Climatic and Geological History of the Chesapeake

Glaciers cover North America, showing different ice sheets
The Chesapeake's water was frozen in glaciers during the last Ice Age.

Base map by Ron Blakey, Colorado Plateau Geosystems, Inc.

The Chesapeake Bay is a geological feature. Like all geological features, the Chesapeake Bay formed slowly over time and will continue to change in the future. How did the Chesapeake Bay form? How did the climate of the region change dramatically from its formation to the present day?

Formation of the Chesapeake Bay

Ancient Rivers Emerge - 340 million years ago

The Earth was a very different place 340 million years ago. Amphibians, insects, and sharks were plentiful, while mammals had not yet come to be. It was in this world that the Susquehanna River formed - the river that would one day become the Chesapeake Bay as we know it today.

Continents Collide - 300 million years ago

Over the course of millions of years, the continents and oceans that make up the Earth's surface have shifted dramatically with the movement of tectonic plates. About 300 million years ago, the land that is now Eastern North America collided with what is now West Africa. This caused the land to uplift, forming the Appalachian Mountains. It also caused the Susquehanna River to change the direction of its flow - it now ran southeast down from the mountains. Over the next 200 million years, the continents pulled apart again and formed the Atlantic Ocean.

Bolide Impact - 35 million years ago

A bolide is an extremely bright meteor. Tens of thousands of bolides enter Earth's atmosphere every year, though few are large enough for anybody to notice. However, about 35 million years ago, a large bolide collided with the Earth near what is now the lower Delmarva peninsula. This impact created a wide crater that is now hidden under the waters of the lower Chesapeake Bay. This large depression helped determine the location of the Chesapeake Bay when it began forming millions of years later.

NASA Chesapeake Bay aerial photograph
Aerial image of the Chesapeake Bay


Glaciers Melt and Form the Bay - 18,000 years ago

Between 29,000 and 19,000 years ago, the Earth was engulfed in an Ice Age, or glacial period. At this time, the position of the Earth's continents and oceans were mostly the same as they are today. However, large glaciers covered some 8% of the Earth's surface. 18,000 years ago, the climate began to warm up, causing the glaciers to gradually melt. Slight, natural variations in the Earth's orientation towards the sun are thought to cause these cycles of glaciation, or Ice Age. As the Ice Age came to an end, the melting ice caused the landscape to transform.

Here in the mid-Atlantic region, the Susquehanna became flooded with massive amounts of water flowing from the melting ice sheet. The entire area that was once the river's valley gradually filled with water and became a wide Bay. Geologists refer to this as a "ria," or drowned river valley. The Bay continued to take form, reaching its modern-day shape about 3,000 years ago.

A Warming Climate and Eroding Shore - From 18,000 years ago to Present

From the Chesapeake's beginnings to the present day, the climate has continued to warm in the wake of the last Ice Age. Today, the geology of the Chesapeake Bay is defined by erosion and sedimentation. The complex pattern of the Bay's tides and currents have caused islands and shorelines to gradually erode. In recent decades, sediment, or dirt, from construction and agriculture has entered waterways in abnormally high amounts, causing some rivers and channels to become more shallow and cloudy. Geological factors along with the removal of groundwater has caused gradual land subsidence - this is the sinking of land due to underground shifts. Finally, human activities have resulted in climate change. Increased average global temperatures will lead to sea level rise and other changes for the watershed. Learn more about the effects of climate change in the Chesapeake Bay here.

Last updated: March 25, 2024

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