Leave your car or bike in the south parking lot
of SV Open Space, walk the trail uphill a hundred yards of so, and stop at the
crest and look to the East. The giant
red sandstone rockshelter or overhang staring back at you, called Crescent Rockshelter by
the archeologists who sifted dirt here a few years ago, belongs to what
geologists call the Fountain Formation, a sedimentary rock composed of sand and
gravel that crumbled off the Ancestral Front Range as it began to rise between
315 and 300 million years ago.
The existence of this Ancestral Front Range is something of a mystery. 315 million years ago, when tectonic activity pushed Pangea together, the continental collision pushed up a chain of mountains on the eastern seaboard of modern America, from Appalachia to the Ozarks; but from there, inexplicably, the chain bent northward, running north-west through New Mexico and Colorado. This Ancestral Front Range stood in the same place the modern Front Range stands, except that Colorado was an island in a great inland shallow tropical sea, something we know because in the Colorado Springs area scientists found Fountain Formation gravel and sand interbedded with mudstone and limestone containing marine fossils.
The existence of this Ancestral Front Range is something of a mystery. 315 million years ago, when tectonic activity pushed Pangea together, the continental collision pushed up a chain of mountains on the eastern seaboard of modern America, from Appalachia to the Ozarks; but from there, inexplicably, the chain bent northward, running north-west through New Mexico and Colorado. This Ancestral Front Range stood in the same place the modern Front Range stands, except that Colorado was an island in a great inland shallow tropical sea, something we know because in the Colorado Springs area scientists found Fountain Formation gravel and sand interbedded with mudstone and limestone containing marine fossils.
Crescent Rockshelter was uplifted from the Golden Fault. It contains loose sand cobbles of quartz and granite that were combined in riverbeds to form the distinctive sandstone of the Fountain Formation. The red color comes from a high concentration of iron within the rock.
Look beyond Crescent and you will see a grayish, pink-veined Hogback Ridge. This is the second-to-last ridge crest before
the Great Plains sweep toward the East. It is 280 million
years old and was deposited on top of the Fountain Formation about 20 million
years after the uplifting of the Ancestral Front Range. The Lyons outcrop consists mainly of
sandstone, and the pebbles are much smaller than in the Fountain Formation,
which indicates that in those intervening 20 million years a period of acute aridity
had slowed and narrowed the huge, tumbling rivers draining the Ancestral
Front Range. By then the mountains had
probably been eroded to knobby stumps or crumpled foothills, greatly
mellowing the rivers’ gradients. Originally, the dense sandstone layers of the Lyons Formations were sand dunes. The slow-motion process of lithification locked
the wind-stirred crossbeds of sand into stone; since this is seen from Montana to
Arizona, geographers believe the desert was once the size of the Sahara. The Ancestral Front Range, however, continued
to collect enough moisture to produce a few straggly creeks meandering through
the dunes, depositing the tiny pebbles that can be seen today.
References
Lon Abbott and Terri Cook, Geology Underfoot: Along Colorado’s Front Range (Mountain Press
Publishing Company: Missoula, MT, 2012).
Ann M. Johnson, Archaeological Investigations at the Ken-Caryl Ranch, Colorado
(Memoir Number 6 of the Colorado Archeological Society: Denver, 1997.)
This takes you to the homepage of an archeological dig at Crescent Rockshelter. The archeologists, of course, don't want us to poke around their sites, but it's pretty clear where this outcrop is located. This is a great little website for anyone curious about the people who wintered in the Front Range for thousands and thousands of years before we pillaged and ruined everything. To get a good look at Crescent Rockshelter (and the tawny reds and salmon pinks of the Fountain Formation), click on Contexts.http://archaeology.csumb.edu/SlideShows/Crescent/RockShelter.swf
Finally, photos!
Crescent Rockshelter, Fountain Formation
Lyons Formation, north-south running ridge
