

112 STRUCTURAL AND FIELD GEOLOGY 
and interosculating lenticular sheets. Thicker and more continuous 
strata of sedimentary origin behave in precisely the same way, the beds 
of coarsest materials thickening out and thinning off more rapidly than 
the fine-grained deposits. Such being the manner in which strata are 
arranged, it is obvious that sections taken across the same series of 
strata at different places will not often show the same number of beds, 
or if all be present, they will probably vary in thickness. In the following 
section (Fig. 7), for example, we have from @ to d@ an apparently con- 


FIG. 7.—THINNING-OUT OF STRATA. 
secutive series of beds, and there is nothing at that end of the section 
to show that the several separating planes of stratification do not 
represent similar intervals. Yet we see that one plane (x, +) really 
indicates a longer interruption or pause in the process of sedimentation 
than the others, a pause of sufficient duration to permit of the accumula- 
tion of the beds bracketed at 2. 
Contemporaneous Erosion.—The accumulation of rela- 
tively shallow-water deposits rarely goes on without interrup- 
tion, for the currents which transport and lay down sediment 
not infrequently vary this action by scouring it out again 
and retransporting it elsewhere. Thus, during the formation 
of lacustrine, estuarine, and littoral and sublittoral deposits, 
accumulation and erosion often alternate. In the accompany- 
ing section (see Fig. 8) we have a series of begs) aime 
accumulation of which has obviously been arrested at intervals. 
The bottom stratum, consisting of sandy clay (¢), points to 
deposition in relatively quiet water. After such conditions 
had obtained for some time, the accumulation of fine sediment 
suddenly ceased, and the area of deposition was traversed 
by a stronger current which trenched and furrowed the 
stratum of sandy clay. As the force of this current declined, 
sand (s) began to be distributed over the denuded surface 
of the clay, and eventually attained a considerable thickness. 
Eventually, however, the speed of the current once more 
