How the Susquehanna River and the Chesapeake Bay were formed

The Susquehanna River river drops 1,191 feet during its journey from Cooperstown, New York, to the Chesapeake Bay. It has seen many changes in the landscape through which it flows over the past 325 million years.

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The Susquehanna River as seen from the Breezyview Overlook north of Columbia. Accomac Point is on the left side.

Today, the Susquehanna River is considered one of the oldest rivers in the United States — if not the oldest. The river flows over rocks that are younger than itself — and that is unique.

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Rewind the earth’s geologic video to at least 325 million years ago. In the vicinity of Baltimore, Maryland, and Wilmington, Delaware, there were the Taconic Mountains, formed from an island arc collision some 425 million years ago. The Appalachian Mountains did not yet exist. An ocean occupied the area where that mountain range is located today. Although the Taconic Mountains have eroded, the river was flowing in the opposite direction of today, creating its early channel.

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Pangea

Fast forward to 300 million years ago, and a huge collision between Africa and North America occurred to help in forming the supercontinent Pangaea. This collision is known to geologists as the Alleghanian Orogeny, which seriously deformed our crust into a series of folds and broken pieces of crust containing faults. The Appalachian Mountains were pushed up during this episode, which lasted about 20 million years. The Appalachian Mountains were once as high as the present-day Rockies. Now, with the Appalachian Mountains the high point, the Susquehanna River reversed its course and flowed in a southeastern direction.

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For the next 150 million years, the constant downward erosion of the river cut into the crust, establishing its channel and claiming its real estate.

About 200 million years ago, Pangaea was well underway to splitting apart into the world we know today.  This series of rifts did not play a big role in the life of the Susquehanna. The river was flowing through a relatively flat plain as the crust would either go with Africa or stay with North America.

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Example of a pothole in diabase north of the Falmouth Boat Launch and south of Three Mile Island. Roxann Barnett for scale.

Meteor helped form bay

Advancing the video ahead to post Pangaea, severe weathering and erosion worked on the landscape, and the Susquehanna River carried much sediment downslope onto a continental shelf of the newly born North America. Some researchers suggest that some 4 to 5 miles of the crust were removed from over our heads by these processes with assistance by the river.

About 35.5 million years ago, our area was greeted with an extraterrestrial visitor.  An asteroid or meteor collided with Earth in what is today the southern portion of the Chesapeake Bay. Traveling at about 144,000 miles per hour, the object created a 25-mile wide crater a half mile deep. This event was the first step in creating the Chesapeake Bay. The Chesapeake Bay Impact is one of the best-preserved craters in the world, although today sediments have slumped into the large depression.

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“The Deeps” river channel below the Holtwood Dam on the Susquehanna River. The river is about 100 feet deep in this area.

Ice age

The final chapter of this story involves Pennsylvania’s last geologic event known as the Ice Age. From at least 770,000 to 17,000 years ago, four ice advancements brought a one-mile-high wall of ice into northern and northwestern Pennsylvania. Meltwater from each advancement found its way down the Susquehanna River, adding to its erosional rate.

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This geological graph shows the slope of the Susquehanna River through much of Pennsylvania.

Particularly at the end of the Ice Age, the additional meltwater from the glaciers created severe downcutting of the Susquehanna River, forming large potholes in the bedrock. It can be shown where the river once flowed about 200 feet higher than its present-day elevation. 

Eventually, around 10,000 to 8,000 years ago, the Susquehanna River had formed a 400-foot deep canyon where it flowed into the Chesapeake Bay. Today, sedimentation has filled in the canyon and helped create what we know of today as the Atlantic Coastal Plain.

Jeri Jones is geologist who lives in York County, Pennsylvania.