PHYSIOGRAPHY OF THE BEAVERDELL MAP-AREA. 
25 
E. with subordinate groups striking east-west and north N. 20° 
W. The groups of faults, however, do not agree in strike 
very closely, nor can their direction be compared in detail with 
that of the trends of the groups of parallel valley stretches. 
This may be due to the fact that strikes were determined upon 
short surfaces of the fracture planes only, and that 75 per cent 
of the faults and fracture zones were determined within a small 
area of quartz diorite on Wallace mountain. The rough paral- 
lelism of valley stretches and fault zones, and their general accord- 
ance in direction is, however, significant. Moreover, fault 
planes were actually found outcropping along the bottoms of cer- 
tain small draws on the side of the West Fork valley, and in the 
bottom of Hall Creek valley, and valleys upon the upland were 
found to lie parallel to the trend of fracture zones in the ad- 
jacent rocks. 
A relationship between topography and structure upon the 
areas underlain by Tertiary rocks is brought out by the topo- 
graphic change which occurs in passing from one Tertiary rock 
formation to another, and from the Tertiary rocks to the older. 
It is suggested further by the finding of evidences of undeveloped 
drainage in the Tertiary. 
On Wallace mountain the white, fine-grained tuffs of the 
Tertiary overlie conglomerates of the same age; the tuffs at 
this place lie nearly flat, and form a terrace about 30 feet high, 
with a steep front upon the conglomerates. There is thus a 
change in topography at the contact between conglomerates 
and tuffs. 
The Tertiary lavas, which overlie the sediments here, and 
the older formations elsewhere in the quadrangle, generally 
stand out fron? the surface immediately surrounding them to 
form rugged buttes or mesas. The flat top and cliff topography 
developed upon them differs from the gentler slopes on the older 
formations (Plate IV). Their contact with the older formations 
is also quite often marked by a cliff in the lava, although the cliff 
is not always immediately at the contact, but may lie some dis- 
tance back of the contact. In places the lava surfaces suggest 
evidences of undeveloped drainage. On the northern of the 
two China buttes, for instance, there is a semicircular depression 
