SAND-WAVES IN TIDAL CURRENTS 295 



above the bridge, the greatest depth of water upon 

 it, according to the chart, being 5 feet at high 

 water of neap tides and 9 feet at high water of 

 spring tides. The thickness of the sandbank itself, 

 or at all events of the soft deposits overlying rocky 

 bottom, is here about 60 feet, as has been proved 

 in sinking for a solid foundation to the piers of 

 the railway bridge. 



Similar sand-waves were seen on the shoals ex- 

 posed at low tide on each side of the narrow loW- 

 water channel between the island of Ynys y Bawd 

 and the town of Barmouth, but whereas on the 

 town side the ridges faced with the flood, on the 

 other side of the channel they faced with the ebb. 

 The Mawdach estuary has a very narrow entrance, 

 but when the tide has risen above the level of the 

 sandbanks it spreads over a wide interior basin. 

 Another character of the estuary is that a shingle 

 spit has grown out from the south, which is the 

 side on which there is the more open expanse of 

 sea, forcing the low-water channel close against 

 the hills on the north. This shape of estuary is 

 connected with the formation and preservation of 

 sand-waves upon the drying shoals, for it has the 

 effect of checking the current over them during the 

 first part of the flood tide and last part of the 

 ebb, allowing the currents, however, to flow strongly 



