52 C. R. Longwell — Geology of 



boring highlands of considerable elevation to furnish the 

 materials. The sandstone corresponds to the La Plata 

 Gronp of Arizona, New Mexico, and Utah ; and it is prob- 

 able that these widespread deposits of remarkably similar 

 character had a common source in a mountain range to 

 the south, perhaps the same highlands which furnished 

 materials for the Chinle formation. The sandstone is 

 here referred to the Jurassic period to agree with the 

 present classification of the La Plata Group. 



Cenozoic Formations. 



Tertiary deposits. — The general structural conformity 

 of sedimentary beds in the Muddy Mountains ends with 

 the Jurassic sandstone. Younger sediments record a new 

 order. A widespread surface of erosion was cut across 

 the upturned edges of Mesozoic and Paleozoic strata, and 

 on this surface the first record is found in a coarse, heavy 

 alluvial fan deposit which embodies fragments of all the 

 older local rocks. In its most typical phase the deposit 

 consists mainly of limestone fragments, angular or 

 slightly worn, with sand filling the interstices, and with a 

 firm cement of calcium carbonate. Beds are lenticular, 

 and fragments of many sizes are jumbled together with 

 evidence of only the rudest attempt at sorting. Lime- 

 stone masses 3 to 25 feet in diameter are not uncommon. 

 Exposures of the deposit are distributed over the entire 

 area, but the thickness varies greatly. Near Bitter 

 Spring it is 20 feet thick, whereas near Overton the thick- 

 ness is more than 3,000 feet. Adjacent to the Virgin 

 Mountains the formation has a nearly uniform thickness, 

 averaging 25 feet. Because of the peculiar character and 

 evident origin of the deposit, Lawson's term fanglomerate 

 is especially appropriate as a lithologic designation, and 

 in this paper the formation will be called the Overton 

 fanglomerate, from its typical development near the set- 

 tlement Overton in the Muddy Valley. 



The Overton deposits indicate rugged topography and 

 arid or semi-arid climate. Near the end of the fan stage 

 relief became more subdued, for the coarse sediments 

 grade upward into fine playa and lake deposits which have 

 a total thickness of at least 2,500 feet. These beds con- 

 sist of sandstone, limestone, magnesium carbonate, shale, 

 tufiP, and gypsum, and the change from one type of sedi- 



