To Get There
Castlewood
Canyon
State Park
Geology
The Journey of the Rocks
Notice the large boulders in Cherry Creek as you
walk the Inner Canyon Trail. Where did they come
from? They began eons ago as molten rock deep
under the surface of the earth and were raised with
the Rocky Mountains. Erosion wore them down
to grains of sand and washed them downstream to
the area of the park, where they became part of the
caprock. Some millions of years ago, they were
broken off from the caprock by the forces of nature
and fell to the canyon bottom. This is not the end of
their journey. Erosion continues to work on them,
and over the millennia, will wear them down to
grains of sand and wash them further downstream–
ultimately to the Gulf of Mexico.
Castlewood Canyon State Park
2989 S. State Highway 83
Franktown, CO 80116
303-688-5242
Email: castlewood.canyon@state.co.us
www.parks.state.co.us
CSP-CAST-200-4/07
Building a Rock Layer Cake
The Layer Worn by Water
Millions of years ago, a tropical rainforest covered
what is now Castlewood Canyon State Park. How
do we know? Because plant and animal fossils
from those tropical forests have been found in the
oldest visible rocks in the park, called Dawson
Arkose. This approximately 55-million-year-old
rock layer can best be seen on the west side of the
park downstream from the dam, brought to light by
the scouring action of the flood waters unleashed
when the dam collapsed in 1933.
The “icing” on the park’s rock layer cake of
Dawson Arkose and rhyolite, and its most
distinguishing geologic feature, is Castle Rock
Conglomerate. These 34 million-year-old rocks,
washed down from the eroding Rocky Mountains,
form the park’s canyon walls and caprock.
Conglomerate rocks are easy to identify–they’re
like cookie dough with bits of chocolate chips
sticking out. The “dough” is sedimentary rock
and the chips are pebbles and boulders that are
smoothed and rounded in ancient rivers and
cemented into the rock by the water’s high
concentration of silicates–nature’s concrete.
Petrified wood is one of the most common fossils
found in the Dawson Arkose. You may come across
a beautiful butter-scotch-colored rock that is, in
fact, petrified wood from an ancient tropical tree.
This type of fossil is so common in the area that
it is often called “Parker wood.” Be sure to leave
fossils and rocks where you find them so others
may enjoy discovering them too.
Let’s see what we can find in the layers above the
Dawson Arkose. The rocks in these layers
are younger by many millions of years, and they
are harder–more resistant to the forces of wind
and weather.
Dawson Arkose
Late Eocene
The Layer Forged in Fire
Although the rocks above the Dawson Arkose
cannot speak they tell the story of a tremendous
volcanic eruption that occurred precisely 36.7
million years ago. The eruption, which happened
about 90 miles away near present-day Salida,
filled the air with a glowing cloud of 2,000 degree
molten rock, ash and poisonous gases. It reached
the area of the park in just a few minutes.
The liquid rock and superheated ash welded into
a thick layer of solid rock as they hit the ground
cooled, and were buried. This rock has several
names: ignimbrite (Latin for “fiery cloud”, Wall
Mountain Tuff (named for the mountain northeast
of Salida where it was first discovered), and
rhyolite. You can find pieces of this once liquid
rock laying all over the park. Look for rocks with
sharp angles and edges, tiny air holes, and shiny
specs. It can be pink, purple, gray or brown in
color. Rhyolite has been mined in this area as a
decorative building material for more than 100
years. Rhyolite blocks were used to build the
outside walls of the park Visitor Center and picnic
shelters and those of the famous Molly Brown
House in Denver.
As you walk the paved trail between Canyon Point
Parking Lot and the Inner Canyon Trailhead on the
park’s east side, do you see anything that makes
you think this area was once covered by water? Do
the rock patterns remind you of the patterns the
surf makes in the sand? Water formed these telltale cross beds millions of years ago and deposited
sand, grave, pebbles, cobbles and stones. Do you
wonder why many of the stones are smooth while
the rhyolite stays angular? Think about the time
each spent in the streams and rivers. The stones
were tossed and turned in the water as they traveled
many miles out of the mountains onto the plains.
The rhyolite came by air, so it was not as worn
down by the action of the water.
Cross Beds
