BISECTION OP MOUNTAIN BLOCKS IN THE GREAT 
BASIN REGION. 
BY CHARLES B. KEYES. 
(Abstract). 
An interesting phase of mountain structure has recently 
come to notice among the block ranges of the basin region 
of the southwest. As is well known, most of the basin 
ranges are long, narrow tilted blocks, arranged subparallel 
to one another, with intervals of twenty to forty miles 
of flat country between . These mountain blocks are fifty 
to one hundred miles in length and are tilted to heights 
of 3,000 to 5,000 feet. 
In central New Mexico there are three ridges — the 
Sandia-Manzano, the Oscura-San Andreas, and San Cris- 
tobal-Caballos — which are essentially parts of single up- 
lifts, but which are cut in two by deep canyons which con- 
nect the plains on either side. Why these ridges should 
be cleft in their highest parts has been puzzling. However, 
an interesting solution has been found. 
In each of the three areas mentioned profound faulting 
has manifestly taken place since the time when the moun- 
tains were upraised, and the trend of the later dislocation 
has been transverse to the major faulting parallel to the 
axis of the range. This has enabled, in each case, an in- 
significant stream to cut down a deep narrow canyon 
directly through the heart of the mountain block. 
In the Sandia-Manzano block the bisection is at the 
Tijeras canyon, where the faulting and relationships of 
the strata are very clearly disclosed. The displacement 
can not be less than 1,000 feet. The Tijeras fault extends 
far beyond the foot of the range, being marked by a con- 
( 165 ) 
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