The origin of these shallow freshwater lakes, which form a characteristic feature in the scenery of East Anglia, has been much debated; but with the knowledge obtained from a study of the submerged forests the explanation is perfectly simple. During this period of slow submergence each of the shallow valleys in which the broads now lie was turned into a wide and deep navigable estuary, which extended inland for many miles. When the subsidence stopped the sea and tides soon formed bars and sand-banks at the mouths of the estuaries, and lateral tributaries pushed their deltas across. The Norfolk rivers, being small and sluggish, were driven to one side, and could neither cut away the sand-banks nor fill up with sediment such wide expanses. These estuaries therefore were silted up with tidal mud and turned into irregular chains of lakes, separated by irregular bars and sand-banks. The lakes, instead of becoming rapidly obliterated and filled up by deltas which crept gradually seaward, remained as freshwater broads; for as soon as a bank became high enough for the growth of reeds and sedges the river mud was strained out and only nearly clean water reached the lagoon behind. Thus a depression once left, provided it was out of the direct course of the river, tended to remain as a freshwater lake until vegetable growth could fill it, and the river mud was spread out over the salt-marshes or went to raise the sand-banks till they became alluvial flats, and thus still more thoroughly isolated the broad.
A few centuries will see the disappearance of the last of the broads, which have silted up to an enormous extent within historic times; but the fact that so many of these broads still exist may be taken as clear evidence of the recent date of the depression which led to their formation.
When we look at ancient records, and notice the rapidity with which the broads and navigable estuaries are becoming obliterated, we cannot help wondering whether the measure of this silting up may not give us the date of the last change of sea-level. It should do so if we could obtain accurate measurements of the amount of sediment deposited annually, of the rate at which the sea is now washing it in, and of the rate at which the rivers are bringing it down. All these factors, however, are uncertain, and it is particularly difficult to ascertain the part played by the muddy tidal stream which flows in after storms and spreads far and wide over the marsh.
Though all the factors are so uncertain, we can form some idea of the date of the submergence. Many years ago I made a series of calculations, founded on the silting up of our east coast estuaries, the growth of the shingle-spits, and the accumulation of sand-dunes. The results were only roughly concordant, but they seemed to show that the subsidence stopped about 2500 years ago and was probably still in progress at a date 500 years earlier. This question of dates will be again referred to in a later chapter.
Before leaving the Broad district we must refer to a boring made at Yarmouth, which, according to Prof. Prestwich, showed that the recent estuarine deposits are there 120 feet thick, and consequently that the ancient valley was far deeper than any recorded in the foregoing pages. There is no doubt, however, that this interpretation is founded on a mistake, for other borings at Yarmouth, Lowestoft, and Beccles came to muddy sands and clays belonging to the upper part of the Crag, now known to thicken greatly eastward. The recent deposits descend only to a depth of about 50 feet at Yarmouth, and consist of sand and shingle; the beds below contain Pliocene mollusca. This emendation is also borne out by the entirely different character of the recent estuarine deposits at Potter Heigham, where we again find a submerged forest at about 56 feet below the marsh-level. The section recorded by Mr Blake is as follows:—
| feet | |
| Bluish-grey loam | 24 |
| Grey silty sand | ½ to 2 |
| Stiff bluish-grey loam, clay, and silt full of cockles, &c. | 13 |
| Black peat, hard, and much compressed | 17 |
| White and buff sand | 2 |
| 58 |
It will be noticed that here only one peat bed was found, and was at the usual depth of the lowest submerged forest. Possibly the white sand below was the bleached top of the Crag; but this point was not cleared up.
If we resume our journey northward along the Norfolk coast we come to the well-known locality of Eccles, where the old church tower described and figured by Lyell in his Principles of Geology long stood on the foreshore, washed by every spring tide. The position of this church formed a striking illustration of the protection afforded by a chain of sand-dunes. The church was originally built on the marshes inside these dunes, at a level just below that of high-water spring tides. But as the dunes were driven inland they gradually overwhelmed the church, till only the top of its tower appeared above the sand. In this state it was pictured by Lyell in the year 1839. Later on (in 1862) it was again sketched by the Rev. S. W. King, and stood on the seaward side of the dune and almost free from sand. For a series of years, from 1877 onward, I watched the advance of the sea, and as the church tower was more and more often reached by the tides, its foundations were laid bare and attacked by the waves, till at last the tower fell.
Not only were the foundations of Eccles church exposed on the foreshore, but an old road across the marshes also appeared on the seaward side of the dunes, giving a still more exact idea of the former great influence of the chain of dunes in damping the oscillations of the tidal wave. The tide outside now rises and falls some 12 or 15 feet; on the marsh within its influence is only felt under exceptional circumstances. A road across the marsh at a level four or five feet below high-water, as this one stood, would still be passable, except during unusual floods.
Eccles Church is an excellent example of the way in which an ancient land-surface may now be found below the level of high-water, and yet no subsidence of the land has taken place. But this coast can give even more curious examples. It does not need a sand-dune to deaden the rise and fall of the tides; even a submerged bank will have much the same effect. Extensive submerged sand-banks extend parallel with the coast, protecting the anchorage known as Yarmouth Roads. These banks rise so nearly to the surface of the sea that not only do they protect the town and anchorage against the waves, they deaden the tidal oscillation to such an extent that its range is much greater outside the bank than within.