352 CAMBRIAN PERIOD 



widespread, indicates that the water was so shallow that the waves 

 were competent to roll sand long distances. Furthermore, the 

 structure of the beds, with their cross-bedding (Fig. 199), ripple- 

 marks, etc., shows that the whole of the thick series from bottom to 

 top was deposited in shallow water, and therefore on a surface which 

 was depressed gradually, relative to sea-level, as the sand accumu- 

 lated. The limestone (chiefly dolomite) in the Upper Cambrian of 

 the southern and southeastern interior, points to clear seas, but per- 

 haps not to deep ones. The adjacent lands were perhaps too low 

 to yield abundant sediment. Limestone is also an important part 

 of the Middle and Upper Cambrian of the west, though clastic rocks 

 predominate in the Lower Cambrian. Where the Upper Cambrian is 

 limestone, it is, as a rule, not sharply differentiated from the over- 

 lying Ordovician. 



Outcrops of Cambrian 



The Cambrian formations were once as widespread as the Cam- 

 brian seas themselves, but they are not now present over all the 

 area they once covered. The areas where they are exposed are not 

 to be confused with the areas where they actually exist. Cambrian 

 formations are exposed, for example, in Wisconsin, Missouri, and 

 Texas; but the strata of Missouri are doubtless continuous, beneath 

 younger formations, with those of Texas, on the one hand, and with 

 those of Wisconsin, on the other (Fig. 310). 



Position of outcrops. The map (Fig. 310) showing the areas 

 where the Cambrian system is now exposed reveals several points 

 of significance: (i) Many of the outcrops are in association with 

 outcrops of the Archean and Proterozoic systems (Fig. 295). In 

 places, the exposed Cambrian lies along one border of the exposed 

 parts of these older systems, while in others it completely surrounds 

 them. This distribution is not peculiar to the Cambrian, but is 

 characteristic of most formations as compared with those of greater 

 age. (2) The exposed areas of Cambrian in the Appalachian Moun- 

 tains occur in parallel or subparallel belts (Fig. 310). This is the 

 result of (a) the folding to which the Cambrian and later strata of 

 this region have been subject, and (b) the erosion which the folds 

 have suffered. Fig. 314 will help to explain the repetition of out- 

 crops. In this diagram, A represents pre-Cambrian strata, - repre- 

 sents the Cambrian, and 0, S, D, and C, the Ordovician, Silurian, 

 Devonian, and Carboniferous systems, respectively. After the 

 strata were folded, erosion cut the folds down. A fold involving 



