GeoLog

estuary

Imaggeo on Mondays: Ice forming on Chesapeake Bay

Imaggeo on Mondays: Ice forming on Chesapeake Bay

Sandwiched between the U.S states of Mayland, Delaware, Pennsylvania, New York State, the District of Columbia and Virginia, lies Chesapeake Bay, the largest estuary in North America. It is of huge ecological importance: “the bay, its rivers, wetlands and forests provide homes, food and protection for countless animals and plants”, says the Chesapeake Bay Program. Up to 150 major rivers and streams feed into the bay’s watershed.

Geologically speaking, Chesapeake Bay isn’t very old. As recently as 18,000 years ago the bay was covered by dry land. Global sea levels were up to 200m lower than they are at present and the last of the great ice sheets to cover America was at its peak. The rivers which flowed along the east of the continent had to cut valleys in what is now the bay bottom, to reach the continental shelf, and drain out to sea.

Fast forward 8,000 years and rising global temperatures caused the ice sheets to melt rapidly. Global sea levels started to rise, flooding the continental shelf and coastal areas, which now make up the modern-day estuary.

The process was helped along by a remarkable, much older geological feature. During the late Eocene, 35 million years ago, the Atlantic margin of the U.S was struck by a 3.5 km bolide (an asteroid or meteorite). The impact crater is located about 200 km south of Washington D.C., buried below 300 -500 m of sediments in Chesapeake Bay. Though the crater didn’t form the estuary, it did create a long-lasting depression in the area which helped determine the location of the bay.

Landsat satellite picture of Chesapeake Bay (centre) and Delaware Bay (upper right) – and Atlantic coast of the central-eastern United States. Credit: Landsat/NASA. Distributed by Wikimedia Commons.

Further reading

The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution: USGS Fact Sheet 049-98.

Chesapeake Bay Program

Landsat Images Offer Clearer Picture of Changes in Chesapeake Watershed (Nasa Landsat Science)

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: A clash of hard and soft landscapes

This week’s Imaggeo on Mondays is brought to you by the photographer himself, who describes the striking scenery of the Conwy estuary in Wales…

After a workshop with salt-marsh specialists in Conwy (Wales, UK), I stayed a couple of days to explore the surroundings. Living and working in The Netherlands, for me intertidal areas are typically wide, flat expanses of land, where the horizon is far away and where sand dunes and dikes are the largest vertical elements.

Here in Wales it is a clash of landscapes: ‘hard’, elevated hills that take ages to change from one shape to another, meeting the ‘soft’, low-lying sedimentary environment of the estuary. The town of Conwy adds a human signature to the hills. The tidal range is macrotidal (greater than 4 metres) here, which guarantees beautiful geometries created by the water, such as these tidal creeks in the soft mud. They are only exposed during low tide; and when the tide comes rushing in again, they will once again be covered. The same tide will lift the boats in the background up several metres.

"Low tide at Conwy estuary" by Alma de Goot, distributed by the EGU under a Creative Commons licence.

“Low tide at Conwy estuary” by Alma de Groot, distributed by the EGU under a Creative Commons licence.

The intertidal areas are a harsh environment for flora and fauna, as they must be tolerant of exposure to the air and submersion in brackish water. This location is too dynamic and/or too low-lying for saltmarsh plants to establish, but the mudflats will undoubtedly be full of life, containing a banquet of bivalves, crustaceans and polychaetes for coastal birds to feed on.

The town of Conwy is known for its suspension bridge and railway bridge crossing the estuary and the photo is taken close to where a highway is tunnels beneath the estuary. The construction of the tunnel yielded a lot of sediment that needed to be disposed of. The tailings from the tunnel were embanked to create a wetland (outside this photo), now protected by the RSPB. Despite its artificial origins, it now hosts a multitude of birds.

By Alma de Groot, Coastal Researcher, IMARES Wageningen UR

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their images to this repository and since it is open access, these photos can be used by scientists for their presentations or publications as well as by the press and public for educational purposes and otherwise. If you submit your images to Imaggeo, you retain full rights of use, since they are licensed and distributed by the EGU under a Creative Commons licence.