TYPES OF ROCKY MOUNTAIN STRUCTURE IN 
SOUTHEASTERN IDAHO™ 
GEORGE ROGERS MANSFIELD 
U. S. Geological Survey, Washington, D.C. 
INTRODUCTION 
GENERAL STRUCTURAL FEATURES 
SPECIAL STRUCTURAL FEATURES 
Noteworthy unconformities 
““Swallowtail” folds 
The Bannock overthrust 
The Blackfoot fault 
Drag folds 
Fan folds 
The Meadow Creek graben 
NoTES ON THE DEFORMATION OF SOUTHEASTERN IDAHO 
Epochs of deformation 
Rocky Mountain geosyncline 
Favorable formations 
Horizontal thrusting ; 
Factors in deformation 
Later deformative epoch 
Relaxation and readjustment 
INTRODUCTION 
Since 1909 the United States Geological Survey has been making 
detailed studies of portions of the western phosphate field, chiefly 
in southeastern Idaho. This region contains a series of sedimentary 
rocks 40,000 feet or more thick, including large bodies of high-grade 
phosphate rock that will prove of great economic importance for 
the future, if not for the present. There is interesting geologic 
structure and a variety of problems covering a wide range of 
geologic and geographic phenomena. 
« Read before the Geological Society of America, December 30, 1919; published 
by permission of the Director of the U.S. Geological Survey. 
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