704 R. W. RICHARDS AND G. R. MANSFIELD 
to the easternmost point on the east margin of the fault block. 
The length of such a line is about 35 miles. This, however, 
neglects the recession produced by erosion along the east margin 
of the fault block. 
AGE OF THE THRUST 
The youngest rocks involved in the faulting are sandstones of 
the Beckwith formation, which may in part be of early Cretaceous 
age. The oldest rocks which have been found concealing its trace 
are the conglomerates of the Almy formation (24a) which represents 
the basal portion of the Wasatch group as defined by Veatch. 
Fic. 5.—Geologic structure section along the line A—B in Figs. 1 and 2. The 
meaning of the letter symbols is explained under Fig. 1 with the exception of Trw, 
which represents Woodside shale. 
The possible range of age is then from late Cretaceous to early 
Eocene, and it is probable that the faulting may have occurred 
during the interval represented by the unconformity between the 
Adaville (24b) and Evanston (24e) formations of Veatch. 
DEFORMATION OF FAULT PLANE 
The arrow points distributed along the fault trace on the map 
(Fig. 1) indicate in a general way the present attitudes of the fault 
plane. The stereogram (Fig. 2) shows the nature of these folds as 
present and reconstructed for a block 18 miles square. The location © 
of Fig. 2 is indicated by the shaded area on the map, Fig. 1. 
The plications comprise two anticlines, two synclines, and portions 
of the adjoining folds. The trend of these folds is slightly west of 
north. Erosion has completely unroofed the western anticline and 
has made a considerable start on the second. The exposure of a 
closed area of the underlying block through a ‘‘window”’ or 
“‘fenster’’ in the overthrust has already been mentioned. | 
Deformation of fault planes is usually attributed to a renewal 
of the compressive forces and the folding tends to continue along 
