Content adapted with permission from the Indiana Geological Survey, a partner in the Natural Heritage of Indiana project. Credit for this article goes to Todd A. Thompson. More information can be found here.

In Indiana, it is not that easy to see bedrock and the bedrock surface. Two-thirds of the state is covered with glacial material. In the northwestern corner of the state, these sediments are as much as 350 feet thick. Only in the southcentral part of the state are rocks exposed at the earth's surface. It is here that we can look at large exposures of rock along stream and river valleys, road cuts, and excavations (quarries and surface mines). Commonly, geologists use chips and cores brought up from holes drilled into the earth as their only source of information.

There is a large variety of rocks that are found in Indiana. All the rocks that are exposed at the bedrock surface, however, are sedimentary rocks. Most consist of sandstone, shale, siltstone, limestone, and dolostone. Other rock types are coal, conglomerate, gypsum, claystone, and chert. Deep wells and exploratory test drillings have encountered granite, gabbro, basalt, andesite, and metasedimentary rocks at depths of 3,500 feet to about 3 miles below the bedrock surface.

Sedimentary rocks occur as parallel or nearly parallel layers, or beds. Beds vary in thickness (<1 to 10s of feet) and spatial distribution (<1 to 10s of miles). Beds that commonly occur together or have similar characteristics and distribution are lumped together by geologists and called "formations." Formations are a shorthand way to describe a collection of rocks that are similar. Typically, formations are tens to hundreds of feet thick, and they can be traced for tens to hundreds of miles. A collection of formations can be lumped together into "groups." Groups are often hundreds to thousands of feet thick and can be traced hundreds of miles. The map to the right shows groups in Indiana. Groups are commonly shown on statewide maps, because formations, and especially beds, would be too thin to draw without making a very large map. Like formations, there is nothing special about the group names. They are useful when you want to talk about large collections of rocks--which is what we are going to do.

Distribution

In the southeastern corner of the state, most of the rocks consist of gray, greenish-gray, and brown shales with a minor amount of shaly limestone. These rocks are part of the Maquoketa Group, and they were deposited during the Upper Ordovician (~440-446 millions of years ago [mya]). They are some of the the oldest exposed rocks along the axis of Indiana's anticline. Here, the anticline is often called the Kankakee or Cincinnati Arch. Throughout the Paleozoic Era, this area was a locus of uplift and erosion.

North and northwest of the Maquoketa Group are rocks attributed to the Silurian Period (~440-410 mya). These rocks are described by a wide variety of names at the group (Salina, Bainbridge), formation (Cataract, Sexton Creek, Wabash, etc.), and member levels across the state. Most consist of limestones and dolostones with varying amounts of fossils and argillaceous material. Of particular importance is the occurrence of bound-together skeletal material interpreted as ancient reefs. Two prominent occurrences of these deposits occur as bands from Fort Wayne to Lake Michigan and Terre Haute to Spencer County.

Rimming the Silurian rocks are Devonian (~ 410-360 mya) carbonates and evaporites of the Muscatatuck Group. These rocks are often used to define that margins of the Michigan Basin to the north and the Illinois Basin to the southwest (see Tectonic Features of Indiana). Consisting of mostly dolomite, the Muscatatuck Group contain granular and fibrous anhydrite and gypsum in the Detroit River Formation.

Overlying the Muscatatuck Group is a thick sequence of Devonian and Mississippian (~360-320 mya) shale known as the New Albany Shale. The New Albany Shale is a formation that is from 100 to 340 feet thick. Parts of the New Albany are brown to black shales that are rich in organic materials. Recently, these black shales have been drilled for possible recovery of natural gas. The New Albany shale occurs across a wide area of Indiana in the northern tier of counties. Here, the shale may be overlain by hundreds of feet of glacial material.

A swath of siltstone, shale, sandstone, and minor amounts of limestone extends north and northwestward from the Ohio River at Floyd County to Benton County on the Illinois border with Indiana. These rocks are called the Borden Group. The erosion of these rocks provide the scenic views in Brown County.

To the southwest of the Borden Group is a sequence of carbonate rocks that is 250 to nearly 500 feet thick and has significant amounts of gypsum, anhydrite, shale, chert, and calcareous sandstone. These rocks make up the Sanders and Blue River Groups. Within the Sanders Group is a formation known by geologists as the Salem Limestone and by architects as "Indiana Limestone." This thickly bedded limestone is quarried for a variety of architectural purposes and is known as one of the premier dimension stones in the world.

Sandwiched between the Blue River Group and overlying Pennsylvanian rocks are 140 to 350 feet of sandstone, limestone, and shale, split into more than 20 formations. These rocks are part of the Buffalo Wallow, Stephensport, and West Baden Groups. Erosion by southwestward-flowing rivers (much like today) at the end of the Mississippian and during the early Pennsylvanian Period has dissected many of these units. The missing rocks between these two periods are known as an unconformity. The Mississippian-Pennsylvanian Unconformity (3-8 my long) is a undulatory surface dissected by V-shaped valleys of up to 125 feet deep.

In the southwestern part of the state the rocks of the Raccoon Creek Group overlie the Mississippian-Pennsylvanian Unconformity. This group consists of mostly sandstone and shale with minor amounts of coal, limestone, clay, and chert. The Raccoon Creek Group is the first of the coal-bearing units in Indiana. Ten named coals occur in this interval.

The outcrop belt of the Carbondale Group extends from Warrick County northward to Vermillion County. The Carbondale Group is similar in composition to the Raccoon Creek Group and includes some laterally persistent limestones and four of Indiana's commerciallyimportant coals. This unit averages about 300 feet in thickness, although it thins northwestward.

Rocks that outcrop in the southwestern corner of Indiana comprise the McLeansboro Group. This group can be as thick as 770 feet and consists of mostly sandstone and shale with discontinous beds of coal and limestone throughout the sequence. More»