THE MUDDY MOUNTAIN OVERTHRUST IN 

 SOUTHEASTERN NEVADA^ 



CHESTER R. LONGWELL 

 Yale University 



INTRODUCTION 



Geologists have long recognized the fact that the region of the 

 Basin Ranges has passed through more than one period of deforma- 

 tion since the middle of the Mesozoic era. The most recent impor- 

 tant disturbance found expression chiefly in great normal faults, 

 which divided the crust into essentially parallel mountain ranges. 

 Within many of these mountain blocks the structure is complex. 

 Tertiary formations are strikingly unconformable among them- 

 selves, and rest with profound unconformity on the older rocks. 

 This great unconformity is a record of prolonged erosion following 

 an epoch of crustal compression in Mesozoic time. The youngest 

 rocks involved in this disturbance appear to be of Jurassic age; and 

 therefore it is probable that the movement was contemporaneous 

 with the folding of the Sierra Nevada and Humboldt Ranges. 



The degree of deformation due to this mountain-making varies 

 greatly in the different ranges. In some the Mesozoic and older 

 strata appear to have been nearly horizontal before the last impor- 

 tant period of faulting. Other ranges preserve a record of the 

 Nevadian movement in folds of varying intensity. In the Muddy 

 Mountains of southeastern Nevada the record consists of over- 

 turned folds and an overthrust of considerable magnitude. These 

 structural features were recognized by the writer in the course of 

 field work during the summer of 1919, and a brief description has 

 appeared in another article.^ The present paper will give a more 

 detailed description of the overthrust (Figs, i and 2), 



' Published by permission of the Director of the United States Geological Survey. 

 ' Amer. Jour, of Sci., Vol. L (Jan., 1921), pp. 39-62. 



, 63 



