KHYTHM8 IN SEDIMENTATION 78?) 



transportation of land waste, the local baselevel is the slope of the graded 

 stream. This is subject to seasonal fluctnation. The depth and strength 

 of current determines how close to the water surface the surface of sedi- 

 mentation will approach. In the season of flood there is a deepening of 

 the baselevel within the channel, as marked by the down-scouring. Over 

 the floodplain a rise of baselevel accompanies the flood waters and deposi- 

 tion results. In the season of low water the action is reversed; then a 

 silting up of the channel and a washing of detritus from the floodplain 

 marks the seasonall}' lower baselevel and slackened current. For the sea- 

 waters the factors leading to the oscillation of wave-base have already been 

 discussed. 



The sediment deposited at any one point is only a small fraction of that 

 which is carried past. This small proportion which remains represents 

 the rhythmic disturbance of a balanced condition. At a time of sink- 

 ing baselevel, using this as synonymous with the surface of deposition, 

 scouring will result, which means that a little more passes beyond the 

 point than comes to it. At a time of rising baselevel the disturbance of 

 the balance becomes additive and the geologic response is deposition. 

 • The maintenance of shallow- water conditions during the accumulation 

 of a formation means that there has been nearly always an excess of sedi- 

 ments above what was needed to maintain the surface at baselevel. This 

 excess has been generally swept to the abyssal slopes of the continental 

 platforms, building a continental terrace wdiich has been the storage 

 ground for the excess of w^aste. The deposits of the terrace have been, 

 however, more commonly hidden from observation by being Avarped or 

 faulted clown toward ocean depths, rather than elevated, and in conse- 

 quence exposed to erosion and observation. 



In the sorting by wave action the coarser materials, as noted, tend to be 

 kept on the landward side of the marine profile and therefore are deposited 

 at wave-base; the suspended matter tends to be deposited in the deeper 

 water farther from shore and partly below wave-base. AVhere there is 

 an excess of sediment leading to deposition on the fore-set slopes of the 

 continental terrace it must be to a large degree the finer material, giving 

 the red and blue muds, mostly settling within a belt 100 miles wide be- 

 yond the outer edge of the shelf seas. A test, then, of the amount of sort- 

 ing and deposition of sands in shallow water may be found by comparing 

 the ratio of sandstones and shales as they occur in exposed geologic sec- 

 tions with the ratio to l)e expected from the erosion of the average igneous 

 rock. Leith and ^Slead have made such a comparison. They find that an 

 average of sections aggregating 520,000 feet, well distributed over North 

 America, contains 46 per cent shale, 323/^ per cent sandstone, and 31% 



