682 R. W. RICHARDS AND G. R. MANSFIELD 
fault which appears to be of unusual extent and magnitude. The 
purpose of this paper is to summarize the stratigraphy of the region 
and describe a portion of the great fault. 
EARLIER WORK 
The first geologic report on the general region is that of Peale 
(18) of the Hayden Survey. The stratigraphy of the region has 
been modified only to the extent naturally incident to a more 
detailed study, but the newly described structural feature is 
scarcely of the same character. It appears that the stratigraphic 
discordance produced by the great thrust was recognized by the 
relations shown between the Jura Trias and the Carboniferous on 
Peale’s map. The accompanying text, however, does not make 
reference to it. The tg909 report on the phosphate deposits by 
Gale and Richards (10) noted the existence of major thrusts in the 
Montpelier and Georgetown districts, but not enough of the 
surrounding country had been mapped at that time to suggest the 
relationship between the two. C. L. Breger (6a) in the same year 
noted the existence of a similar thrust along the valley of Crow 
Creek east of Preuss Range. The 1o11 report (19a) on a portion 
of the Phosphate Reserve described a thrust fault which extended 
through the region west of Bear Lake and north into Nounan 
Valley. It remained, however, for the 1911 fieldwork in adjoining 
areas to develop sufficient data for a clearer understanding of the 
character of the thrust faulting. 
STRATIGRAPHY 
The rock formations of southeastern Idaho, and the adjoining 
portion of Utah, comprise a stratigraphic section in which every 
system from Middle Cambrian to Upper Jurassic or possibly basal 
Cretaceous is present unconformably overlain by Tertiary and 
Quaternary deposits. The latter rocks toward the south are 
sedimentary, but toward the north include extensive basaltic flows 
which are probably in part as late as the early Quaternary. | 
In the vicinity of Ogden, Blackwelder (1) has noted the presence 
of the Algonkian and Archean, while to the east of the Wyoming 
border Veatch (24) and Schultz (20) have described additional 
* The blackface figures in parentheses following a name refer to the Bibliography 
at the end of this paper. 
