Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 19

younger sedimentary rocks (Fig. 2.2). The primary process in continental growth or con-
tinental accretion is the addition of new land to continental cores by mountain building
(Condie 2005).
The great mountain belts are chiefly marine sedimentary rocks (although the rocks
are commonly metamorphosed and injected with igneous material during the course of
mountain building). The sedimentary rocks of these mountains are undeniably marine,
as proven by the presence of fossilized shallow-water seashells high on peaks. This has
confounded everyone who has theorized about mountain genesis. How did fossils typ-
ical of shallow seas get to mountaintops? Marine fossils also occur deep in the Earth,
far below the foot of the mountains. Indeed, sediments underlying mountains go far
deeper than sedimentary accumulations under the surrounding lowlands. In the United
States, for instance, it was observed more than a century ago that the marine sediment-
ary rocks in Iowa and Illinois (which were once covered by ancient shallow seas) were
not as thick as those of the same age under the Appalachians in Pennsylvania and New
York.
FIGURE 2.1 Precambrian shields (Achean and Proterozoic) age (>540 million years) consisting of
crystalline rock are in black. Mountain belts are in shades of gray. Each shade represents a dif-
ferent age: Cenozoic age (the last 65 million years), Mesozoic age (65-225 million years ago), and
Paleozoic age (225-540 million years ago) mountains. (Adapted from various sources.)
The presence of thicker accumulations of sedimentary rock under mountains than
under surrounding lowlands reveals much. Geologists had long puzzled about whether
the great weight of mountains was simply piled on the surface as excess load and was
therefore in an unstable condition that would ultimately “pop up,” or whether moun-
tains have “roots” or foundations that are less dense than the surrounding strata so
that a relatively balanced condition exists. In addition to the thicknesses of sediment-
ary rock, evidence for the presence of mountain roots came during the mid-1800s from