THE STORY OF OUR ROCK FOUNDATION 



43" 



Tjvi Ditto, crodcdalongstrcams (I'l^l 



\a | 1.1 l^wcr Magncsian - - l-l'l 



^A Richmond shale - - | | 



[0| Niagara Limestone - - |[ ||| 



Kcwcenasvan, upper 

 Potsdam Sandstone 

 Lower Magnesian - 



o 



III 



Fig. 28. — Map of the exposed geologi- 

 cal formations in the Great Lakes area. 



St. Peters Sandstone - - - 

 Galena-Platteville Limestone 



Ditto, eroded along streams to [7=7] 

 St. Peters Sandstone - - - \Z\1\ 



Devonian subdivided in l/Vy 

 pari — lower, mid, upper f - /I 



Mississippian, sub- f\vl 



divided ------ ^ \ 1 



Pcnnsylvanian - - - \-\~\ 

 Quaternary . - - • 



