Global sea levels are rising. This shouldn’t be news; they’ve been rising for 20,000 years, going up 300 feet since the last Ice Age, when glaciers covered Siberia and Canada. As it turned out, Britain was pretty awful during the last Ice Age, and people avoided it; mile-thick ice caps had something to do with it. When people did start appearing about 100 centuries ago, sea levels were still 150 feet below the present level, and the new settlers simply walked over from Europe; the Dover-Calais ferries now float high above their paths.

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A lot of water has covered a lot of land since then, disrupting human societies and natural ecosystems all the way. Nearly every continent has a low-lying stretch of coast that has been affected. The United States has several: Georgia’s Sea Islands, the Florida Everglades, Louisiana’s Mississippi Delta. In England, the largest such area is called the Fens, 300,000 acres of flat and sinking land, facing the North Sea from Cambridge to Lincoln.The Fens, like the Louisiana Delta, formed over the last 10 millennia as rivers dumped sediment onto a sinking plain, forming wide marshes. A person might see one of these marshes as something eternal and unchanging, but this appearance is erroneous; in geological terms, these lands are recent and changing rapidly. A person might also think he’s looking at ordinary land that’s been flooded. This, too, turns out to be a serious, common and costly error.

Let’s start with these errors. Daniel Defoe called the Fens “the sink of thirteen counties,” meaning that rivers drained most of Middle England into these low, flat lands. In the spring, these rivers would run in high floods, heavy with sediment. When they hit the flat Fens, they would slow down and drop the heaviest of their sediment load. These sand and clay bars would obstruct the channels and send the rivers into wide meandering patterns, perhaps doubling their length before they hit the North Sea in the Wash, a large shallow bay. Of course, the longer a river took to fall to the sea, the slower its water, and the more sediment it dropped.

This left the Fens with a shifting landscape of sluggish channels choked by sediment banks both new and ancient. In between these clogs, standing water would foster rich marsh vegetation. This in turn caused the formation of peat, nearly pure plant material partially rotted to a brownish black mass, with the rotting halted by lack of oxygen in the standing water. Peat, unlike normal vegetation, will never rot as long as it stays in standing water; over eons, it will turn into coal instead. Moreover, peat will accumulate at a remarkable rate — a foot or two in a decade. Dried peat forms into burnable bricks (it smells like Scotch whisky when it burns), and because it accumulates so rapidly, a small village can stay in equilibrium with its peat fuel supply over many centuries.

For the last 60 centuries, sea levels have risen gradually and slowly, an oddity in a geological record otherwise filled with large sea level oscillations. Even so, the North Sea had a nasty little jump between ad 350 and 550, flooding the coasts of northern Europe with an extra 2 feet of water and sending its inhabitants — folk known as Angles and Saxons — fleeing (although “conquering” might be the better word) into ill-prepared Roman territories. At the start of this rise, the areas we know as the Fens were a well-settled part of Roman Britain ruled from the town of Duroliponte (Cambridge) by its native people, the Christianized Romano-Celtic Iceni. Then the sea level rose, and history’s curtain went down for two centuries. When the curtain came back up, Duroliponte and the Iceni had disappeared, and 300,000 acres of marshlands covered the northwestern flank of the pagan German kingdom of East Anglia. The modern Fens had come into existence.

Now it’s time to take on one last error: that the undrained Fens were empty wastelands of little or no economic use. This was certainly untrue by medieval times and was probably never true. Left alone, the Fens grow over with a dense, brushy scrub vegetation known locally as carr. It seems, however, that medieval Fenland vegetation consisted of vast beds of sedge and reed, with willow and grasslands in the drier spots and ponds in the wetter spots. These ecosystems are not natural; they are human-induced and must be harvested regularly to avoid the transition to carr. People were in the Fens from the start, exploiting the resources and shaping the environment.

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