Geology

THE SUSQUEHANNA RIVER is considered the world’s oldest major river system. Dating back to the Paleozoic Era (543-248 million years ago), geologic evidence suggests the river began flowing before the Appalachian Mountains were formed. While most rivers shape their surrounding valleys over time, the Susquehanna was already flowing, as mountains and valleys formed around it.

Alleghanian Orogeny

The Susquehanna River formed in the Carboniferous Period, a time when dense, swamp-like forests covered the planet. These forests would become peat and then coal, mined millions of years later to fuel the Industrial Revolution.

About 300 million years ago, the Appalachian Mountains formed when Africa and North America collided in what geologists refer to as the Alleghanian Orogeny. An asteroid or meteor is believed to have collided with Earth about 200 million years ago. It caused the crater that is now the southern portion of Chesapeake Bay.
 

World’s Largest Rock Pothole Fields

Ultimately, glacier meltwater from the Ice Age eroded the river path, creating the 1,191-foot drop between Cooperstown and the Atlantic. The northern section of the Susquehanna Basin, located mostly in New York, is composed largely of shale with small sections of limestone and sandstone at its most northern points.

Rapids, rocks and other obstructors have rendered the lower Susquehanna, in Pennsylvania and Maryland, unnavigable to commercial traffic. This area is home to one of the world’s largest rock pothole fields, created by underwater vortexes during extreme glacier melt at the end of the Ice Age.

At low points, smooth rocks are visible, sculpted by tamer waters. Much of the lower river is composed of diabase. During the continental drift 200 million years ago, this hard igneous rock formed from hardening magma.
 

Devonian Age

At 444 miles, the National Park Service ranks the Susquehanna River as the longest in the Eastern United States. As one of its tributaries, the Chemung River runs about 70 miles. It starts in Painted Post, New York, joining the east branch of the Susquehanna at Tioga Point, after flowing 45 miles into northern Pennsylvania.

The Chemung River valley’s composition includes shale, sandstone, and limestone from the Devonian age. Most hilltops were rounded by Ice Age glaciation.