Geology Reference
In-Depth Information
Geo-Focus
Denver's Weather—280 Million Years Ago!
With all of the concern about global
climate change, it might be worthwhile
to step back a bit and look at climate
change from a geologic perspective.
We're all aware that some years are hot-
ter than others and some years we have
more rain, but generally things tend to
average out over time. We know that it
will be hot in the summer in Arizona,
and it will be very cold in Minnesota in
the winter. We also know that scientists,
politicians, and concerned people every-
where are debating whether humans are
partly responsible for the global warm-
ing that Earth seems to be experiencing.
What about long-term climate
change? We know that Earth has ex-
perienced periods of glaciation in the
past—for instance, during the Precam-
brian, the end of the Ordovician Period,
and most recently during the Pleistocene
Epoch. Earth has also undergone large-
scale periods of aridity, such as during
the end of the Permian and beginning of
the Triassic periods. Such long-term cli-
matic changes are probably the result of
slow geographic changes related to plate
tectonic activity. Not only are continents
carried into higher and lower latitudes,
but their movement affects ocean circu-
lation and atmospheric circulation pat-
terns, which in turn affect climate, and
result in climate changes.
Even though we can't physically travel
back in time, geologists can reconstruct
what the climate was like in the past.
The distribution of plants and animals is
controlled, in part, by climate. Plants are
particularly sensitive to climate change
and many can only live in particular
environments. The fossils of plants and
animals can tell us something about the
environment and climate at the time
that these organisms were living. Fur-
thermore, climate-sensitive sedimentary
rocks can be used to interpret past cli-
matic conditions. Desert dunes are typi-
cally well sorted and exhibit large-scale
cross-bedding. Coals form in freshwater
swamps where climatic conditions pro-
mote abundant plant growth. Evaporites
such as rock salt result when evaporation
exceeds precipitation, such as in desert
regions or along hot, dry shorelines.
Tillites (glacial sediments) result from
glacial activity and indicate cold, wet en-
vironments. So by combining all relevant
geologic and paleontologic information,
geologists can reconstruct what the cli-
mate was like in the past and how it has
changed over time at a given locality.
In a recently published book titled
Ancient Denvers: Scenes from the Past
300 Million Years of the Colorado Front
Range
by Kirk R. Johnson depicts what
Denver, Colorado, looked like at 13 dif-
ferent time periods in the past. The time
slices begin during the Pennsylvanian
Period, 300 million years ago, and end
with a view of the Front Range amid a
spreading wave of houses on the south-
ern edge of metropolitan Denver.
The information for piecing together
Denver's geologic past was derived
mainly from a 688-m-deep well drilled
by the Denver Museum of Nature and
Science beneath Kiowa, Colorado, in
1999. Using the information gleaned
from the rocks recovered from the
well, plus additional geologic evidence
from other parts of the area, museum
scientists and artists were able to recon-
struct Denver's geologic past.
Beginning 300 million years ago
(Pennsylvanian Period), the Denver area
had coastlines on its eastern and western
borders and a mountain range (not the
Rocky Mountains of today). The climate
was mostly temperate with lots of seed-
less vascular plants, such as ferns, as well
as very tall scale trees related to the mod-
ern horsetail rush. Huge insects such as
millipedes, cockroaches, and dragonfl ies
shared this region with relatively small
fi n-backed reptiles and a variety of
amphibians.
By 280 million years ago, the area was
covered by huge sand seas, much like the
Sahara is today (
Figure 1). This change
in climate and landscape was the result
of the formation of Pangaea. As the
continents collided, arid and semiarid
conditions prevailed over much of the
supercontinent, and the Denver area was
no exception.
During the late Jurassic (150 million
years ago), herds of plant-eating
dinosaurs such as
Apatosaurus
roamed
throughout the Denver area, feasting on
the succulent and abundant vegetation.
◗
◗
Figure 1
La
Conchita, California, is
located at the base of
a steep sloped terrace.
◗
Figure 1
Denver as it appeared 280 million years ago. As a result
of the collision of continents and the formation of Pangaea, the world's
climate was generally arid, and Denver was no exception. Denver was
probably covered by great seas of sand, much as the Sahara is today.
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