CHAPTER XII. 



THE FISn LAKE PLATEAU— THE AWAPA.— THOUSAND LAKE 



MOUNTAIN, 



Southern extension of the Wasatch monocline across Saliua CaBo:i.— Its bifurcation into the Sevier and 

 Grass Valley faults.— Strawberry Valley.— Ascent of the northern slopes of Fish Lake Plateau.— 

 Summit Valley.— Tertiary exposures.— Fish Lake Plateau.— Its summit.— The great gorge and 

 cliffs. — Sources of the volcanic sheets. — Origin of the gorge. — Fish Lake. — Moraines. — Koversal of 

 the course of the drainage.— Alcoves in the plateau wall.— Succession of beds.— Trachytes and 

 dolerites.- Augitic andesites.- Location of the vents and sources of the lavas.— Outlet of the lake.— 

 Mount Terrill.— Mount Marvinc.— Origin of Summit Valley.— Isolation of Mount Marviuo from 

 its parent mass. — Moraine Valley. — Exposuresof Tertiary beds. — Mount Hilgard. — Gilson's Crest. — 

 Lavas of Mount Hilgard.— The Awapa.— Its general configuration and structure.— Its desolate 

 character. — Great variety of rocks displayed in the Awapa. — Homblendic and granitoid tra- 

 chytes.— Conglomer.ates.—Propylites.— Basaltic fields of ancient date.— Rabbit Valley.— Its 

 structural origin.— Erosion of tlie lava sheets around the borders of the valley.— Accumulation 

 of modem. alluvial conglomerates.- Exposures of Tertiary beds in Eabbit Valley.— Thousand 

 Lake Mountain. — A remnant of the grand erosion of the Plateau Province. — Lava Cap. — Under- 

 lying Tertiary. — Absence of the Cretaceous and unconformity <if the Tertiary with the Jurassic. — 

 The Water Pocket flexure and its age.— Jurassic sandstone.- Triassic beds.— The Shinilrump 

 and its sculptured cliff. — The Bed Gate. — The separation of the mountain from the Aquarius 

 Plat«an. 



The third range of plateaus, including the Fish Lake, the Awapa, and 

 the Aquarius, are not inferior in interest to those already described. Con- 

 nected with them are the masses of Mounts Marvine and Hilgard with the 

 intervening valleys. Far to the northward, in the extension of the same 

 line, is the Wasatch Plateau, of which the structure has already been 

 described. The great monoclinal slope which forms its western flank splits 

 gradually into two displacements in its southward extension, one of which 

 forms the Sevier fault, and the other, passing gradually from a monoclinal 

 into a sharp dislocation, forms the Grass Valley fault on the eastern side of 

 Grass Valley. The uplifting along the course of the Sevier fault has pro- 

 duced the Sevier Plateau. The uj^lifting along the other branch or Grass 

 Valley fault has given rise to the Fish Lake table and the Awapa Plateau, 



257 

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