84 



THE SEVERN TUNNEL SECTION. 



Zone 2. a Lower Clay. 

 /3 Lower Peat. 



Zone 3. a Sands and Mad. 

 j8 Gravel. 



Concerning those deposits, Prof. Sollas says ■.^— 

 " The presence of glaciated stones in tlie gravel is very 

 suggestive. . . . Tlie sand is evidently a marine or 

 tidal deposit, like the blue clay, from wliioli it differs 

 chiefly in containing no argillaceous sediment, or scarcely 

 any. . . . The lower peat must evidently have accu- 

 mulated at a time when the relative level of laud and sea 

 was different from that of the present day. At New 

 Passag(;, for instance, it lies at half-tide level, and would 

 have been, under existing conditions, always submerged for 

 one-half the day. An elevation of twenty feet, however, 

 is all that is required to bring the layer of peat np to tlie 

 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 scom 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 depression ; ' and we can also account 

 for the peat by supposing tliat the downward movement 

 ceased for an interval, during which deposition continued 

 and led to the silting up of the creeks and bays of the 

 estuary to above the high-water level of ordinary spring 

 tides ; extraordinary spring tides would raise tho level 

 somewhat above 'high- water spring' mark, and a marsh- 



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