540 F. T. THWAITES 



Wisconsin fresh rock is found immediately below the drift, so that 

 the characteristic weathered phases of the west are wanting. 

 The base of the Black River is sandy in many localities, and along 

 a belt which runs south from Milwaukee parallel with the Lake 

 Michigan shore, there is a distinct bed of sandstone near the bottom 

 of the group; this is known to well drillers as the "Trenton Stray 

 Sand." It has a maximum known thickness of about 30 feet and 

 consists of rather coarse-grained gray calcareous sandstone. This 

 rock differs from the St. Peter in greater size of grain, lack of chert 

 fragments, and in its dolomite cement which is locally sufficient to 

 cause the rock to break in thin chips under the drill. The "stray 

 sand" is confined, so far as known, to an area where the thickness of 

 the Trenton and Black River groups is over 300 feet, although 

 it is not present everywhere that the dolomites reach that thickness. 

 It is probable that locally it lies directly upon the St. Peter, but 

 where it can be distinguished with certainty, there are a few feet 

 of more or less sandy dolomite between the two sandstones. There 

 are some evidences that the "stray sand" is present elsewhere in 

 Wisconsin. The thickness of the entire Trenton and Black River 

 groups varies from 179 to 450 feet. 



Big Buffalo Series 

 st. peter formation 



Distribution. — The St. Peter formation outcrops in the Galena- 

 Black River escarpment, along valleys which cut the back slope of 

 the cuesta, and in small inliers as far south as La Salle, Illinois. 

 Being a soft formation it forms either steep slopes with occasional 

 cliffs and crags, or low broken country with many small knolls. 

 The extent in Illinois is greater than indicated on existing geological 

 maps, for well records show it to lie immediately below the drift 

 of valleys as far south as Rockford. 



Character. — The St. Peter is dominantly a light gray or yellowish 

 gray, fine to medium grained, more or less dolomitic sandstone; 

 below the sandstone are beds of purplish red and green shale 

 interstratified with layers of white disintegrated chert, and con- 

 glomerate with chert and limestone pebbles in a matrix of fine to 

 coarse sand. These basal beds cave very badly in wells and must 

 generally be cased off. At Shullsburg, Wisconsin; Galena, Illinois, 



