The process is not, of course, always the same, nor are the exact influences, which made the English Fens, generally, operating in precisely the same manner here, but the main principle is the same, and the lesson taught by the improvement of the Fens is perfectly applicable in our case.
"This great level extends itself into the six counties of Cambridge, Lincoln, Huntington, Northampton, Suffolk and Norfolk, being bounded by the highlands of each. It is about seventy miles in length, and varies from twenty to forty miles in breadth, having an area of more than 680,000 acres. Through this vast extent of flat country, there flow six large rivers, with their tributary streams; namely, the Ouse, the Cam, the Nene, the Welland, the Glen, and the Witham.
"These were, originally, natural channels for conveying the upland waters to the sea, and whenever a heavier downfall of rain than usual occurred, and the swollen springs and rivulets caused the rivers to overflow, they must necessarily have overflowed the land to a great extent.
"This, however, was not the principal cause of the inundation of the Fens: these rivers were not allowed a free passage to the ocean, being thus made incapable of carrying off even the ordinary amount of upland water which, consequently, flowed over the land. The obstruction was two-fold; first, the outfalls became blocked up by the deposits of silt from the sea waters, which accumulated to an amazing thickness. The well known instances of boats found in 1635 eight feet below the Wisbeck River, and the smith's forge and tools found at Skirbeck Shoals, near Boston, buried with silt sixteen feet deep, show what an astonishing quantity of sediment[pg 194] formerly choked up the mouths of these great rivers. But the chief hindrance caused by the ocean, arose from the tide rushing twice every day for a very great distance up these channels, driving back the fresh waters, and overflowing with them, so that the whole level became deluged with deep water, and was, in fact, one great bay.
"In considering the state of this region as it first attracted the enterprise of man to its improvement, we are to conceive a vast, wild morass, with only small, detached portions of cultivated soil, or islands, raised above the general inundation; a most desolate picture when contrasted with its present state of matchless fertility."
Salt marshes are formed of the silty deposits of rivers and of the sea. The former bring down vegetable mould and fine earth from the uplands, and the latter contribute sea weeds and grasses, sand and shells, and millions of animalculæ which, born for life in salt water only, die, and are deposited with the other matters, at those points where, from admixture with the fresh flow of the rivers, the water ceases to be suitable for their support. It is estimated that these animalculæ alone are the chief cause of the obstructions at the mouths of the rivers of Holland, which retard their flow, and cause them to spread over the flat country adjoining their banks. It is less important, however, for the purposes of this chapter, to consider the manner in which salt marshes are formed, than to discuss the means by which they may be reclaimed and made available for the uses of agriculture. The improvement may be conveniently considered under three heads:—
First—The exclusion of the sea water.
Second—The removal of the causes of inundation from the upland.
Third—The removal of the rain-fall and water of filtration.
The Exclusion of the Sea is of the first importance, because not only does it saturate the land with water,—but this water, being salt, renders it unfertile for the plants of ordinary cultivation, and causes it to produce others which are of little, or no value.