446 GEORGE ROGERS MANSFIELD 
Three semi-detailed reports’ and a number of shorter papers 
have already been published and a fourth report? is now in press. 
An additional, more extended report is well advanced in preparation 
and includes a discussion of the geography, geology, and mineral 
resources of the seven quadrangles named. The purpose of this 
paper is to present in advance of the detailed report some of the 
striking structural types of the region and to discuss briefly certain 
conditions that attended the development of these structures. 
The maps used in illustration of the structural features are extracted 
from the detailed geologic maps of the quadrangles mentioned. 
Their locations are shown on the index map, Figure tr. 
GENERAL STRUCTURAL FEATURES 
The stratigraphic series in southeastern Idaho includes more 
than sixteen recognized unconformities. Most of them do not 
appear to record great crustal disturbances, but a few indicate 
changes of considerable magnitude. Several are very striking, 
both as seen in the field and in cartographic representation 
The region is traversed by many folds, some of which exceed 
50 miles in length. The more important folds are synclinoria 
with relatively narrower intervening anticlines or anticlinoria, 
usually unsymmetrical and inclined or even overturned eastward or 
northeastward. ‘The axes for long distances are nearly horizontal 
or slightly undulatory, due to the presence of relatively broad and 
low transverse folds, and the pitch is gentle, generally toward 
the north or northwest. The trend of the folds is convex toward 
the northeast, bending from a little east of north in the Montpelier 
quadrangle to northwest in the Lanes Creek quadrangle and 
beyond. This arrangement gives rise to long nearly parallel folds 
t See especially H. S. Gale and R. W. Richards, “Preliminary Report on the 
Phosphate Deposits in Southeastern Idaho and Adjacent Parts of Wyoming and Utah,” 
U.S. Geol. Survey Bull. 430 (1910), pp. 457-535; R. W. Richards and G. R. Mansfeld, 
‘“‘Preliminary Report on a Portion of the Idaho Phosphate Reserve,” U.S. Geol. 
Survey Bull. 470 (1911), pp. 371-451; R. W. Richards and G. R. Mansfield, “Geology 
of the Phosphate Deposits Northeast of Georgetown, Idaho,” U.S. Geol. Survey Bull. 
577, 1914. 
2G. R. Mansfield, ‘‘The Geography, Geology and Mineral Resources of the 
Fort Hall Indian Reservation, Idaho, with a Chapter on Water Resources, by W. B. 
Heroy,” U.S. Geol. Survey Bull. 773. 
