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THE main story of the formation of the Fenland depression is fairly clear. There was a time before the Ice Age when the Chalk Uplands of Lincolnshire were continuous with those of Norfolk, and through gaps in this chalk ridge there flowed eastward the early representatives of the modern rivercourses. This ridge disappeared under the action of two forces.1 The rivergaps, enlarged by the normal processes of erosion, reduced the one-time escarpment to little more than a line of isolated outliers, while the submergence of the land completed the work so that the sea gained access to a western plain formed by the outcrops of soft Jurassic clays. The Fenland in its origin was essentially a flooded alluvial plain which had for its limits the harder rocks around, chalk to the north and south and, in the west, resistant oolite grits. The surface of this Jurassic plain was uneven, and in its higher portions projected above the general level to form the islands of the historical period, Ely, March, and so on. The region emerged from the complicated events of the Ice Ages with its basin character unchanged, and all subsequent time has witnessed its filling up by the various agents of sedimentation?normal and otherwise?leaving the Wash of the present day as the remnant of a much wider opening of the sea. The mediaeval registers often record additions of or which owing to the seas leaving thereof, become firm ground,2 thereby yielding an increase of land surface. The boundary of the depression thus formed may be defined by a line running from Firsby through Tattershall to Lincoln, and then by Billinghay and Peterborough almost due south to Earith, whence it curved northward through Cottenham and Burwell to Lynn. The regularity of this line was broken by the extension of fen up the rivers that, leaving the surrounding upland, wandered slowly3 through the plain by devious4 channels and sundry branches5 before they reached the sea. The chief of these were the Ouse, the Nene, the Welland, and the Witham; the modern drainage system is almost entirely artificial.6 The sediments carried down by these streams and deposited where their velocity was checked, or brought up-channel by the tides from the sea oftentimes so up with silt and sand7 the river-courses that their waters were forced to seek new outlets, winding back upon themselves maybe or interlacing with other streams. Such was the case in the Wisbech estuary which during the thirteenth century was filled up by and sand brought in by the sea-tides, until a mere slender stream flowed out to sea leaving a great stretch of marsh on either side; thus Wisbech, the point of discharge for
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