SEDIMENTARY ROCKS 467 



character are due to varying conditions of deposition; the variation in 

 thickness in part to this, but for the most part to the removal by erosion, 

 in Jurassic time, of the Triassic, Permian, and Pennsylvanian rocks 

 from northwestern Montana. 



The important, changes in character are mainly within the Quadrant 

 formation, which, in the Yellowstone National Park and central Mon- 

 tana, includes equivalents of the Amsden and Tensleep formations. The 

 Amsden, Tensleep, and Bmbar formations and their approximate 

 Montana equivalents, the Quadrant and Phosphoria, show great variation 

 in thickness, even when this can not have been caused by the Jurassic 

 erosion. The Amsden member is most persistent, being everywhere rep- 

 resented by a thin red shale at the base of the Quadrant. The Tensleep 

 sandstone, on the other hand, diminishes from .several hundred feet in 

 northern Wyoming and southwestern Montana to a few feet only, near 

 Billings, and either disappears entirely or is represented by shale in 

 central Montana. The Embar, or impure limestone phase, with the 

 phosphate horizon, merges eastward into the lower members of the Chug- 

 water, as Condit has described^^, but northward becomes thicker and 

 more shaly, exceeding 600 feet in Devils Basin, Cat Creek, and the Snowy 

 Mountains. From this maximum it thins rapidly to the northwest, still 

 uneroded, until in the vicinity of LewistoAvn it loses the protection of 

 the overlying Chugwater formation and is stripped from the region north 

 and west by the Jurassic erosion. 



The Embar (and partly equivalent Phosphoria) is the probable source 

 of most of the heavy oils found below the Cretaceous in Wyoming and 

 Montana. Its distribution and character bear directly on the explora- 

 tion for oil in the lower horizons. Hence its decrease in thickness in 

 north-central Montana and its removal from the northern part of the 

 State are facts of much importance. 



The Chugwater formation, overlying the Embar, bore in Montana the 

 brunt of the Jurassic erosion and is accordingly less widespread than the 

 Quadrant. Thickest to the southeast, where it reaches 600 feet, it de- 

 creases northward, with the beveling off of its upper surface, to 400 

 feet near Billings (reference, Duck CVeek well log), 320 feet in the 

 Snowy Mountains (measured section), and 250 feet near Cat (*reek 

 (West Dome well, number 1). A thin-section of red beds with gypsum, 

 overlying the Quadrant limestone at Belt Creek, may be an outlying 

 remnant. Westward the Chugwater decreases to 200 feet in the Yellow- 

 stone Park and disappears near Gardiner. To the west and north of 



^« U. S. Geol. Survey Prof. Taper 98, 1916, p. 263. 



