When someone mentions the geology of Iowa, you probably don't even give it a second thought. There are no spectacular mountain ranges or deserts, no volcanoes or geothermal occurrences, so how can the geology possibly be interesting? The processes and non-processes that contributed to the creation of the landforms of what is now the state of Iowa are varied and fascinating. Those of us who live here and are familiar with that geology know that it is much deeper than it appears (pun intended).

Life has inhabited the earth for over 3 billion years. The fossils that occur throughout the Driftless Area are the remains of those creatures which lived during the Paleozoic Era, beginning about 600 million years ago. The Paleozoic Era lasted 375 million years, until the beginning of the Permian Period 225 million years ago.

The lifeforms in the seas that covered what is now northeast Iowa were vastly different from those of today. Some of the fossils found in the Paleozoic deposits of Allamakee, Clayton and Fayette counties are trilobites, cephalopods, gastropods, brachiopods, bryozoans, echinoderms, graptolites, stromatoporoids, nautiloids and corals.

The study of invertebrate paleontology may not first appear as exciting as the study of the dinosaurs. However, once you begin to appreciate the accessibility and variety of the fossils occurring in the driftless area, you begin to understand the fascinating diversity of life which made up the Paleozoic Era. And you might as well face it; without these simple (and sometimes complex and bizarre) invertebrates, there would not have been any dinosaurs - indeed, there would not even be man.

During the Pleistocene, it is believed there were eleven separate advances and retreats of the continental ice sheets. In North America, the glaciers extended as far south as the Ohio and Missouri Rivers and actually shaped the general courses of those rivers.

Although there are still theories being proposed and discoveries being made, continually defining and refining the precise glacial history of this area, there is one thing that scientists have known and generally agreed upon for many years. The area centered on the mouth of the Wisconsin River at the Mississippi River and extending to the approximate limits of the map above is unique in the glacial history of North America. We owe the formation of this beautiful and rugged landscape to what was basically a "non-process"; the area was not formed by the direct action of the most recent glacier, rather it was formed by the indirect action of the Pleistocene glaciers. The last glacial ice only skirted this area, so it deposited little or no drift or till. Hence the term "driftless" area.

The four named advances or stages are:
(1) Nebraskan - 800,000 [±300,000] years ago, {yellow and green}
(2) Kansan - 600,000 [±200,000] years ago, {blue}
(3) Illinoian - Possibly from 250,000 to 135,000 years ago) {mauve and green}
(4) Wisconsinan - prob. lasting from 100,000 to 5,000 years ago) {speckled white}

Some scientists do contend some of the earlier advances and retreats during the Pleistocene did cover portions of the driftless area, especially those areas west of the Mississippi River. Even some of the western hills here in Allamakee County show some deposits of drift material (fragments of crystalline rock scraped from the bedrock of northern Minnesota, Wisconsin and Canada) from the Nebraskan (the first glacial advance of the Pleistocene). In fact, it was the erosional process of the pre-Illinoian glaciers on the landscape that shaped the Paleozoic bedrock we see exposed in the driftless area today.

The shallow sedimentary bedrock so prominent in the driftless area was laid down as sediment on the sea floor of the Cambrian, Ordovician, Silurian and Devonian ages some 300 to 550 million years ago. In the course of geological time, those sediments hardened into brittle rock strata that were later deformed by the earth's crustal movements. Most of the bedrock here in Allamakee and Clayton Counties features a series of deep vertical cracks. These cracks are a "signature" of the bedrock throughout the driftless area and result in weakened planes (running both parallel and at right angles to each other) within the structure of the bedrock. Known geologically as joints, these cracks are the reason our landscape is made up of so many sheer faces along the rock bluffs and block-like formations on top of prominent hills.

Some of the resistant layers occur as waterfalls and rapids breaking the flow of some rivers and streams in the driftless area. The fact the strata erode in their own unique ways results in a landscape that is clearly definable to its parent strata. The erosional processes are so unique, it is quite easy to trace the geologic formations across the landscape. The higher vantage points throughout Allamakee and Clayton Counties afford some remarkable sweeping views of rolling hills apparently connected to lower levels or to valley floodplains by steep cliffs and bluffs. These are the levels that produce the angular, stepped skyline and trademark plateaus of the driftless area.

There is a steep and nearly continuous rim of east-to-northeast-facing rock following a line from central Fayette County to south-eastern Jackson County. The western edge of the rim and the sudden drop in the landscape at that point clearly defines the limits of the Paleozoic Plateau (readily apparent in the image below as the pink shaded area in the northeast corner of the state). This rim is the most easily observed portion of the Silurian Escarpment, the leading edge (cuesta) of the upturned bedrock sculpted by the pre-Illinoian glaciers.

At the point along the rim of the Silurian Escarpment where the land surface drops from the gentle rolling hills of the Iowan Surface, one can clearly see the dramatic change in the landscape ahead. If a line running northwest to southeast was drawn along the land surface from West Union through Fayette, Strawberry Point, Edgewood, Colesburg, and Peosta it would mark the beginning of the Paleozoic Plateau as you traveled eastward. The western side of the line marks the beginning of the point where the Devonian limestones overlie the Cambrian, Ordovician and Silurian layers of the Paleozoic Plateau.

