GEOLOGIC HISTORY 467 



sands and clays witli marsh vegetation. The first deposits now constitute 

 the Morrison formation — a widespread mantle of massive sandy shales 

 which extends through the Eocky Mountain province from Montana to 

 New Mexico. The materials were laid down in a shallow l)ody of fresh 

 water witli wide mud-flats, and the deposits Avere- mixtures of clay and 

 fine sand witli tlnn, irregular bodies of coarser sand deposited by streams 

 or currents. Occasional thin beds of impure carbonate of lime were also 

 formed. Huge dinosaurs were numerous, for their remains are now 

 found in abundance in the formation. 



Morrison time was succeeded in the early Cretaceous l^y a rapid change 

 to widespread coastal plain conditions, under which the coarse-grained, 

 massively bedded conglomerates and sandstones of the Cloverly (Lakota) 

 were deposited. Although the deposits change abruptly and there is occa- 

 sional local channeling of the surface of the soft Morrison shale, there 

 appears to have been only the small amount of erosion that would be ex- 

 pected from the strong currents which carried the coarse Cloverly de- 

 posits. If there had been any great interval of uplift and erosion follow- 

 ing Morrison deposition, the soft clay would have been widely removed. 

 The coarse deposits of the Cloverly sandstone were derived from sources 

 not clearly located and spread liy currents over a wide area. The coarse- 

 grained lower member, usually about 50 to GO feet thick, gives place to a 

 medial member of clay mostly of purplish color, not unlike the Morrison 

 beds, which is believed to represent the Fuson formation of the Black 

 hills. It appears to extend southward into a member of the Comanche 

 group. The top sandstone, resembling the Dakota sandstone of other re- 

 gions, indicates a resumption of the strong currents which deposited the 

 sands of the basal member of the formation. 



Following the deposition of the great sheet of sandy sediments of 

 Cloverly-Dakota time, there was a rapid change to clay deposition, of 

 which the first representative is the widespread Benton shale. This in- 

 augurated the vast Later Cretaceous submergence in which marine condi- 

 tions prevailed, and it continued until several thousand feet of clay were 

 deposited during the Benton, Niobrara, and Pierre epochs. In Benton 

 time there were occasional deposits of sand, and one of them, in the latter 

 part of the epoch, was general over the greater part of the northern Rocky 

 ^Fountain region. Anotlier marked episode was that which resulted in 

 the deposition of the thin Greenhorn limestone in the middle of the Ben- 

 ton sediments along the east front of the Rocky mountains from the Black 

 liills to New Mexico and far eastward into Iowa. The great extent of 

 this higlily characteristic limestone is an impressive feature, for it indi- 



XLIII — Blll. Geul. Soc. A.\r., Vol. ID, 1!)()7 



