THE OLD EROSION SURFACE IN IDAHO 229 
that “faulting and folding have affected the plateau area of central 
and eastern Idaho since its last elevation”’ but it has not destroyed 
its plateau character and “through all, the integrity of the old 
surface has persisted in a remarkable degree.” From Salmon City 
to Gilmore, a distance of about 90 miles, the shore-line of the 
Miocene lake rises from 5,700 to 7,200 feet above sea, and along the 
same traverse the summits rise from 8,500 to 10,000 feet. Within 
this general rise to the southeast, however, are several subordinate 
anticlines and synclines. 
A surface determined by the combined crestlines of the region 
would be “‘an undulating plain” dipping gently to the northwest 
and not a surface with maximum “‘declivities of but a small fraction 
of one degree” as Mr. Blackwelder seems to conceive it. 
The casual reference which Mr. Blackwelder makes to the 
Payette formation as of late Eocene age, though not followed to its 
logical conclusion by him, touches upon what I believe to be the 
only essential weakness in my earlier paper. If the Payette 
formation is really of late Eocene age, either it must have been 
deposited in a different physiographic province or the old erosion 
surface must be crowded back, probably into the Cretaceous. I 
believe that the valley occupied by the Payette formation has a 
history coincident with the several valleys which I have studied— 
in fact that they are tributary to it. Near Hailey Mr. Lindgren 
found lacustrine deposits which, from their relations to the lavas 
and also from floral remains (Sequoia angustifolia), he believed to 
be Payette.t I have visited the Hailey locality and feel sure that 
the beds here mapped occupy a valley tributary to the one which 
earlier in this paper is described as extending from Camas Prairie 
nearly to Salmon City. I. C. Russel,’ in his studies of the Snake 
River plains, was led to infer that the Payette formation extended 
eastward beneath the lavas—an inference supported by the relation 
of the plains to several of the old valleys which I have visited. 
From these observations it seems probable that the Payette 
formation, the lake beds at Salmon City, those at Challis, and the 
t Waldemar Lindgren, Twentieth Ann. Rept. U.S. Geol. Survey, Pt. IIL (1898-99), 
Pp. 197. 
21. C. Russel, Bull. U.S. Geol. Survey 199 (1902), p. 51- 
