THE TOM AH AND SPARTA QUADRANGLES 627 



undescribed stratigraphic unit.^ This supposition does not appear 

 to be correct. 



Where the St. Lawrence is not overlain by higher strata there 

 develops a rolling topography of gentle outlines. Where higher 

 strata are present there is a gently sloping surface on which rela- 

 tively resistant strata near the middle of the formation give rise 

 to discontinuous terraces. Quite commonly the upper beds of the 

 St. Lawrence form the basal portions of the higher tier of cliffs. 



Relations to the Franconia formation. — At all places where the 

 contact between the Franconia and the St. Lawrence has been seen 

 there is present an edgewise conglomerate composed of pebbles 

 derived from the upper greensand member of the Franconia. 

 These pebbles appear to have been indurated before their deposi- 

 tion, thus indicating the exposure and erosion of Franconia rocks 

 at the beginning of the St. Lawrence deposition. If such erosion 

 has occurred, as appears quite probable, it follows that the two 

 formations are separated by a disconformity. The variation in 

 thickness of the Franconia is in harmony with this conclusion. 



Characteristics.— In the exposures of the Tomah and Sparta 

 quadrangles the St. Lawrence formation consists largely of sand- 

 stones. Some of the beds of the lower half are more or less dolo- 

 mitic and shaley and others contain small quantities of greensand. 

 Farther to the southeast in the Wisconsin Valley the dolomite 

 content is greater, and one member of the formation is a dolomite. 



It is possible to divide the St. Lawrence into two members: 

 a lower, composed of shaley and fine-grained, light-yellow to light- 

 brown sandstone in which there are some dolomitic layers with 

 local lenses (i foot maximum thickness) of gray- and purple-spotted 

 dolomite and also a little greensand, and an upper, consisting of 

 fine-grained yellow sandstones in which are several layers (6 inches 

 to 4 feet) of conglomerate composed of fine-grained yellow sand- 

 stone pebbles of lenticular shapes in a matrix of yellow sandstone 

 of coarser grain. The pebbles are commonly in horizontal position, 

 but a conglomerate near the middle of the formation has them in 

 edgewise position with inclinations varying to nearly vertical. The 



'W. D. Shipton, "A New Stratigraphic Horizon in the Cambrian System of 

 Wisconsin," Proc. Iowa Acad. Set., XXIII (1916), 142-45. 



