388 DYNAMICAL GEOLOGY. — [Boox IIL 
la Beche described an example from the coast of South Wales where 
two streams, the Towey and Nedd (a and }, Fig. 128), enter Swansea 
Bay, bearing with them a considerable amount of sandy and muddy 
sediment. ‘The fine mud is carried by the ebb-tide (¢¢ ¢) into the 
sheltered bay between Swansea (c) and the Mumble Rocks (e), but is — 
partly swept round this headland into the Bristol Channel. The — 

Fig. 128.—Action or Rivers, TIDES, AND WINDS IN SWANSEA Bay (B.). 
coarser sandy sediment, more rapidly thrown down, is stirred up and: 
driven shorewards by the breakers caused by the prevalent west and 
south-west winds (w). ‘The sandy flats thereby formed are partly — 
uncovered at low water, and being then dried by the wind, supply it. 
with the sand which it blows inland to form the lines of sand-— 
dunes (f/).* 
(f) Deltas in the Sea.—The tendency of sediment to accumulate 
in a tongue of flat land when a river loses itself in a lake is exhibited | 
on a far vaster scale where the great rivers of the continents enter — 
the sea. It was to one of these maritime accumulations, that of the 
Nile, that the Greeks gave the name Delta, from its resemblance to 
their letter A, with the apex pointing up the river, and the base front- 
ing the sea. This shape being the common one in all such alluvial — 
deposits at river mouths, the term delta has become their general 
designation. A delta consists of successive layers of detritus, brought. 
down from the land and spread out in the sea at the mouth of a river 
until they reach the surface, and then, partly by growth of vegeta- 
tion and partly by flooding of the river, form a plain, of which the. 
inner and higher portion comes eventually to be above the reach of 
floods. Large quantities of drift-wood are often carried down, and, 
bodies of animals are swept off to be buried in the delta, or even to 
be floated out to sea. Hence, in deposits formed at the mouths of. 
rivers, we may always expect to find terrestrial organic remains. __ 
A delta does not necessarily form at every river-mouth, even, 
where there is plenty of sediment. In particular, where the coast- 
* “Geological Observer,” p. 88. » 