Incredibly, the Silurian Escarpment here in Iowa is but a tiny portion of the immense plates that extend eastward through the United States. Before the Mississippi River was born, these Silurian exposures continued unbroken to the east across the northern United States. Today, they exist only as scattered erosional outliers capping steep, isolated hills in the driftless area. Two good examples of these outliers are Sherrill Mound in Dubuque County and Sinsinewa Mound in southwestern Wisconsin. Sinsinewa Mound is visible on a clear day from Balltown Ridge here in Clayton County. The Silurian escarpment reappears in northwest Illinois and continues north to the shores of Lake Michigan, the northern shore of Lake Huron and the southern shore of Lake Ontario. It is the Silurian Escarpment that makes up the resistant bedrock responsible for the formation of Niagara Falls at the eastern end of Lake Erie. If the existence of Niagara Falls is not a good enough example of the erosional resistance of these rocks, then consider this - the bulge at the eastern border of Iowa is a result of the Mississippi River having to change direction because it wasn't powerful enough to cut away the Silurian bedrock.

When one stands upon the edge of any bluff overlooking the Mississippi River, it is easy to see the immense erosional processes of the Mississippi in the width and depth of the valley she has carved out for herself. When I say the Mississippi wasn't powerful enough to erode the Silurian bedrock in one paragraph and then say how immense the erosional processes were in the next, I don't mean to contradict myself. The best way to put it in to perspective is by quoting Mr. William J. Burke from his book "The Upper Mississippi Valley: How the Landscape Shaped Our Heritage" -

"The most interesting and dramatic land-forming geologic activity in the Upper Mississippi Valley was the result of the turbulent waters rushing southward from the melting glaciers. While this melt-water flooding occurred on and off over eons of time, the most clearly identified events were the glacial melt waters released from several temporary glacial lakes. Among these glacial lakes were Lake Agassiz, which occupied parts of northern Minnesota, North Dakota, most of Manitoba, and glacial Lake Wisconsin in central Wisconsin, and glacial Lake Superior.

Water melting off the glaciers collected in these lakes and was released in a torrent as plugged outlets in the lakes gave way. Lake Agassiz drained southward cutting out the Minnesota River gorge and continued southward in the location of the future Mississippi River. Lake Wisconsin drained northwestward to the location of the Black River and then down the Mississippi gorge, and glacial Lake Superior emptied into the Mississippi by way of the St. Croix River.

It (has) been estimated that the emptying of Lake Agassiz alone, sent a torrent of five to ten million cubic feet of water per second flowing southward. By comparison, the 1965 record flood on the Upper Mississippi River was only about 300,000 cubic feet per second between LaCrosse and Prairie du Chien; and the average flow in the Upper Mississippi River is only about 67,500 cubic feet per second. This ancient glacial river has been referred to as the River Warren."

This map dramatically illustrates the rugged topography of the driftless area here in Allamakee and Clayton Counties. If you look straight north out of Monona on the map, you can see a series of drainages, all eventually feeding into the Yellow River which in turn eventually feeds in to the Mississippi. The gorges and valleys cut into the bedrock by these drainages serve for some spectacular vistas as one drives or hikes down into the Yellow River Valley. In addition, they have served well to help create a "living history" of the area - if you know how to read it. Many of the gravel roads in this area go along for a mile or two and then suddenly top out on a ridge overlooking a secluded valley.

As one descends the road into the valley, you begin to realize just how hidden and well protected from the weather these hollows are. Here lies the fist entry into the "living history" journal; many of these roads are named for the original settlers who built their here. One of my favorite roads here, just north of town is Jackson Hollow Road. Along this drive, geological history is laid out in easy to read layers; from the top down you can travel through over 140,000,000 years in less than a mile. Directly across the road from a favorite Ordovician collecting site of mine is the limestone foundation and remains of a root cellar from the original Jackson Family cabin.

There are Cambrian exposures throughout Clayton and Allamakee counties. The earliest fossils occurring in abundance (and even recognizable as fossils to the beginner) do so in Cambrian outcrops. Although Cambrian fossils are found in this area, they occur in much greater numbers in Minnesota and Wisconsin. Most of the Cambrian exposures are found near the town of Lansing and contain mostly trace fossils in the form of burrows. The oldest exposed Paleozoic deposit in the state of Iowa can be found along Highway 26, just slightly less than 4 miles north of Lansing. There is an exposure of coarse-grained sandstone along the stream on the north side of the bridge. This an exposure of the Ironton Member of the Wonewoc Formation. Another exposure of the Ironton Member is found 10 miles north of Lansing along county road A-26. At this location, I have found the ever-present burrows in great numbers, but also found occasional fragments of both brachiopods and trilobites. Preservation of the latter is too poor to accurately identify species.

Another interesting series of exposures found in Allamakee County is a series of road cuts along the western side of Highway 76, just south of the Upper Iowa River bridge. If you continue south from that point for approximately 1/2 mile, you will find the Cambrian/Ordovician contact exposed. The column for this contact site is as follows (Anderson et al. 1979) -

I. Ordovician Period (Canadian Series)

A. Prairie du Chien Group

1. Oneota Formation: a few feet of dolomite at the top of the cut; gastropods and burrows at the base of the unit

II. Cambrian Period

A. Trempealeau Group

1. Jordan Formation

a. Coon Valley Member; approximately 17 feet of brown, sandy dolomite and dolomitic sandstone

b. Van Oser Member; a few feet of coarse sandstone exposed at the base of the cut