KHYTHMS IN SEDIMENTATION 779 



tinental platforms, ^\liereas mud tends in larger degree to be deposited 

 beloAv wave-base on tlie slopes of the submarine continental terrace. 



Wave-base is deeper with heavier wind action, witli l)readth of water 

 surface, with, distance from shore, and with fineness of sediment : a gravel 

 or sand would lie below wave-base, because remaining unagitated, in the 

 same locality and deptli where silt oi' mud woidd be well ^vithin Avave-base. 

 Conceive a shallow cireidar basin of water withoid" outlet. The action of 

 \vaves will tend to shift the bottom sediment and establish a ''profile of 

 equilibrium'^ witli steepest slope near shore and fiat in the middle. This 

 profile wdll var}^ in magnitude from that which is in adjustment with the 

 i-ollers of the open ocean to that in adjustment with the ^vm^i ripples on 

 the surface of a shallow pool, but for these wddely different scales it will 

 remain a curve of the same character. For purposes of comparison the 

 following is adopted as an empirical curve for the margin of a wide shelf 

 sea in mid temperate latitudes with a beach of sand and fine gravel. Far 

 offshore the bottom is takeji as line sand and mud. 



Profile of Equilibrium for ^helf Sea 



Distance from shore in miles 1 2 3*5 10 20 100 



Depth in fathoms 7.5 11 13.5 15.5 18 23 50 



For wide lagoons, such as those behind barrier beaches, both ordinates 

 and abscissas of this curve should be divided by about 20. Thus, at a 

 distance of five miles offshore the full depth of afiout 2.5 fathoms w^ould 

 ])e attained. 



The symmetry of this curve is greatly disturbed by a number of factors 

 which are always present to some degree. The curve is flatter on a shore 

 toward which sediment is driven; undertow sweeps the bottom material 

 in certain directions, giving current effects; Aai-iations in storms~^nd in 

 depth of water modify it from one scale to anotber. in general, the rise 

 of sealevel since late Glacial times, a rise which is generally estimated 

 at as much as 20 fathoms, has overdeepened tJie orean profiles and only 

 where there is a great abundance of unconsolidated l)ottoni mateiial has 

 the profile been adjusted to the present higher water level. The lack of 

 recognition of this condition has gi\("ji ;i general impression that wave 

 action is effective io a greater depth tlian is the case. The limit is com- 

 monly taken as 100 fathoms. Tu a few jfiaces, as west of the British Isles, 

 this is appit>ximately the case, but an inspection of coast charts in general 

 shows a downward curvature of the bottom profile, indicating the cessation 

 of effective wave action, at about 50 fathoms. Daly, Vaughan, and the 

 writer have come to the conclusion that this would be a better general 



