STRATIGRAPHIC RELATIONSHIPS 



235 



shades by gradual merging of character and each break or interval 

 terminates in wedge-like form. 



In this Paleozoic interior basin the Mississippian sea of Cambric time 

 advanced slowly from the southward and in its marginal encroachment 

 spread out the great basal conglomerate and sandstone series that is so 

 prominent in the Upper Mississippi valley and the Great Lake region, 

 where it is of Upper Cambric to Lower Ordovicic age. With successive 

 oscillations of level, and accompanying variation in character and supply 

 of sediment, the whole series accumulated. They are largely sandstones 

 near the base and prevailingly limestones .at higher levels and at greater 

 distances from shore; but almost without exception the margin was 

 everywhere gravel and sand — a continually advancing sheet of conglom- 

 erate and sandstone, growing younger step by step with the northward 

 advance, a continuous lithologic formation, but, in its successive stages, of 

 very unlike time equivalence. It probably ranged from lowest Cambric 



Figure 3. 



■Generalised Sketch illustrating Relationship of interbedded Sandstones to 

 great Basal Sandstone Formation. 



in the southern United States continuously to and including the Saint 

 Peter of the Ordovicic on its original margins. 



With each recurring period of elevation of the land, or retreat of the 

 sea, or excessive waste supply, the sands crept seaward over the deeper sea 

 types of deposit; and with each subsidence and consequent advance of the 

 sea the sand was left behind as a continuous bed, to be covered in large 

 part by other sediments. Naturally enough the nearer to the margin of 

 the sea a locality is the greater prevalence of sand is to be noted in its 

 series of formations, and, other things being equal, the greater the number 

 and the greater the thickness of the interbedded wedge-like sandstone 

 formations. 



The Saint Peter is the fifth and last one of these sandstone formations 

 in the Upper Mississippi valley. 



Of great significance is the comparative thickness of the involved 

 formations in different parts of the region. In this discussion the 

 formations both above and below the Saint Peter must be included. 



