SYLAMORE SANDSTONE. 



SYLAMORE SANDSTONE 



Southward the Grassy Creek shales thin and seem to dis- 

 appear in northern Lincoln County. A few miles south of their 

 southern margin the Sylamore sandstone occupies the same 

 horizon and has the same relationships to the underlying and 

 overlying strata. The Sylamore is less than one foot thick at 

 most of its outcrops, though in narrow elongated areas it is 30 

 to 70 feet thick and in a few places it reaches a thickness of 40 

 feet over several square miles. It is composed of medium to 

 fine-grained sand for the most part, but conglomerates are not 

 rare and many lenses of shale have been found. It is usually 

 gray to light brown in color but a light green phase is frequently 

 met with. It lies with marked unconformity on the Snyder 

 Creek shales, Callaway limestone, Mineola, Cooper, Plattin, 

 Kimmswick, and Joachim formations. The evidences of un- 

 conformity are overlap on many formations, striking irregularity 

 of the bottom, complete break in life, conglomerates at the 

 bottom, complete change in sediments, Sylamore sand filling 

 cracks in underlying rock, Sylamore sand filling old caves in 

 underlying rock. The Sylamore shows positive evidences of 

 having been deposited in a sea advancing over extensively eroded 

 rocks, some of which were of very late Devonian age. There can 

 be no question that the line between Devonian and Mississippian 

 should be placed below the Sylamore in central Missouri, and 

 that the Snyder Creek shale, the youngest underlying formation, 

 is Upper Devonian in age. 



Devonian seems to be absent from southwestern Missouri. 

 The Phelps sandstone referred to the Devonian by Shepard and 

 Ulrich on the basis of the Ptyctodus remains in it, is equivalent 

 to the Sylamore in central Missouri. The shales associated with 

 the Phelps seem to be only another phase of the same formation, 

 as they are in central Missouri. The Eureka shale of southwest- 

 ern Missouri should be referred to the Mississippian as the 

 Grassy Creek shales are referred in the northeast. 



The Eureka shales associated with the Sylamore are the 

 deposits of an advancing sea. The underlying rocks are St. 

 Peter sandstone or Ordovician dolomites in northern Arkansas 

 and southern Missouri as they are from Pettis County south- 

 ward and in many places in Cooper, Moniteau, Phelps, Boone, 

 Callaway, Montgomery and Warren counties. In the above 

 ^amed group of counties, the Sylamore usually rests on the 



