Approximately 99.9 percent of geologic time spans back farther than we can easily imagine. If you put geologic time in the perspective of one day, humans came in during the last minute of the day. Most of Iowa's geologic history covers the last 25 percent of geologic time.

As we mentioned previously, there are three basic rock types: igneous, metamorphic and sedimentary. Iowa's rock deposits consist mainly of sedimentary rocks: limestone, dolomite or gravel. Two main processes form sedimentary rocks. The first and most common process involves the accumulation and compaction of the skeletons of sea animals on the ocean floor. Part of this process can involve erosion and redeposition of pre-existing rocks by wind, water or ice. The other process involves the precipitation of limestone out of a body of water.

Iowa's sedimentary rocks were originally formed in several different environments. These environments vary from oceans to coral reefs, shorelines and swamps, and rivers and glaciers.

The processes that formed Iowa's rocks are still going on today around the world. New sedimentary deposits are being formed constantly. The forces of wind, water and ice are forming these deposits. These forces erode sediments from one area and redeposit them in another area.

Glaciers deposited most of what we see at the surface in Iowa. Most of what lies under the glacial deposits was formed in other environments. River deposits are also found at the surface. We find them mainly along existing rivers, but we can also find them along old river channels that have been abandoned by the river; for example, in oxbow lakes.

Iowa's geologic past, especially the glacial past, directly affects the landscapes we see in Iowa. In areas that have seen long periods of erosion since the glaciers left, there are deep valleys and high hills for example, those found in southern Iowa. In areas that haven't seen such long periods of erosion, there are lower, more gently rolling hills such as those found in north-central Iowa. In certain areas, where there is very little to no glacial deposits left, there are tall cliffs and bluffs, which are found in northeast Iowa.

TO: F. Unit 1 Glossary