THE TOMAH AND SPARTA QUADRANGLES 617 



have been drilled to this contact suggest that the pre-Cambrian 

 lands had been reduced to slight relief before the deposition of 

 the Cambrian sands. 



THE CAMBEIAN SYSTEM 



Sandstones compose nearly the whole of the Cambrian rocks of 

 the two quadrangles. Some of the finer-grained sandstones have 

 been called shales, but were most of such in association with true 

 shales they quite certainly would be called sandstones. True clay 

 shales are present only as thin laminae and lenses. Dolomite and 

 dolomitic sandstones of local and horizontal distribution also occur. 

 The basal strata of those exposed consist almost wholly of quartz 

 sandstones. The sands are commonly clean and well sorted. The 

 higher Cambrian strata are also largely composed of quartz sands, 

 but some portions contain an abundance of greensand. The top 

 of the Cambrian is quite sharply defined from the base of the 

 Ordovician by the rather general occurrence near the contact of 

 quartzite-like gray sandstones and by the great differences between 

 the dolomite of the basal Ordovician and the sandstone of the 

 highest member of the Cambrian. 



Four Cambrian formations are exposed. These, in ascending 

 order, are (i) the Dresbach sandstones, (2) the Franconia glauco- 

 nitic and shaley sandstones, (3) the St. Lawrence sandstones and 

 dolomitic shales with occasional dolomite layers, and (4) the 

 Jordan sandstones. There is a possibility that a fifth Cambrian 

 formation is present, since at the top of the Cambrian sequence 

 there are a few feet of strata which somewhat resemble the Madison 

 sandstone. The 350 feet of unexposed strata may contain the 

 Eau Claire and Mount Simon formations of Dr. Ulrich's section. 

 The total thickness of the Cambrian is about 900 feet (Fig. 2). 



The Cambrian sediments were deposited in shallow water above 

 wave-base. This is proved by the abundant occurrence of both 

 wave and current ripple mark and the wonderful development of 

 cross-lamination in which there are steep fore-sets up to 50 feet 

 in length. Mud cracks, which are present in at least three horizons, 

 prove that at times the sediments were exposed to the drying 

 effects of the sun. The rounding, sorting, and cross-lamination of 

 some of the sands suggest local eolian deposition. 



