb@co.marshall.ia.us

Robert H. Mohlenbrock, professor emeritus of plant biology at Southern Illinois University, Carbondale, explores the biological and geological highlights of U. S. national forests and other parklands.