By the 7th century AD, the long-term impact of post-glacial rebound (the "ice age bounce back") had created a distinct, slow-motion environmental shift for Southern Britain and Northern Germany: 

the north was rising (emerging), while the south was sinking (subsiding). 

As Scotland and Scandinavia rose after the removal of the ice sheet's weight, the land further south—acting like the other end of a seesaw—subsided, causing a relative rise in sea level and increased vulnerability to flooding. 

Impact on Southern Britain (7th Century)

In the 7th century, Southern Britain was in a phase of slow, steady subsidence. While the most dramatic flooding events occurred thousands of years earlier (when Doggerland was submerged around 6000 BC), the ongoing, slower "peripheral bulge" subsidence continued to shape the region. 

• Sinking Land and Flooding: The southern half of Britain was dropping by roughly 5 cm per century. By the 7th century, this contributed to the encroaching sea, particularly in low-lying coastal and estuary areas.

• Submerged Forests and Shorelines: Prehistoric forests, which had grown in the early Holocene, were submerged by this subsidence, creating "submerged forests" visible on coastlines (like in East Anglia and the Thames Estuary).

• River Behavior: The rising sea level at the coast caused inland rivers to struggle to drain, increasing the risk of siltation and higher water tables in inland areas. 

Impact on Northern Germany (7th Century)

Northern Germany, particularly the North German Basin, was similarly affected by the "peripheral bulge" subsidence. 

• Coastal Inundation: The coastlines of the North Sea were experiencing relative sea-level rise. This required early medieval communities to constantly adapt to changing shorelines and rising water levels.

• Land Surface Changes: The re-adjustment of individual fault blocks during post-glacial relaxation led to differential, crust-dependent subsidence, often affecting low-lying coastal plains.

• Migration of Settlements: Early settlements in coastal regions had to be constantly relocated as the land subsided, with many areas that were previously dry becoming salt marshes or sea, as seen in the broader Holocene, which continued into the early medieval period. 

Summary Table: 7th Century Rebound Dynamics

Feature 

Southern Britain

Northern Germany

Crustal Motion

Subsidence (Sinking)

Subsidence (Sinking)

Relative Sea Level

Rising (Relative Sea Level Increase)

Rising (Relative Sea Level Increase)

Key Impact

Increased flooding (Thames/East Anglia)

Coastal/Wetland erosion (North Sea)

Long-term trend

~5 cm/century sinking

Marginal subsidence/Re-adjustment

By the 7th century, these areas were in a mature phase of Post-Glacial Rebound, where the dominant effect was "subsidence," ensuring the landscape was becoming increasingly maritime and vulnerable to coastal flooding. 

In the 7th century (c. 600–700 AD), the coastlines of southern 

Britain and northern Europe were broadly similar to modern contours, though significant local differences existed due to extensive wetlands and lack of modern drainage. 

1. Southern Britain Coastline

The general shape of the island was established by this period, but several key areas were markedly different:

• The Fens: A massive area of eastern England (between modern Cambridge and the Wash) was a vast, marshy wetland rather than dry land.

• The Kentish Marshes: Areas around the Thames Estuary and the eastern coast of Kent were extensively marshy; the Isle of Thanet was still a true island, separated from the mainland by the Wantsum Channel.

• The Somerset Levels: In the southwest, large portions of Somerset were tidal marshes and wetlands, often underwater or only navigable by boat.

• Erosion and Deposition: Soft-rock coastlines, such as those in East Anglia (e.g., Dunwich) and the Holderness coast, extended further into the sea than they do today before centuries of erosion. 

2. Northern Europe Coastline

The continental coast directly opposite southern Britain also featured significant differences:

• The Low Countries (Netherlands/Belgium): Much of the modern-day Netherlands and northern Belgium consisted of tidal flats, peat bogs, and shifting river deltas. Large areas now protected by dykes were then subject to regular flooding.

• Frisian Coast: The Wadden Sea islands were more fluid in shape, and the coastline was characterized by extensive salt marshes.

• Submerged Doggerland: By the 7th century, the "land bridge" of Doggerland was long gone, having been completely submerged by approximately 5000 BC. The North Sea between Britain and Europe was fully formed. 

3. Political Geography (7th Century Context)

Maps from this era typically show the following territorial divisions: 

• The Heptarchy (Britain): Dominated by the seven Anglo-Saxon kingdoms: Wessex(South), Sussex (South Coast), Kent (Southeast), Essex (East), East Anglia (East), Mercia (Midlands), and Northumbria (North).

• Celtic Regions: The western edges of Britain remained under British (Celtic) control, including Dumnonia (modern Cornwall and Devon) and the various kingdoms of Wales.

• Frankish Coast: The northern coast of modern France and the Low Countries was largely under the control of the Merovingian Franks. 

Actionable Resources

• Historical Accuracy: To see a detailed reconstruction of how wetlands changed the usable landmass, view the research on Britain's coastline now vs. then by Nicola Griffith.

• Digital Archives: The History Files provides a clickable, high-resolution map of Britain specifically for 700 AD.

• Cartographic Prints: If looking for physical representations, the Anglo-Saxon Map by Jan Jansson (though a 17th-century interpretation) is a standard historical reference for the 7th-century Heptarchy.