The rocky outcrops of limestone along the river were noted by many of the early explorers that passed through this area as early as the late 1600s. Much of the scenic beauty that we enjoy today has been forming for hundreds of years. Human history is barely a moment compared to the history found in the rocks and soil deposits in the monument. The clues to the past are evident along the trails, in secluded wooded valleys and in the picturesque bluffs along the Mississippi River.
- Along the highway south of the Visitor Center, Cambrian sandstones make up a type of sedimentary rock laid down by wave action in a shallow sea some 500 million years ago. This same type of sandstone is found at the base of the bluff below Hanging Rock. (Hanging Rock is located in the North Unit of Effigy Mounds National Monument.) This soft rock, Jordan sandstone, weathers or breaks down easily as the quartz crystals are weakly cemented together. Prehistoric Indians used the eroded Cambrian "caves" as rockshelters and carved petroglyphs in the soft rock.
Most of the rock making up the bluffs of the Mississippi River valley in the area are made up of Ordovician rock called Prairie du Chien dolomite. This limestone, rich in magnesium, is much more resistant to weathering than sandstone. This rock was formed by clay deposits deposited in a shallow water environment. Many varieties of invertebrate fossils such as brachiopods, cephalopods and corals are found within these deposits. These fossils give us a clue to the type of life existing in the area around 450 million years ago.
Along the trails hikers can find layers of chert, which is an impure form of flint found embedded within the limestone. Gathered from erosional deposits, this material was used to make projectile points, scrapers and other tools.
In the South Unit, an outcropping of St. Peter sandstone can be found on Rattlesnake Knob and on the old Military Road below the Marching bear Group of mounds.
The ridge and valley topography of this area is very much different than most of Iowa and the Midwest. Continental glaciers covered most of what is now the Midwest five times in the last million years, ending about ten thousand years ago. This stretch of the Mississippi River Valley lies in the Driftless Area that escaped the planing effect of those icesheets. Driftless refers to the lack of glacial rock brought into glaciated areas, such as granite brought in from the north. When the ice melted in areas to the north, the flood waters deepened the river valley and later deposited up to 250 feet of material in the old river bed. It is believed that the river valley was much deeper before glacial deposits were deposited by braided stream channels.
Windblown silt called loess probably originated in the glacial Missouri River basin and covers the ridgetops at a depth of one to six feet. This loess soil was used by prehistoric Indians to build most of the ridgetop mounds.
The area's interesting geologic history began hundreds of millions of years ago and the forces of uplift and weathering continue to shape the landscape today. The Driftless Area shaped a unique collection of plant life that was instrumental in the development of prehistoric cultures such as the Mound Builders.