Index Page | Site Map | Earth Systems Education | Ohio Sea Grant | Contact Us

    This page presents the geologic characteristics and origin of lake basins and river valleys in the Great Lakes. The sedimentary rocks in the Great Lakes Basin formed in an ocean that covered most of the basin beginning about 600 million years ago. Different types of rocks formed as the depth and shape of the ocean changed over a long time. The ocean disappeared from the area about 225 million years ago because all adjacent land was raised above sea level. So, sediments were no longer deposited and erosion dominated everywhere. As the mountains rose, many rivers developed on their slopes. Of these rivers, the main one flowing westward was the Teays, which followed a northwestward course across West Virginia, Ohio, Indiana and Illinois to join a very small preglacial Mississippi River. This river no longer exists, and its valleys are now deeply buried by glacial deposits. In other words, before the Great Lakes formed, the Great Lakes basin was occupied by a river with many branches.
   A million years ago, the preglacial rivers in the Great Lakes area were blocked by the ice and in many cases were destroyed. As the ice retreated back toward the north, a whole series of ice-dammed lakes was formed in each of the Great Lakes basins (Forsyth, 1996).
As the glacier retreated, large volumes of meltwater occurred along the front of the ice. Because the land was greatly depressed at this time from the weight of the glacier, large glacial lakes formed. These lakes were much larger than the present Great Lakes. Their legacy can still be seen in the form of beach ridges, eroded bluffs and flat plains located hundreds of metres above present lake levels. Glacial lake plains known as 'lacustrine plains' occur around Saginaw Bay and west and north of Lake Erie. As the glacier receded, the land began to rise. This uplift (at times relatively rapid) and the shifting ice fronts caused dramatic changes in the depth, size and drainage patterns of the glacial lakes. Drainage from the lakes occurred variously through the Illinois River Valley (towards the Mississippi River), the Hudson River Valley, the Kawartha Lakes (Trent River) and the Ottawa River Valley before entering their present outlet through the St. Lawrence River Valley. Although the uplift has slowed considerably, it is still occurring in the northern portion of the basin. This, along with changing long-term weather patterns, suggests that the lakes are not static and will continue to evolve (The Great Lakes: An Environmental Atlas and Resource Book, 1995).

GS-017 Kelley's Island glacial grooves.

EP-079 The Great Lake Erie. 
Chaper 2. The Geological Setting of the Great Lakes 

EP-014 Geography of The Great Lakes

FS-019 Lake Erie shore erosion

To request these materials, please visit OSG website. 

•  When did the rocks in the Great Lakes basin form? 
•  How were sedimentary rocks in the Great Lakes basin formed? 
•  How did rocks and rivers shape the Great Lakes? 
•  What evidence of glaciation exists in the Great Lakes region?
• What evidence of glaciation and geologic processes can be found on Great Lakes beaches?      

Relief, Drainage and Urban Areas
Source: The Great Lakes: An Environmental Atlas and Resource Book (1995)

6. Factors Affecting Great Lakes-St. Lawrence System Flows
http://www.great-lakes.net/envt/water/flowfact.html