Aubrey Manning presents the great arc. Source: BBC Talking Landscapes: The Fens

Changes in near-surface geology beneath the fens back him up:

It's perfectly natural. Source: British Geological Survey (BGS)

His arc really does look like part of a circle:

Except for the perfectly circular bits. Source: British Geological Survey

This BGS map of the Fens shows superficial geology - more or less surface level layers of sand, gravel and peat-soil. The deeper bedrock looks very different.

But Manning is correct. His arc is most obvious in the south-west. It is a close fit for about 80 degrees of circle. And a near-fit for another 40 degrees of the circle.

Then at the southern end of the Lincolnshire Wolds - indicated by the top part of the red circle - the arc continues through another change in geology, extending the near-circle.

Is this a coincidence?

It's a good question. Source: BBC Talking Landscapes: The Fens

There are many clues towards an answer. Some clues can be found in geographers' research into Lincolnshire's main river: the River Witham. According to them, the River Witham's course is:

most unusual

Source: Lincolnshire - Gaps and More Gaps, Allen Straw, 2002, page 1.

anomalous

Source: The Geology of Lincolnshire, HH Swinnerton and PE Kent, 1981, page 97.

abnormal

Source: The Landforms of Lincolnshire, David Linton, in Geography, Vol. 39, No. 2, April 1954, page 75.

The enigmatic course of the River Witham:

Blurry, blue and very hard to explain.

It's not just the Witham's abrupt turns... it's the 70-mile U-turn and the Witham's studious avoidance of at least three shorter routes to lower ground.

But there must be an explanation.

Perhaps - as Raymond Selkirk controversially suggested - the Romans engineered water on a much bigger scale than they've been credited with.

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