FORMATIONS OF THE FRONT RANGE 433 



shale. At the top is a chalky bed, which weathers to a characteristic 

 bright straw yellow color. 



PIERRE SHALE 



Nearly all of the Great Plains portion of Colorado north of the Arkan- 

 sas river is underlain by a sheet of this formation, and it extends along 

 the flanks of the Rocky mountains south from Florence to Trinidad. 

 Its thickness is great, from 4,000 to over 7,000 feet, adjoining the moun- 

 tains; but it is much less to the east. About Denver and to the south 

 and north it is overlain by Laramie and still later formations. Its 

 character is remarkably uniform throughout, a dark colored clay or 

 shale, with local slight variations in color and stratigraphic com- 

 ponents. 



Lime concretions of various sizes are of frequent occurrence, and some 

 local beds of sandstone are included, which in the vicinity of Florence 

 and Boulder yield petroleum in considerable amount. The lime con- 

 cretions begin to be abundant above the first 400 or 500 feet of basal 

 members of the formation and continue to its top. In the Pueblo re- 

 gion these concretions are particularly abundant in a zone about 600 

 feet thick, lying from 400 to 500 feet above the base of the formation, 

 and carry fossils typical of the Pierre formation. Above this zone the 

 shales are paler in color and finer grained and concretions are abun- 

 dant, generally larger, and much more fossiliferous than those occurring 

 lower down. The fossils are often well preserved, showing the actual 

 shells with much of their original pearly luster. Baculites compressus 

 and large Inoceramus sagensis abound. In this zone to the south there 

 are found occasional large limestone concretions which give rise to 

 tepee buttes.* The characteristic fossil in the limestone is a small shell 

 known as Lucina occidentalism which is about an inch in width. Other 

 Pierre fossils also occur in the limestone. Above the zone of shales 

 containing the limestone lenses of tepee buttes there are several hundred 

 feet of dark shales to the top. In the Florence basin the formation 

 contains many thin, sandy beds carrying the oil, and its thickness is at 

 least 4,000 feet. To the south about Trinidad it is from 1,200 to 1,300 

 feet, according to R. C. Hills. West of the Denver basin the Pierre 

 shale appears to have a thickness of 7,700 feet, which is more than ob- 

 served anywhere else in the Great Plains region. Here the formation 

 contains local beds of limestone which are thin and a local bed of sand- 

 stone which varies from 100 to 350 feet thick. The sandstone is a soft, 

 friable, yellowish gray rock composed of quartz sand and more or less 



♦Described by G. K. Gilbert: Seventeenth Ann. Rept. U. S. Geol. Survey, part ii, p. 569. 



