THE CAMBRIAN PERIOD 485 



area was largest and highest, and slower in the later stages after 

 the land surface had been worn down by erosion and narrowed by 

 the encroachment of the sea. Sedimentation was probably greater 

 near the" land, and less far from the shores in deep water. 



Sources and kinds of sediments. As in other geologic periods, 

 the land-derived sediments came from all formations then exposed 

 to erosion. The sediments along the immediate borders of the 

 land were doubtless different from those deposited farther from it. 

 Even along shore there were considerable variations, both because 

 of variations in the sources of the sediments, and because of differ- 

 ences in wave, river, and current action. 



The Cambrian formations include all common phases of sedi- 

 mentary rocks. There are conglomerates, presumably accumu- 

 lated near the shores of the time; there are sandstones, the sand 

 of which was deposited in shallow water where the waves were 

 sufficiently vigorous to keep the mud from settling; shales repre- 

 senting the deposits made in stiller or deeper water; and beds of 

 limestone representing, for the most part, the accumulations of 

 shells, etc., where terrigenous sediments were not carried in 

 quantity. 



Geographic variations in the sediments. The distribution of 

 these various sorts of sedimentary rocks shows that various kinds 

 of detrital beds were accumulating in different places at the same 

 time, and at the same place at different times. Not only this, but 

 they were accumulated at very different, rates. Thus the full sec- 

 tion of the Middle Cambrian (all that was deposited during the whole 

 of the Middle Cambrian epoch) seems to be present at many points, 

 yet the thickness of the Middle Cambrian strata is far from uniform. 

 Equal thicknesses of rock do not necessarily accumulate in equal 

 periods of time. 



The fact that in the northern interior of the United States the 

 Upper Cambrian formation is generally of sandstone, and that this 

 sandstone is wide-spread, indicates that the water was so shallow 

 during its deposition that the waves were competent to roll sand 

 long distances. Furthermore, the structure of the strata, with 

 their cross-bedding (Fig. 266), ripple-marks, etc., shows that the 

 whole pf the thick series from bottom to top was deposited in shallow 



