THE OLD EROSION SURFACE IN IDAHO 413 
which are matters of general knowledge, that seem to indicate a 
much later age than Eocene for the plateau surface described by 
Mr. Umpleby. These necessitate a short introduction for the sake 
of clearness. 
It is said that ‘“‘faulting and folding have affected the plateau 
area of central and eastern Idaho since its last elevation, but through 
all, the integrity of the old surface has persisted in a remarkable 
degree.’’ In the preceding paragraph, however, it is pointed out 
that the remnants of the peneplain do not vary greatly in altitude, 
the maximum being 10,000 feet in central Idaho and from that 
falling off very gradually to 8,000, 7,000, and even 5,000 feet at a 
distance of some 400 miles. Unless there is some other evidence 
of the supposed folding and faulting, the reader is justified in con- 
cluding from the facts presented that the plain has been subject 
merely to very gentle changes of level, which may be termed mild 
warping, rather than folding. This very slightly warped condition 
of the old peneplain should be compared with the much more 
pronounced deformation visible in the late Eocene sediments and 
lavas of closely adjacent regions on several sides.. Thus, imme- 
diately southeast of the region under consideration, the late Eocene 
and Oligocene beds of northwestern Wyoming have an average dip 
of ro° and in some places form anticlines with 25° dips on either 
limb. In addition to this, they have been broken by normal 
faults of several thousand feet displacement. Again, on the south- 
western confines of the central Idaho region itself, the Payette 
formation, which seems to be safely identified by fossil plants as 
of late Eocene age, varies in elevation from less than 1,000 feet 
above sea-level near Weiser, to nearly 6,900 feet above sea- 
leveleast of Boise'™ In the same region the dip of the fine 
plant-bearing shales, which were doubtless deposited in horizontal 
position, is now generally 1o-15°, and not rarely 25°; while at 
a one point it rises as high as 80°. Still more striking is the 
condition in west-central Washington, less than too miles from the 
Republic district in which the old peneplain is said to be readily 
identified. There the Eocene and even the late Miocene formations 
1 W. Lindgren and N. F. Drake, U.S. Geol. Survey, Folio 104, Silver City, Idaho, 
1904. 
