May 12, 1898] 



NATURE 



43 



waves. It is owing to its persistent motion that sea-sand accu- 

 mulates in vast banks where it is flung by the sudden bending 

 or 1 checking of currents {e.g. at tidal nodes), or where it is 

 dropped during tumultuous mixing of waters. 



The wash of the waves, owing to percolation, piles up the 

 pebbles thrown forward by the breaker, forming a bank, or 

 ridge, or Full, and this is the action proper to the sea on a shore 

 of shingle. 



The piling up of the ridge goes on, its height and steepness 

 increasing, until the wash can reach no higher, and the steepness 

 of the ridge at each point is such that the assistance which gravity 

 gives to the down-flowing surface stream counterbalances the 

 loss of transporting power due to percolation at that level. This 

 is the equilibrium profile or regimen of the Full. Now, the greater 

 the volume of water flung forward by the breaker, the greater 

 is the depth of the back-flowing surface stream, and thus for the 

 same size of beach material the carrying power of the back- 

 wash is more nearly equal to that of the on-wash. Consequently, 

 in a given locality, the regimen slope of beach proper to a rough 

 sea is not so steep as that for a quiet sea. 



It is evident that the greatest amount of transport can occur 

 when the sea acts upon the greatest quantity of shingle — that is 

 to say, when the sea is at its highest level. The transporting 

 power increases in a more rapid 

 ratio than the rise of level, owing 

 to the circumstance that most of the 

 shingle is accumulated on the land- 

 ward side of the beach, where its 



thickness is greatest. It follows that 



a wind blowing in the direction of 



the flood tide will have an advantage 



in shingle-transport over the wind 



which blows with the ebb ; for the 



former, by opposing the turn of the 



tide, tends to increase the duration of 



tidal high water, and to diminish the 



duration of tidal low water. Thus, 



although the forces of currents may 



be equal and opposite in the two 



cases, the opportunities of action on 



shingle are greater when the wind 



blows with the flood tide. Again, 



the waves break most violently on 



the steep beach near high -tide mark, 



which further increases the effect of 



prolonged high water in promoting 



transport The along-shore wind 



which is accompanied by a low 



barometer has a corresponding ad- 

 vantage of opportunity over the 



along-shore wind which is accom- 

 panied by a high barometer, and 



the wind along-shore which blows 



from the greater expanse of water 



over the wind which blows from the 



number of fine grains than by the attrition of a small number of 

 coarse grains. This increase in coarseness will continue until 

 the beach material is brought to a uniform size, when the grading 

 proceeds as in i . 



Law 3. — If the material be mostly coarse stuff", with a small 

 admixture of fine stuff", then, as we recede from the extremity, 

 the grain of the beach will become finer, for the attrition of a 

 great number of large particles has a greater effect upon the 

 average size of the material than the removal of a small number 

 of fine particles. 



By combining 2 and 3 we can deduce corollaries applicable to 

 the case of a beach fed from both extremities. 



Zaw 4. — The grain of the beach is {ceteris paribus) coarser 

 where the beach is exposed to the heaviest breakers. This law 

 follows from what has been said on the action of the back-wash, 

 and on a "critical size " of beach material. 



Law 5. — The grain of the beach is {ceteris paribus) coarser 

 near the "weather" end of a promontory. Thus, if west be the 

 weather side, and the end of a long beach is protected from the 

 east by a headland at the eastern extremity, then both large and 

 small pebbles will travel eastward along the beach in a westerly 

 wind, but only the small ones are carried back from the pro- 

 montory during an east wind, so that the proportion of large 



[Ii'>i»ii a photogiaph by ihf author. 



-East end of Chesil Beach. 



No stony particle of less than a certain critical size can remain 

 permanently on a beach, but is ultimately swept out to sea. 

 This critical size is greater on a coarse-grained than on a fine- 

 grained beach, for the regimen slope of the former is steeper, 

 and gravity therefore gives greater assistance to the back-wash. 

 It is well iinown that every particle upon the surface of a beach 

 suff"ers attrition, whence the conclusion has been too hastily 

 drawn that the grain of an isolated beach naturally becomes 

 finer as the distance increases from the extremity where the beach 

 is fed with detritus. Now, it is to be noted that whereas the 

 attrition of the particles tends to lower the average size of the 

 shingle, and hence to make the grain of the beach finer, the 

 removal of particles of less than the critical size raises the average 

 dimension of the shingle. Hence we may deduce the following 

 laws of grading of beach shingle applicable to a beach fed 

 entirely at one extremity, whence the material travels along the 

 beach : — 



Law I. — If the material be of uniform size, the grain of the 

 beach becomes finer as we recede from the extremity. 



Law 2. — If the material be mostly fine stuffs, with a small ad- 

 mixture of coarse stuff, then (unless the coarse stuff" be very 

 friable, and the fine stuff" very durable) the grain of the 

 beach will become coarser as we recede from the extremity, for 

 the average size is more aflfected by the removal of a large 



NO. 1489, VOL. 58] 



pebbles to small is increased as we near the promontory from 

 the west. This is, in fact, similar to the case of the sorting of 

 sand from shingle by unsymmetrical oscillation. 



The author considers that the chief factors which determine 

 the observed grading of the Chesil Beach are as follows : — 



(i) The beach is fed at both ends (Bridport and Chesilton). 



(2) The material fed in at the west end is mostly fine, owing 

 chiefly to the natural groynes at Golden Cap and Thorncombe. 



(3) The material fed in at the east end is mostly coarse, owing 

 to the nature of the local rock and the mode in which it is 

 supplied to the foreshore. 



(4) The main drift of water is easterly, but 



(5) Of the fine shingle carried eastward from Bridport, much 

 is brought back by waves from the east ; whereas 



(6) The strong outset at Chesilton removes such fine stuff" as 

 may be there supplied from Portland. 



(7) The largest waves converge on Chesilton from both sides. 

 The formation of a beach-ridge, or Full of sand, is well seen 



when the sand is being brought in during off-shore winds. Sand 

 being readily raised by upward-swirling water (which is 

 equivalent to suction dredging), the building up of a Full of 

 sand in front of the breaker is accompanied by the excava- 

 tion of a trough, or Low, at the back of the breaker. This is 

 roughly similar to the simultaneous excavation and elevation 



