354 



CAMBRIAN PERIOD 



Width of outcrops. The widest outcrops of the Cambrian 

 (Fig. 310) are in Wisconsin; yet there the Upper Cambrian only is 

 present, with a thickness of less than 1,000 feet, while in the Appala- 

 chian Mountains, where the system has an aggregate thickness of sev- 

 eral thousand feet, it appears at the surface in narrow belts ; that is, 

 the outcrops are narrow in the east where the system is thick, and 

 wide in the interior where it is thin. The explanation of this appar- 

 ent anomaly is found in the attitude of the strata. In Wisconsin 

 they are nearly horizontal, while in the mountain regions, both east 

 and west, they are tilted at high angles. Where strata are vertical, 



the width of their 

 outcrop on a hori- 

 zontal surface is 

 about the same as 

 the thickness of 

 the beds (, right- 

 hand side of Fig. 

 316); where they 

 are nearly hori- 



Fig. 316. Diagram illustrating the influence of dip 

 on the width of outcrop. The Cambrian beds, , to the 

 left have a much wider outcrop than the Cambrian beds 

 to the right, though the thickness is the same. 



zontal, (~, left-hand side of Fig.) the width of outcrop on a hori- 

 zontal surface is much greater. It is not to be inferred, however, 

 that horizontal strata always have a wide outcrop. The width of 

 outcrop is also influenced by topography, as shown in Fig. 317. 



aJ 



Fig. 317. Diagram illustrating the effect of topography on width of outcrop. 



Here the horizontal stratum between B and C has about the same 

 thickness as - of Fig. 316, but its outcrop is narrow. In general, 

 the width of outcrop, so far as determined by topography, depends 

 on the angle between the bedding-planes and the surface where the 

 formation outcrops. The width of the outcrop decreases as this 

 angle increases. 



Changes in Sediments Since Deposition 



The sediments of the Cambrian system have undergone change 

 since their deposition. In most regions they have been compacted 

 and cemented into solid rock. Over great areas in the interior 



