MIDDLE AND UPPER ORDOVICIAN. 175 



Section of the Valley limestones near Lexington, Va. 



Ordovician: Feet. 



Liberty Hall limestone 1,000± 



Murat limestone 100-150 



Natural Bridge limestone 3,500+ 



The correlation of these divisions with those recognized both north and south 

 depends on detailed studies, in progress, from which it appears that the limestone 

 formations present differences of sequence and facies not previously suspected. 

 Bassler "" states : 



The name Shenandoah limestone proposed by Darton for the Valley limestone of early 

 geologists was made to include all the limestones in the Valley of Virginia occupying the interval 

 between the Cambrian quartzites and the Upper Ordovician shales. The lower portion of the 

 great limestone series had been found by Mr. Walcott to include Lower, Middle, and Upper 

 Cambrian rocks, but the Ordovician portion had been determined only to the extent that Tren- 

 ton strata were supposed to occur at the top. The work of the writer in Virginia brought out 

 the fact that the geologic succession of the Ordovician division was quite different in various 

 parts of the valley. ■ In northwestern Virginia a great thickness of Beekmantown is overlain by 

 1,000 feet of Stones River, and this in turn by 400 feet of Black River, while the strata bearing 

 Trenton fossils form the lowest division of the overlying shales. In central western Virginia the 

 Black River alone rests upon the Beekmantown, but in southwestern Virginia two distract 

 arrangements were noted. Along the western edge of the valley the Beekmantown is followed 

 by 1,000 or more feet of Stones River but no Black River, while along the eastern side only the 

 Black River occupies the interval between the overlying shales and the Beekmantown. In 

 each case the Trenton does not form the upper part of the limestone, but is the basal member 

 of the overlying shales. 



The Shenandoah limestone, therefore, is a broad term, embracing strata of Cambrian and 

 Ordovician age, the geologic succession of the latter varying in different parts of even the type 

 area. 



The classification of the Ordovician limestones of southern Pennsylvania as 

 worked out by Stose and Ulrich is given in Chapter III (pp. 107-111). There is no 

 reason to look for any marked difference in the section where the belt crosses 

 Maryland, but the divisions have not been traced. 



K 10. TAYLOBSyiLLE DISTRICT, CALIFGBNIA. 



The Grizzly formation, which occupies the summit of Grizzly Peak, in the Lassen 

 Peak quadrangle, California, is an unfossiliferous formation, 400 feet thick, which 

 underlies the Montgomery limestone. As the latter is considered by Walcott '^''^^' ^'^^ 

 to be of Niagara age, it is possible that the Grizzly formation is of Ordovician age. 



K 12. WASATCH AND TJINTA RANGES, UTAH. 



The quartzite of the Wasatch and Uinta ranges, distinguished by the Fortieth 

 Parallel Survey as Ogden quartzite and mapped as Devonian, is correlated by Weeks ^^ 

 and Ulrich as Ordovician. In the Wasatch section it rests upon limestone of Cam- 

 brian and Ordovician age, is 800 to 1,200 feet thick, and is overlain by Silurian 

 limestone, at least in Cache Valley. Along the south side of the Uinta Range it lies 

 upon the Lodore shale (Cambrian) and is immediately followed by limestone of lower 

 Mississippian age. No equivalent is known on the north side of the Uinta Range. 



