Instead of a single large thrust in the Muddy Mountain area, Nevada, as reported from earlier field study, the writer distinguishes two superposed thrusts which may represent distinct orogenic episodes separated by a considerable time interval. Both thrusts “root” to the west. The structurally lower thrust (for which the name Muddy Mountain thrust is retained) is the more extensive; as reported previously, it has brought Paleozoic carbonate formations over Jurassic sandstone, with the heave-component of slip at least 15 miles. The higher thrust (here called the Glendale thrust) has heave-displacement of at least 5 miles. Together with associated smaller thrusts, it involves formations of early Upper Cretaceous age, as well as thick piedmont deposits that may be considerably younger. “Orogenic deposits” several thousand feet thick were laid down in front of the Glendale thrust as it advanced. Accurate dating of these strata would give an exceptional record of structural events.

Conglomerate at the base of the Upper Cretaceous section, containing boulders and cobbles derived from resistant units in older systems as low as the Permian, indicates earlier strong deformation not far west of the Muddy Mountain area. This earlier orogenic episode may have included development of the Muddy Mountain and other large thrusts in the region which are not known to involve formations younger than Jurassic. Therefore the earlier orogeny can now be dated merely as post-Jurassic and pre-Upper Cretaceous.

Overturned and faulted folds associated with the Glendale thrust rival in complexity some structural features of the Swiss Alps. Important transverse faults with large strike-slip component pose problems of origin; the largest of these displaces the Muddy Mountain thrust plate as much as 2 miles vertically and may be genetically related to the Glendale thrust. Numerous normal faults, variously oriented, bear witness to movements ranging in date from the Glendale thrusting episode to late Cenozoic time.

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