So strong is the barrier which these sea-borne sands oppose to the river-borne ooze, that as soon as a seabank is built—as the projectors of the ‘Victoria County’ have built them—across any part of the estuary, the mud caught by it soon ‘warps’ the space within into firm and rich dry land. But that same barrier, ere the fen was drained, backed up for ages not only the silt, but the very water of the fens; and spread it inland into a labyrinth of shifting streams, shallow meres, and vast peat bogs, on those impervious clays which floor the fen. Each river contributed to the formation of those bogs and meres, instead of draining them away; repeating on a huge scale the process which may be seen in many a highland strath, where the ground at the edge of the stream is firm and high; the meadows near the hillfoot, a few hundred yards away, bogland lower than the bank of the stream. For each flood deposits its silt upon the immediate bank of the river, raising it year by year; till—as in the case of the ‘Levée’ of the Mississippi, and probably of every one of the old fen rivers—the stream runs at last between two natural dykes, at a level considerably higher than that of the now swamped and undrainable lands right and left of it. If we add to this, a slope in the fen rivers so extraordinarily slight, that the river at Cambridge is only thirteen and a half feet above the mean sea level, five-and-thirty miles away, and that if the great sea-sluice of Denver, the key of all the eastern fen, were washed away, the tide would back up the Cam to within ten miles of Cambridge; if we add again the rainfall upon that vast flat area, utterly unable to escape through rivers which have enough to do to drain the hills around; it is easy to understand how peat, the certain product of standing water, has slowly overwhelmed the rich alluvium, fattened by the washing of those phosphatic greensand beds, which (discovered by the science of the lamented Professor Henslow) are now yielding round Cambridge supplies of manure seemingly inexhaustible. Easy it is to understand how the all-devouring, yet all-preserving peat-moss swallowed up gradually the stately forests of fir and oak, ash and poplar, hazel and yew, which once grew on that rank land; how trees, torn down by flood or storm, floated and lodged in rafts, damming the waters back still more; how streams, bewildered in the flats, changed their channels, mingling silt and sand with the peat-moss; how Nature, left to herself, ran into wild riot and chaos more and more; till the whole fen became one ‘Dismal Swamp,’ in which the ‘Last of the English’ (like Dred in Mrs. Stowe’s tale) took refuge from their tyrants, and lived, like him, a free and joyous life awhile.