624: PROE. W. J. SOLLAS OK THE ESTUARIES OF 



required to bring the layer of peat up to the same level as that of 

 the existing surface of the ground ; and more than this cannot, I 

 think, well be allowed, for the tidal deposits lying conformably 

 above and below both beds of peat seem to point to a simple movement 

 of depression interrupted by occasional pauses. The tidal mud 

 beneath the peat proves that during its formation the margin of the 

 land was below water, just as surely as the layer of peat proves 

 that during its formation it was above. We can account for the 

 silt on the theory that it was a case of " deposition during de- 

 pression ; " and we can also account for the peat by supposing that 

 the downward movement ceased for an interval ; during which 

 deposition continued and led to the sil ting-up of the creeks and 

 bays of the estuary to above the high- water level of ordinary 

 spring tides : extraordinary spring tides would raise the level some- 

 what above " high-water spring " mark ; and a marsh-growth 

 would raise it somewhat higher still. The cleansing of muddy 

 water on passing through the reedy margin of a marsh has been 

 alluded to by Sir Charles Lyell, and may be invoked here to account 

 for the general purity of the peat, which, nevertheless, does contain 

 some sponge-spicules, Foraminifera, and sand-grains, though few. 



After a long pause, during which the lower peat, with its asso- 

 ciated remains of forests, was formed, subsidence set in again, and 

 the lower blue clay gradually accumulated over the layer of vege- 

 table matter. 



Another interval of rest succeeded, during which a new alluvial 

 surface rose above the tide, and the upper bed of peat was pro- 

 duced ; again the movement of depression was renewed, and appa- 

 rently not uniformly, since the lower part of the upper clay contains 

 scattered fragments of plants which were probably derived from some 

 exposed portion of the peat, which the waves ploughed to pieces and 

 distributed far and wide. During the continuance of the depression 

 the upper gravel bed at Bridgewater was laid down, probably through 

 local current-action due to the change in the configuration of the 

 estuary consequent on its depression. Finally, the upper part of 

 the blue clay was deposited ; and this is more arenaceous than the 

 lower part of the same deposit, because the continued depression had 

 brought the sea nearer the area of deposition. The tidal waters, as 

 the land sunk, extended further and further up the estuary and its 

 tributaries ; and, meeting the upland waters further inland, sooner 

 received their supplies of sediment. But it was from these muddy 

 tidal waters that the silt of the alluvial flats was supplied, and thus, 

 the source of sediment being shifted nearer the area of deposition, 

 we have, according to a well-understood rule, coarser succeeding to 

 finer sediments. 



Thus then we explain the origin of the alluvial tracts of the 

 Severn. Save for differences of level, they have been formed in the 

 same way as tidal deposits are accumulating at the present day ; in 

 the past as in the present the tidal waters rose and fell in the 

 estuary, transporting with them sediment of a double origin, the 

 joint product of the land and sea. 



