3g UNIVERSITY OF MISSOURI STUDIES 



the rule. In the Central district, where the Gasconade forma- 

 tion is well exposed, .thin beds predominate. The very thin beds 



are shaly. 



Fossils, aside from those in the chert, were found in a shaly 

 member of the Gasconade formation about 75 feet from the base. 

 These were found on a small tributary of Rocky Branch. They 

 were mainly Unguals. Associated with them are fucoids. The 

 trilobites and especially the gasteropods are much more numerous 

 in the chert than in the shaly member. 



The dolomite weathers into angular blocks or else into platy 

 forms. Much of the sandstone weathers into large blocks, some 

 of which are quartzitic. These sometimes show case-hardening. 

 The chert weathers into all kinds of shapes and various sizes, the 

 breccia fragments being the largest. Temperature changes 

 break it up into very small fragments. 



Age and correlation. — The Gasconade formation is probably 

 Lower Ordovician in age. While only a few fossils have been 

 found in it, those which appear (gasteropods, trilobites, and bra- 

 chiopods) are Lower Ordovician species. In central Missouri 

 there is a sandstone at the base of the Gasconade formation call- 

 ed the Gunter sandstone, which is included with the Gasconade 

 formation. In the Washington County area this sandstone was 

 not recognized. The sandstone members of this and the overly- 

 ing formation, the Roubidoux, are lenticular in character, and it 

 is not strange that the Gunter standstone does not extend to 

 Washington County. 



This formation is very persistent over a large part of the 

 Ozarks. Ulrich^ places the Gasconade in the Ozarkian, between 

 the Saratogan and the Upper Ozarkian, with an unconformity 

 above and below. If such unconformities exist they are not to 

 be seen in this region. 



Roubidoux formation 



There is a very small amount of the Roubidoux formation 

 (largely sandstone) in the district, its total area being probably 

 not more than a quarter of a square mile. It is in the extreme 

 southwestern corner of the area on the ridges above the North 



• Ulrich, E. O., ibid, p. 608, pi. 27. 



