ON THE ESTUARIES OF THE SEVERN AND ITS TRIBUTAEIES. 611 



35. The Estuaeles of the Severn and its Tributaries ; an Inquiry 

 into the Nature and Origin of their Tidal Sediment and Allu- 

 yial Flats. By Professor W. J. Sollas, M.A., E.R.S.E., E.G.S., 

 Fellow of St. John's College, Cambridge. (Read June 6, 1883.) 



The tidal channel of the Severn is notorious for its mud. At high 

 tide it is filled with a sea of turbid water, thick and opaque with 

 tawny-coloured sediment; as the tide ebbs a broad expanse of 

 shining mud flats is revealed fringing the coast ; but so like is the 

 water to the mud that, seen from a distance, it is often hard to tell 

 where the sea ends and the shore begins. It is the same with its 

 tributaries, the Wye, the Usk, Ely, and Rhymney on the Welsh 

 side, the Avon, Teo, Parrot, and others on the English coast. 



The source of this mud has been made a subject of much dispute. 



That it is chiefly supplied by the rivers themselves to their re- 

 spective estuaries might sound to geologists like an obvious truth ; 

 but such is certainly not the opinion of those who have most closely 

 inquired into the matter. Engineers like Mr. C. Richardson and 

 Mr. Howard have long been of opinion that the sediment of the 

 tidal Avon is furnished to it by the Severn ; the like is asserted of 

 the Parrot, and I do not think one stands in any fear of contradiction 

 when stating as a general truth that all the estuaries opening into 

 the Severn derive their mud at least immediately from the main 

 channel. This being so, whence then has the Severn obtained it ? 

 The answers given to this inquiry by engineers are various : some 

 attribute it to the sea, meaning, it is to be supposed, the mouth of 

 the Bristol Channel ; some to the mud shoals of the estuary ; some to 

 its bordering cliffs ; and others to the fresh water of its tributary 

 rivers. 



There is, no doubt, truth in all these opinions, and the only mistake- 

 lies in regarding them as mutually exclusive, or in assigning to any 

 one source a larger share than its due. 



With regard to the fluctuating mud-banks in the channel, they 

 have been deposited by the tidal water, and will in time be washed 

 away again, and redeposited, and so on again and again. However 

 obviously a source of mud, they are certainly a long way from 

 being an ultimate source, and nothing is to be gained from their 

 further consideration. 



With regard to the relative share contributed by the remaining 

 agents, the view which geologists would take on general grounds is 

 no doubt correct ; the rivers which discharge into the Severn estuary, 

 draining, as they do, a catchment basin of 9193 square miles, are the 

 chief sources of supply ; but that much is produced by the waves 

 which wash the shores of the estuary, assisted, as they are, by sub- 

 aerial agents, is also clear, and to this the cliffs of Penarth, Aust, and 

 Portishead bear striking testimony. That the distant sea has contri- 

 buted anything at all is not an idea likely at first sight to find much 



