4 
MUSEUM BULLETIN NO. 11. 
more detailed part following, which is intended for reference only, 
may with advantage be omitted. The last part is concerned 
with the genesis of the Plateau topography. The discussion of 
the physiographic development of the Plateaus in this portion 
is preceded by a statement of the hypothesis of the “Geographic 
Cycle" and of the value of the criterion of regional slopes in 
classifying old land surfaces. A summary of the conclusions 
reached regarding the physiography of the Plateaus is given at 
the end of the bulletin. 
Geological Formations and Structures. 
The following is a short description of the rock basement upon 
which the action of water and ice has carved and moulded the 
present topography of the Beaverdell area. 
The oldest rocks in the area are the Wallace group, which 
occupy about one-third of it, and generally outcrop on the upland. 
They consist of limestone, argillites, andesites, tuffs, schists, 
and basic plutonic rocks; the andesites and tuffs forming perhaps 
80 per cent in bulk of the whole. The sediments, andesites, and 
tuffs are bedded and in some cases have been thrown into open 
folds. The andesites and tuffs occur also in irregular masses 
in which bedding planes are not apparent. The coarser igneous 
rocks are in dykes, sheets, or irregular masses. The group is 
provisionally classed as of Triassic- Jurassic age. 
Into this group there is intruded an extremely irregular 
batholith, the West Fork quartz diorite, which is thought to be 
of Jurassic age. In the Eocene, another batholith, the Beaver- 
dell quartz monzonite came into place. Each of the two batho- 
liths occupies nearly one-third of the area of the sheet. 
After a period of erosion in which parts of the Beaverdell 
batholith were exposed at the surface, a series of thick-bedded 
coarse conglomerates overlain by thin-bedded, fine-grained tuffs 
were laid down, probably in local basins. They are of Oli- 
gocene age and have been called the Curry Creek series. A series 
of lavas, the Nipple Mountain series, follows the Curry Creek 
and are of Miocene, perhaps partly of Oligocene age. 
