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sandstone when the area was above sea level, shale when the
area was a coastal bay or lagoon, and limestone when the region
was beneath a shallow sea. These rocks have been progressively
exposed in the past 10 million years because the Colorado River
has been downcutting. To put this period of downcutting in the
context of the time analogy just discussed, consider that it be-
gan on December 31 at about 5 a.m. (Table 12.2).
Another place to appreciate deep Earth history is a place
called the Spanish Peaks in southern Colorado because this area
has a complex geologic history that spans a relatively recent in-
terval of time. In addition, you can see how the rock cycle fits
within the context of the geologic timescale here. The Spanish
Peaks lie on the eastern flank of the Sangre de Cristo mountain
range (Figure 12.22a) and rise about 2134 m (7000 ft) above the
Great Plains (Figure 12.22b), which lie to the east. The evolution
of this landscape began sometime before 75 million years ago
when thick sedimentary deposits accumulated in a marine envi-
ronment that fluctuated in depth. These deposits, consisting of
alternating layers of sand, clay, and calcium carbonate, may have
originated from other rocks and slowly compressed and lithified
over millions of years to form sandstone, shale, and limestone,
respectively, after the ocean receded.
Approximately 75 million years ago, during the late
Cretaceous Period, the nearby Sangre de Cristo Mountains
began to form (Figure 12.23). Over the course of the next
50 million years, until about 25 million years ago, this moun-
tain range slowly rose above the surrounding plains. The force
of this uplift caused large fractures to appear in the preexisting
Ocean
Alternating layers of sand,
clay, and calcium carbonate
that form sandstone, shale,
and limestone, respectively.
(a) Earlier than 75 million years ago
Radial dike forms
in fractured rock
Formation of
Sangre de Cristo
Mountains
Sedimentary rock
Stocks
Magma intrusion
about 25 million years ago
(b) 75-25 million years ago
Modern
Sangre de Cristo
Mountains
Spanish Peaks
Radial dike
Magma cools to
form batholith,
which is then uncovered
Sedimentary rock
(c) 25 million years ago to present
Figure 12.23 Evolution of the landscape at the Spanish Peaks. (a) Before 75 million years ago, the region was
covered by a fluctuating ocean, which deposited alternating layers of sediment that became sandstones, shales, and
limestones. (b) Formation of the Sangre de Cristo Mountains occurred between about 75 and 25 million years ago. The
force of this uplift bent and fractured the sedimentary rocks to the east. Between about 25 and 14 million years ago
magma intruded into these fractures. The magma subsequently cooled, forming a batholith, two stocks, and a num-
ber of radial dikes. During this same interval of time, the Sangre de Cristo range began to erode intensively and wear
down. (c) In the past 5 million years the sedimentary rocks overlying the igneous features have been eroded, which has
exposed the stocks and radial dikes seen in Figure 12.22b.
 
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