218 PHYSICAL GEOLOGY 



prevailing winds which determine the direction of the greatest shore 

 drift. Large particles are not carried far by the shore currents, but 

 the finer sand may be transported many hundreds of miles. 



Tidal Currents. — Tidal currents are often of great importance in 

 the removal of sediment (p. 221). When the tide flows through 

 narrow passages, as between islands, or in V-shaped bays, swift cur- 

 rents are developed which erode and carry away the mud, sand, and 

 gravel which come within their reach. Some tidal currents run so 

 strongly that divers are unable to stand against them. The out- 

 going tide has greater power than the inflowing, since the latter mov- 

 ing in as a great wave fills the bays above their normal level and 

 backs up the water of the rivers, often for long distances. On ac- 

 count of this accumulation of water an outflowing current begins 

 along the bottom before the tide is wholly in, and when the tide changes 

 this adds to the strong current which has already begun. Such 

 strong, outflowing currents tend to keep the channels deep and open, 

 and carry the mud and sand into deeper water. 



The transporting and erosive powers of the outgoing and incoming tides are, how- 

 ever, sometimes almost equally strong, as was shown by an examination of a steamer 

 which was sunk off the mouth of the Gironde River in France. The vessel rested on 

 her keel in 36 feet of water. At the end of the ebb tide the sands were so scoured as 

 to leave the hull supported only in the middle, but at the end of the flood tide the 

 vessel was again completely covered, the sand beds extending 100 yards fore and aft 

 of the vessel and 50 yards from each side. (Partiot.) 



Tides not only scour out channels but may also cause the deposi- 

 tion of the sediment which the rivers are carrying to the sea. It 

 often happens that sand flats are formed at the entrances of bays. 

 If a point projects on the side of the river mouth first reached by the 

 incoming tide, the tidal flow may carry the sediment far beyond the 

 mouth of the river ; but if no such point exists, the entrance may be- 

 come more or less choked. 



Features Resulting from Transportation 



Beaches. — When the sea has cut a rock terrace so wide that a strip 

 of sand and gravel is left between the clifF and the sea, a beach is 

 formed. Along coasts exposed to strong waves the breadth of the 

 wave-cut terrace must be much wider before sand is left on it to form 

 a beach than in quiet water, since in the former the sand and gravel 

 may be swept away as fast as formed even when the terrace is several 

 hundred feet wide. Wide beaches are usually first formed within 



