PALEOZOIC ROCKS FOUND IN DEEP WELLS 545 



Mendota Formation 



Distribution. — The Mendota dolomite as defined by Ulrich 

 outcrops only in two small areas, one near Madison and the other 

 near Baraboo, Wisconsin, where it caps small hills and terraces. 

 It has not been definitely followed underground for more than a 

 score of miles to the east of Madison, but may extend much farther. 



Character. — The Mendota formation is a gray dolomite with 

 purple and greenish gray blotches; glauconite is sparingly present 

 and chert is nowhere found. In southeastern Wisconsin and 

 northeastern Illinois there is a considerable thickness of chert-free, 

 purple-spotted dolomite beneath the typical Oneota with its oolitic 

 chert and green shale specks. If, as is known to be the case with 

 the Jordan, the Madison loses its lithologic identity when followed 

 down the dip from its outcrop, this rock may be in part at least of 

 Mendota age.^ The maximum proved thickness of the Mendota 

 is about 20 feet, but if some of the non-cherty dolomite of Ilhnois 

 is of the same age, the thickness may be several times as much. 



^ The stratigraphic position of the Mendota has caused more diificulty than any 

 other problem within the area under discussion. Near Madison it is overlain by the 

 Madison sandstone. Two or three feet of dolomite below the massive layers of the 

 Mendota are regarded by Ulrich as the equivalent of the St. Lawrence or Black Earth 

 dolomite member of the Trempealeau formation of western Wisconsin but contain 

 no fossUs. Under these layers is the Mazomanie sandstone with the same character, 

 thickness, and with its top at exactly the same distance below the base of the Oneota 

 as in the western section. At the west end of Lake Mendota we have in ascending 

 order (a) Mazomanie sandstone, {h) red shale and glauconitic sandstone, (c) St. 

 Lawrence or Black Earth dolomite member, {d) Lodi sandy dolomite member of 

 Trempealeau formation, (e) Jordan sandstone, (/) Madison sandstone, and (g) 

 Oneota dolomite. Two mUes east we have a section so similar in lithologic char- 

 acter that were it not for the fossils described by Ulrich it would never have occurred 

 to anyone to question its equivalence. The only difference is that {d) is thin or locally 

 absent. According to Ulrich the testimony of the fossils places the Mendota dolomite 

 of the south and east shores of Lake Mendota in his proposed Ozarkian system, in 

 other words as younger than the Jordan sandstone. The fossils of the St. Lawrence 

 or Black Earth member of the Trempealeau formation are strikingly similar to those 

 of the Mendota although they show certain differences. The paleontological evidence 

 is, therefore, at the present date entirely unsupported by stratigraphic data in this 

 district. The exposures of Mendota dolomite near Baraboo do not show the same 

 adjoining formations as at Madison; indeed the stratigraphic section close to the 

 quartzite islands is in many respects quite different from elsewhere. It is therefore a 

 question whether or not the Mendota of Baraboo is the same as the original Mendota 

 dolomite at Madison; the correlation rests solely upon fossils. The entire question 

 is fortunately of Uttle importance in the study of well records since the Mendota is of 

 very limited occurrence. 



