NEW CAMBRIAN HORIZON IN WISCONSIN 14:'. 



ii; a gully one-half mile north of Middle Ridge in the north- 

 west corner of section 2, township 15 north, range 5 west. An- 

 other good exposure occurs along the road in Pine Hollow two 

 and one-half miles southeast of Melvina in the northwestern cor- 

 ner of section 19, township 15 north, range 3 west. 



The beds of the Sparta member consist of argillaceous layers 



of sandstone alternating with thin, fissile, arenaceous and cal- 

 careous layers, all with more than fifty per cent sand. The 

 arenaceous beds are mostly thin, but a few reach two feet in 

 thickness; the more limy layers are rarely more than one inch 

 thick. The layers apparently become more calcareous near the 

 Madison-Sparta contact. The fissile shales vary in color. Some 

 of the beds are of green glauconitie color, due to disseminated 

 grains of glauconite. Other beds which contain minute glis- 

 tening micalike scales and small black particles grade from a 

 light graj to a dark gray color. Where the beds are mainly 

 glauconite a greenish color is imparted to the soil. The layers 

 are distinctly laminated and break into thin plates. The laminae 

 in most places are horizontal; in many places a minor cross- 

 bedding is visible. The rocks tend to split alone- the lamina?, 

 which are formed apparently of the green grains of glauconite, 

 the diminutive micalike specks, and the minute dark particles. 

 Some fragments of calcite are found. The most shaly beds are 

 nicely ripple-marked, the markings being asymmetrical. A 

 type locality for the ripple-marked layers is two miles north - 

 31 of Sparta, along the Big Creek road in the eastern pari ol 

 section 9, township 17 north, range 4 west. 



The Sparta beds are used for quarrying purposes; the mem- 

 ber is called "Free Rock"-owing to its being quarried so easily, 

 the term being of strictly local application. 



This peculiar phase of the Potsdam, here called the Sparta 

 diale, appears not to have been given a distinct stratigraphie 

 horizon previously. Chamberlin 2 notes a stratum of shales at- 

 taining a known thickness of 80 feet somewhat above the middle 

 of the Potsdam formation. Above the shale is 150 feel of 

 sandstone which is overlain by 35 feel of shale and Limestone (the 

 Mendota limestone). In the Sparta region the shale stratum 

 reaches a total thickness of 200 feel and is overlain by the .Mad- 

 ison sandstone; the Mendota apparently is missing. This may 



2 Geology of Wisconsin, Vol. I, pp. 121-122. 



