4 Iowa Geology

Last fall, I purchased a Honda CTX700 motorcycle, a black beauty with a double-clutch transmission and more power than anything I’ve owned before. Admittedly, I last rode a scooter. Now I’m passing through the final stages of middle age, even feeling old some days, and I have my hottest bike ever.

Suppose at some point in the future, I, like many other riders, take off my helmet to let the wind flow through my hair, only to lay the bike down to avoid a collision. As I slide across the asphalt on my face, imagine what it looks like—the bones in my cheek and jaw protruding, the soft parts gouged out, and the flesh dragged to the lower part of my face. That is Minnesota. As the glaciers ravaged its face, they dragged the ground up rock-flesh to Iowa to weather into the great soils that now grow corn and soybeans. The gouged-out spots filled with water, forming the Land of 10,000 Lakes. And a few hard parts were left sticking up to form low mountains, including the Sawtooth Range.

The geology of Iowa is dominated by the very old and the very new, geologically speaking. (For an overview of the geologic time scale, see [here.]) Hundreds of millions of years ago, shallow oceans covered and uncovered the Midwest multiple times. From those seas, layers of sediment, much of it from the body parts of once-living organisms, accumulated on the ocean bottom, compressing and cementing into the sedimentary rocks that now underlie much of Iowa.

But when Iowa remained above sea level, as it did during the time of the dinosaurs (the Mesozoic), sediments were eroded, not deposited, and few rocks of that age or the early part of the time of the mammals (the Cenozoic) remain. Only near the end of the Age of Mammals during the Pleistocene Era did the great glacial ice sheets gouge and grind up and redistribute older rocks, forming a layer of glacial drift over much of Iowa, rock flour that weathered as the glaciers retreated into the good soils that now grow corn and beans. The last time the glaciers advanced, a tongue of ice licked the belly of Iowa, forming the Des Moines lobe.

However, here in Dubuque we are on the edge of the Driftless Area, which includes a bit of Eastern Iowa and Northwest Illinois but much of Southern Wisconsin. The glaciers missed us, and the result is some of the prettiest landscape in the Midwest. The Mississippi River stayed in its course, deepened, and formed the beautiful bluffs alongside. Streams emptying into the Mississippi cut down (incised) as it did, forming rolling hills. And the incising Mississippi exposed deposits of galena, a mineral containing lead.

When Julien Dubuque, a French fur-trapper, came to the area, he met Native Americans who used the galena for producing vi-brant body paint, its vibrancy being a property of lead paint that eventually led to its widespread use across the nation. (Unfortunately, lead also causes lots of problems, including neurological damage.) Dubuque obtained the mining rights in the area now designated as the Mines of Spain State Recreational Area.

Before the Civil War, the area near Dubuque and in southwest Wisconsin and northwest Illinois produced about three-fourths of the nation’s lead. Down by the river stands the Shottower, not the pizza place but a tall building where molten lead was poured through a sieve, dropping 35 feet or so into water, forming spheres as it fell, before being quenched in a pool of water below. The pellets were then sorted and used for lead shot—thus, the name.

In conjunction with the Mississippi River, lead formed the foundation of the economy of Dubuque, the town named after that early fur-trapper.

Through the years, mining has gradually declined, the last mine in the area closing in 1979. Meanwhile, agriculture came to dominate. Much of Iowas was covered with tall-grass prairie when the first European settlers came to the area. Most of that prairie is now gone. Instead, row crops like corn and soybeans dominate. With them come today’s main environmental problems:

soil loss: Iowa’s wealth lies in its rich soil, but much of that soil has eroded, especially in the hillier regions, such as the Driftless Area. A variety of methods can be used to reduce soil loss, such as planting grass in waterways, plowing along hill-sides rather than up and down, planting trees to slow wind, and alternating crops, both in time and space.

water pollution: Fertilizer, manure, and other nutrients entering waterways creates problems all the way into the Gulf of Mexico. Nutrients cause algae to grow rapidly. When it dies, bacteria break down the algae, in the process using up available oxygen in the water—eutrophication. In the Gulf of Mexico, the Dead Zone forms seasonally, as much as 9,000 square miles in size, with oxygen levels too low near the bottom to support sea life. See [here.]

energy costs: The costs of agricultural goods is directly related to the cost of energy—fuel for tractors, trucks, and transportation, and natural gas (a raw material for the nitrogen in fertilizer.) In addition, if people spend more of their pay-check on gasoline, less is left for groceries. Profit for farmers is cut on both ends.

Notes on Glaciers:

There have been many periods of extensive glaciation, called ice ages, throughout geologic history. We’ll talk about their causes later in the course. The most recent ice ages have occured during the time epoch geologists call the Pleistocene, beginning 2.6 million years ago and ending about 11,700 years ago. During that time, glaciers advanced and retreated multiple times, leaving behind evidence of their presence:

moraines: long, sinuous lines or dirt tens of feet high that mark the edges and end of the glaciers. They outline the last location of the Des Moines lobe. And if you drive from Dubuque toward Madison, most of the route passes through the Driftless Area, with deep stream valleys and rolling hills. But just outside Madison, in Verona, the road crosses the terminal moraine of the glaciers, and as one continues into the city, the landscape is flattened, with multiple lakes.

glacial erratics: The glaciers forming the continental ice sheets were thousands of feet thick, and they transported huge boulders, made of rock from faraway locations. When the glaciers melted, the rocks were left behind where they didn’t fit with the local geology. These boulders are called erratics.

drift: the rocks, sediment, and ground-up rock flour transported by glaciers. In Iowa, the weathering of the drift scattered across the state led to the great soils we have, essentially imported from Minnesota.

incised stream: when a stream cuts down deep into the landscape, geologists say it is incised, Because glaciers did not come to the Dubuque area, the Mississippi River stayed in it course ( not the case further downstream past Clinton) and cut down over long periods of time, aided by meltwater rushing through from retreating glaciers.

U-shaped valleys: In more mountainous areas (obviously not the Midwest) glaciers deepen and widen valleys into a broad Ushape. Valleys cut by running water have more of a V shape.