xiii Action of Water on tJie Land 249 



marine formations consisting of shingle or pebbles ridged 

 by the waves, but most of them are due to a combination 

 of river and sea action. When rivers enter a tidal sea by a 

 comparatively wide shallow estuary, such as the Tay, Mersey, 

 or Thames, sandbanks are formed, the size, position, and 

 shape of which depend on the amount of sediment brought 

 down and the form of the coast -lines which guide the tidal 

 currents. Professor Osborne Reynolds, in a series of beauti- 

 ful experiments, shows how, in a small flat -bottomed model 

 of an estuary, the floor of which was strewn with fine sand, it 

 was possible, by causing mimic tides to stream to and fro 

 in rapid succession, to rearrange the sand in banks with 

 channels between', precisely like those of the real estuary 

 represented. 1 In lakes and seas not subject to strong tides, 

 such as the Baltic, Black Sea, Mediterranean, and Gulf of 

 Mexico, the sediment thrown down by rivers is not swept 

 away, but accumulates like a railway embankment in course 

 of formation until it rises to the level of the sea. The 

 action of waves piles up the deposited mud into low islands 

 on which vegetation takes root and assists to raise the level 

 by forming vegetable mould. These islands split the river 

 into numerous branches, which interlace with one another 

 sometimes in a very complicated way. The typical delta of 

 the Nile originated the name, for below Cairo the river 

 splits into two main branches which enclose a triangular 

 piece of land like the Greek letter A (delta) in form, the 

 broad growing edge of the delta, 180 miles long on the 

 Mediterranean, being the base of the triangle. The 

 Mississippi delta grows much more rapidly than that of 

 the Nile. It forms a long narrow peninsula spreading out 

 into a series of branches, each traversed by an arm of 

 the river and all constantly varying in size and position. 

 When the amount of sediment is very great, deltas are formed 

 even in tidal seas, as, for example, where the Ganges and 

 Brahmaputra meet at the head of the Bay of Bengal. 

 The Adriatic Sea is being filled up so rapidly by the 

 sediment of rivers descending from the Alps and Apen- 

 nines that the coast is lined by a broad belt of new land 

 interposing a stretch of 14 miles between the present 



