THE SHORE FAUNA OF CARDIGAN BAY. 103 



Large amounts of muddy fresh water are poured into Cardigan Bay- 

 by tlie numerous torrential rivers and streams which flow from the 

 mountain regions where the rainfall is heavy. Some of these (from N, to 

 S.) are the Soch, Rhyd-hir, Erch, Wen, Glaslyn, Dwyryd, Artro, Mawd- 

 dach, Dysynni, Dyfi (with subsidiary streams Einon, Clettwr and Leri), 

 Rheidol, Ystwyth, Wyre, Aeron, Teifi, and Nevern. Of these, the Glaslyn 

 and Dwyryd, the Mawddach, the Dyfi and subsidiaries, and the Teifi, 

 form estuaries of considerable extent. After heavy rain, the inshore 

 waters of the Bay are discoloured for some distance from the river- 

 mouths. This discolouration is chiefly to the northward of the river 

 mouths, owing to the surface waters being driven in that direction 

 by the prevailing rain winds. The junction between the surface 

 of the muddy fresh water (floating over sea water) and the clear 

 sea is frequently plainly visible. The Bay is bounded for the most 

 part by high land, but the continuity is broken by the deep clefts 

 and estuaries of numerous rivers. The coast-line comprises an alternat- 

 ing series of (1) steep, rocky clifls, consisting of Cambrian Ordovician 

 and Siliu'ian grits and shales, though there are some exposures 

 of igneous rock, (2) drift clift's of lower elevation, and (3) estuaries. 

 In each of these the characteristics of the tidal area are different. In 

 the first it is chiefly reef and hard erosion plane, with great variability in 

 local conditions according to the strike and dip of the rock, the amount 

 of exposure, and the quantity of detritus which washes to and fro in the 

 gullies between the rock-ridges. If the strike of the rocks is fairly 

 parallel with the coast, the dip of the rocks becomes an important factor 

 as regards the Fauna. If it is low, the whole surface is exposed to wave 

 action and the rocks are barren ; if it is high with a landward dip, they 

 are also barren ; but if high and seaward, there may be a fairly good 

 Fauna on the more sheltered landward slope. Outlying reefs may provide 

 shelter, even if submerged, because they break the force of a ground 

 swell and lessen the amount of wave-borne detritus. The shingle derived 

 from boulder clay and carried along the coast is often largely augmented 

 by detritus from the grit clifls of the locality. 



(2) Where drift clifls prevail, much of the foreshore consists of shingle 

 with large stretches, or low reefs of boulders, and local patches of coarse 

 sand. 



(3) In the estuaries, and often for some distance on either side, the 

 tidal area is sandy or muddy and the foreshore is dune-capped. More 

 recently there appears to have been a considerable influx of finer sand 

 (presumably from deeper water) along the greater part of the shore-line, 



