336 PROTEROZOIC ERA 



average depth of four miles below the surface, and if the angle of 

 deposition were 2, the computed thickness of the deposits, accord- 

 ing to the common methods of measurement, would be about 7 

 miles. If the delta were built out 1,000 miles, the computed depth 

 would be 35 miles, though the basin was but four miles deep. If a 

 delta were built half-way across a lake basin 100 miles wide and 1,000 

 feet deep, the angle of deposition being 3, the thickness of the 

 series, measured by the above method, would be 13,800 feet, though 

 the basin was but 1,000 feet deep. With these points in mind it is 

 clear that caution must be used in interpreting the great thicknesses 

 sometimes assigned to sedimentary systems. 



The sedimentary part of the Keweenawan system has commonly 

 been assumed to imply marine submergence; but so far as now 

 known the sediments may have been accumulated in an interior 

 basin, or may be partly subaerial. 



s 



Fig. 308. Diagram illustrating the development of the Lake Superior syncline. 

 JR, Archean; H and A, Huronian and Animikean; K, Keweenawan. (Irving, U. S. 

 Geol. Surv.) 



Deformative movements. About the close of the Keweenawan 

 period, the rocks of the system were somewhat deformed, and the 

 deformation in the Lake Superior region was perhaps contempo- 

 raneous with deformation in other parts of the continent. These 

 changes are regarded as marking the beginning of the end of the 

 Proterozoic era. As a result of these deformations, some parts 

 of the area where Keweenawan sediments had been deposited were 

 brought into such an attitude as to be eroded, but the changes did 

 not, as a rule, involve great folding or faulting of the strata. In 

 keeping with their structure, the rocks are not greatly metamor- 

 phosed. 



After the warping which followed the deposition of the Kewee- 

 nawan system, the exposed surfaces of this and older systems suf- 

 fered protracted erosion. Ultimately the land about Lake Superior 

 sank again, and when the sea came back, a new series of sedimentary 

 beds was deposited unconformably on the eroded surface of the 

 older. The waters of the returning sea teemed with life, for the 

 formation then made contains abundant fossils. This abundantly 



