Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 52

are caused by compressive forces that tend to squeeze and push one segment of the
Earth's surface over another. Low-angle faults can pass upward to the surface at high
angles. A series of high-angle faults or an imbricated thrust system will produce a more
or less vertical stacking of beds that can erode differentially into a series of parallel
ridges and valleys. The result is sharp and rugged topography, as in the Sawtooth Range
of Montana, where thrust and reverse faulting produced juxtaposed harder and softer
rocks (Fig. 2.20).
Low-angle thrust faulting tends to produce nearly horizontal displacement as low-ly-
ing older rocks are pushed over younger strata. The rock units involved may vary from
a few tens to hundreds of meters in thickness, and they can override the adjoining sur-
face for hundreds of kilometers. Erosion can then remove the weaker strata, leaving
the more resistant strata as isolated remnants. Extremely confusing stratigraphic rela-
tionships are produced: A mountain peak can be composed of rocks older than under-
lying strata and not found elsewhere in the vicinity; alternatively, a mountain peak can
be composed of rocks identical to those exposed far below in an adjacent valley, while
sandwiched between are several hundred meters of younger rock.
Many overthrust faults occur in major mountain ranges. In the Lewis Overthrust
near Glacier National Park, Montana, ancient Precambrian rocks were uplifted and
thrust a distance of 24 km (15 mi) over younger shales and sandstones of Mesozoic age
(King 1977). Chief Mountain is an erosional outlier remnant, or klippe, of that over-
thrust sheet (Fig. 2.21). Results similar to overthrust faulting can also be accomplished
through other processes, such as gravity sliding and overturned folding (discussed be-
low). All three of these processes can be interrelated, and it is commonly difficult to
identify the relative influence of each in a given situation.