GRASSY CREEK SHALE. 5 



million and ten million years in different sections. During 

 this time some 5,000 feet of sediments were deposited in places 

 in New York and Pennsylvania. Only in Pike County does the 

 Devonian rest on rocks as young as Silurian and every place 

 else it is on Ordovician formations. In most places the Ordovician 

 rocks were thrown into small folds before they were eroded and 

 the Devonian seas came over them. Commonly the Devonian 

 limestones rest on dolomites or sandstones, but in some cases 

 they are on limestones. 



The unconformity between Devonian and older rocks is 

 indicated in several ways. A clear-cut erosion surface of the 

 older formations showing erosion channels and truncated folds 

 is common. (See Plate D, a.) The overlap of Devonian forma- 

 tions on several older formations is the rule. In Callaway County 

 The Callaway limestone rests on Jefferson City, St. Peter, 

 Joachim, Plattin, and Mineola formations, all in a radius of ten 

 miles. Any distance of ten miles would show Devonian rocks 

 on two or three older formations. There is an almost complete 

 break in life between the underlying formations and the Devon- 

 ian. 



The top of the Devonian is not as well defined as the bottom 

 and is placed at the contact with the Grassy Creek shales in 

 northeastern Missouri and at the contact with the Sylamore 

 sandstone in the central part of the State. 



GRASSY CREEK SHALE 



The Grassy Creek shales have usually been classed with 

 the Devonian, but seem to belong with the Mississippian. By 

 this I do not mean to imply that they may not be of the same 

 age as some of the formations east of the Cincinnati arch that 

 have usually been classed as Devonian, but that they were 

 formed in a sea that advanced widely following a widespread 

 land emergence and considerable deformation, and their sedi- 

 mentation was continuous with that of the Mississippian. 



The Grassy Creek shales range from a few inches to more 

 than 60 feet thick, and consist mainly of black and blue shales 

 with which thin layers of sandstone are imbedded near the top. 

 The shales are exposed only in Lincoln, Pike, Ralls, and Marion 

 counties and lie unconformably on Mineola and Cooper of the 

 Middle Devonian, Edgewood and Sexton Creek of the Silurian, 

 Kimmswick and Flattin of the Ordovician. The unconformity 



