As soon as the sea began to ebb and flow through the opening in the barrier, the conditions were greatly altered and we see the results of the conflict between the mud-carrying upland waters and the beach-forming sea.
Turbiferous and Areniferous Series.
The Fen Beds belong to the last stage and, notwithstanding their great local differences, seem all to belong to one continuous series. Seeing then that their chief characteristic is that they commonly contain beds of peat it may be convenient to form a word from the late Latin turba, turf or peat, and call them Turbiferous to distinguish them from the Areniferous series which consists almost entirely of sands and gravels.
When the land had sunk so far that the velocity of the streams was checked over the widening estuary and on the other hand the tide and wind waves had more free access, some outfalls got choked and others opened; turbid water sometimes spread over the flats and left mud or was elsewhere filtered through rank plant growth so that it stood clear in meres and swamps, allowing the formation of peat unmixed with earthy sediment.
Banks are naturally formed along the margin of rivers by the settling down of sand and mud when the waters overflow, as seen on a large scale along the Mississippi, the Po, as well as along the Humber and its tributaries.
The effect of a break down of the banks is very different. A great hole is scooped out by the outrush, and the mud, sand and gravel deposited in a fanshape according to its degree of coarseness and specific gravity.
A good example of this was seen in the disastrous Mid-Level flood at Lynn in 1862[1] and the more recent outburst near Denver in the winter of 1914-15[2] , of which accounts were published in contemporary newspapers. The varied accompanying phenomena can be well studied in the process of warping in Yorkshire or the colmata in Italy.
This was a much commoner catastrophe in old times, before the banks were artificially raised, and, as the streams could never get back into their old raised channel, this accounts for the network of ancient river beds which intersect the Fens.
The bottom of the Turbiferous alluvium is always, as far as my experience goes, sharply defined. This of course cannot be seen in a borehole or very small section.
The surface of the older deposits seems to have been often washed clean either by the encroaching sea or by the upland flood waters.