86 G. H. Kinahan — On Wind and Currents. 



observed to travel upwards in oblique lines, formed of a series of 

 curves, while others move in curving lines along the beach. 



The following is the usual arrangement in a steeply-sloping beach 

 formed of mixed detritus. The highest spring-tide limits are usually 

 indicated by an accumulation of coarse gravel and shingle. A similar 

 band of coarser shingle in greater quantity being also found near the 

 low- water line of neap-tides. This lower band usually travels along 

 its own level, but some of its pebbles may travel upwards obliquely 

 to the spring-tide line, where they remain until disturbed by extra- 

 ordinary tides or storms. 



This travelling of beaches accounts for the accumulation of shingle 

 on that side of an artificial groin against which the current sets, 

 the larger fragments ascending a beach where the groins are near 

 together, being prevented descending by the groin, while the smaller 

 fragments are withdrawn by the back wash of the waves of trans- 

 lation. 



Should two groins be far apart, the materials will be sorted by off- 

 shoots from the on-shore waves, so that the large fragments will lie 

 against the second groin, while the finer materials will be deposited 

 behind the first. 



Many n-atural groins produce similar results; but as some of these 

 extend into deep water beyond the margin of the beach, the effects 

 will be different. A beach travelling with the general tidal current 

 will be drawn out into deep water to be carried in obliquely into 

 the next bay. If, however, the bay is long, narrow, and regularly 

 formed, the wash will l)e directly in and out, and at the heads of 

 such bays larger beaches generally accumulate than in more open ones. 



If there are a succession of bays, and the headlands between them 

 are formed of hard rocks, from which shingle is not easily derived, 

 the materials of the beaches will decrease in size from wear and 

 tear till they are eventually composed of fine sand, in the last 

 bays of a series, furthest from the source of the tidal current. This 

 refers solely to the driftage immediately along a coast-line, for there 

 might be a deep-sea driftage of coarse materials striking obliquely 

 on the coast, which would modify the above-mentioned results. 



So far the driftage considered was that of the tidal currents. The 

 action of these, however, could be modified or augmented by wind- 

 waves. 



Wind-waves, as shown by Scott Eussell, have usually no driftage 

 power, but locally they may be waves of translation. Incoming 

 tidal waves augmented by wind-waves proceeding in the same direc- 

 tion are capable of performing the maximum amount of driftage on 

 a coast-line ; while if the v/ind opposes the incoming tidal current, 

 it modifies the operation of the tidal waves or may temporarily 

 arrest it altogether. 



Tidal-waves and wind-waves when opposed to each other pile the 

 gravel and sand in ridges on the beach. The axes of these ridges 

 and intervening hollows are at right angles to the direction of the 

 opposing currents, when these have equal force and assume an 

 obliquity when one or the other current has a predominant strength. 



