Formation of the Cincinnati Arch

Excerpted from  Geologic History

The Earth is dynamic, consisting of plates that are made of rigid continental and oceanic lithosphere overlying a churning, plastically flowing asthenosphere (Figure 1.2) and are constantly in motion.

Figure 1.2: Compositionally, the Earth is divided into three

basic layers: the very dense core, the mantle, and the crust.

The crust is the outermost layer and the least dense. Based

on physical properties, the Earth is divided up another way:

the lithosphere, asthenosphere, mesosphere, outer core, and

inner core. The rigid crust of the lithosphere, is constantly

moving over the plastically flowing asthenosphere. Figure by J.


Plates are pulling apart, colliding, or sliding past one another with great force, creating strings of volcanic islands, new ocean floor, earthquakes, and mountains. The continents are likewise continuously shifting position because they are part of the moving plates. This not only shapes the land, but also affects the distribution of rocks and minerals, natural resources, climate and life. 

Compression from colliding plates, stretching from plates pulling apart, addition of land to North America, weathering and erosion have combined to slowly sculpt the form of the continent. 

Scientists can reconstruct what the ancient Earth may have looked like by studying rocks, fossils and other geologic features. More than a billion years ago narrow strips of land collided and compressed to form the beginnings of the North American continent and what is now the Precambrian shield.

From this proto-North America, sediment was eroded and transported by rivers and streams across the ancient continental margins and then into the adjacent oceans. The sediment deposited in the ocean waters on the eastern margin of proto-North America comprise what is presently called the Grenville belt.  Over 1 billion years ago, proto-North America collided with other continents to form a large supercontinent that we now call Rodinia. Such collisions of continental crust result in mountains. The Grenville sediment that had been deposited in the ocean was caught in the collision and thrust up onto the side of proto-North America.  The collision crumpled the crust, creating a tall (Himalaya-size) mountain range that stretched from Canada to Mexico: the Grenville Mountains.

Figure 1.7: Grenville Mountain Building

-North America collides with other continents, forming a supercontinent (Rodinia)

-Grenville belt of ocean sediment pushed onto the side of ancient North America

-Grenville Mountains form from the collision


Though the exposures of Grenville-age rocks in the Southeast are relatively sparse, they are important because of the geologic story they tell about the region. Grenville-age rocks are literally the foundation of the inner continent and because in many areas they are mostly overlain by sedimentary rocks, they are called basement rocks.

[Basement rocks, not depicted in the colorful graphics by KGS, are the foundation that underlies the surface geology depicted in the preceding page.]

Following Grenville Mountain building, the North American plate began to break away from the other continental plates because of tensional forces in the Late Precambrian and early Cambrian. A series of cracks in the crust formed, known as rifts. The continents finally split apart completely at a major rift that eventually was flooded by ocean water. This ocean is called the Iapetus (or Proto-Atlantic) because several hundred million years later the modern Atlantic Ocean opened up in a similar way in a similar position relative to modern day North America and Europe. Rifts are cracks in the crust, which can be caused by tensional stress as a landmass breaks apart into separate plates. If you hold a Snickers candy bar at both ends  and slowly pull it apart into two pieces, you will notice a series of cracks ("rifts") in the chocolate. At a major rift near the middle, the candy bar will break apart completely. The way that the  candy bar breaks is analogous to how the continental plate separated in the Precambrian to form the Iapetus Ocean.)


Precambrian rift basins

Rift basins that formed in the crust when the continental plates were pulling apart during the Precambrian have played an important role in the geologic makeup, shape, structure and sedimentary deposits of the Southeast.  Many basins formed during this time into which sediment was deposited in the Precambrian and throughout later mountain building events. The Cincinnati Arch, a structural feature that stretches through Ohio, Kentucky, and Tennessee, was originally a rift basin. In the Cambrian, the rift basin was uplifted and became an arch. The Nashville and Jessamine Domes of Tennessee and Kentucky are bumps along the Cincinnati Arch.


Additional references   picture of Cincinnati Arch on Pg 2.


University of Cincinnati Geology Professor Paul Edwin Potter has written a book which all of us interested in the local Cincinnati Geology should obtain: Exploring the Geology of the Cincinnati/Northern Kentucky Region. This 115 page soft cover volume is a Kentucky Geological Survey publication - Special Publication 22, Series XI, 1996. This can be obtained directly from the Survey for about $10.00 plus shipping.  Re-publication, with update occurred in 2007.  Publication 13872