ESTUARINE DEPOSITS 663 



the Welsh side, the Avon, Yeo, Parrot and others on the EngHsh 

 coast." (SolIas-89:d/7.) 



The origin of this mud has been a subject of much dispute. In 

 part no doubt it is suppHed by the rivers which have a catchment 

 basin of 9,193 square miles (English), but much is also produced 

 by the waves washing the shores of the estuary. The water in the 

 tidal portion of the Severn Channel flows up and down twice daily 

 at the rate of from 6 to 12 miles an hour, a velocity much greater 

 than that required to move along large boulders of rocks. As a 

 result scouring of the channel occurs in many places. The devel- 

 opment of the currents has been described by Mr. W. R. Browne 

 (13) : "In ordinary tidal channels such as the Avon below Bristol, 

 the course of events during an ebb seems to be as follows. At first 

 the slope of the surface is exceedingly small (in the Avon it was 

 about 13^ feet in yy^ miles), and, while the velocity at the surface 

 is considerable, it diminishes rapidly from thence downward, and 

 at some distance from the bottom becomes nil. This continues for 

 about two-thirds of the ebb, the surface velocity increasing up to a 

 certain point, and then becoming nearly constant. During all this 

 time not only is no scour going on at the bottom, but, if the waters 

 be muddy, an actual deposition of silt is taking place. At this time, 

 after about two-thirds of the ebb, the water has fallen about three- 

 quarters of its total height, the slope of its surface has considera- 

 bly increased, and the conditions approximate to those of an ordi- 

 nary river. The bottom layers of the water then spring suddenly 

 into motion, the surface velocity diminishes steadily as the tidal 

 waters disappear, until it assumes the normal rate of the low-water 

 flow. During this period a scour of the bottom is of course going 

 on ; but, as this velocity is not much higher than in the subsequent 

 period of low- water flow, the rate of scour will not be much greater ; 

 and the actual scour will be insufficient to compensate for the 

 amount of deposit from the tidal waters which has taken place, not 

 only during the period of high water, but also during the first two- 

 thirds of the ebb. It must follow, therefore, that the scouring effect 

 of the tide is little or nothing, and the observed incapacity of tidal 

 flows to sweep away the silt they have deposited is amply and satis- 

 factorily explained." 



Sollas thinks that eventually much of the silt will find its way 

 out to sea, owing to the constant outward pressure of the normal 

 river current. As a part of the material is carried seaward from 

 the constant mass moved back and forth, new material is supplied by 

 the rivers. 



The material of tliis estuarine mud consists of "a variable quan- 



