Boulder OverView
A
cache
by AnnyJ
Hidden
:
9/9/2008
Difficulty:
Terrain:
Size: 
(Not chosen)
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This EarthCache is located at a scenic turnout from westbound US-36
between Denver and Boulder, just past Louisville.
The stop is atop a ridge, which gives a brilliant view of the Front
Range, Boulder Valley, and the area’s most beloved geological
feature – the Flatirons.
Taking a full, 360-degree look around you, you will see that you
stand more or less at the intersection of two great geological
entities: the Great Plains, and the might Rocky Mountains. What is
not so obvious is that you are also standing at the intersection of
two generations of geologic strata (geologic what?? .....
rocks.)
WHAT
To the East, the bulk of what you see (or would see, if you could
sweep off the dirt, dust, and man-made objects) is either Upper
Cretacious Sandstone, or the Denver formation. Below these two is a
blanket of the older Pierre Shale, which can be found on the
surface closer to the mountains. These three strata (layers of
rock) date to the Cretacious Age in geologic time: that is, roughly
70 – 150 million years ago (mya). Let not the numbers fool
you – in our time scale, this is very young!
To the West are exposed many layers of older rock: most notably
sandstone strata known as the Morrison (~200-250 mya), the Lyons
(250-280 mya), and Fountain (280-320 mya) formations, repectively.
Each of these may be seen from this EarthCache location (barring
rainy or foggy days), and are pointed out on the informational
panel at the site. On a clear day, you can see peaks beyond the
foothills – these are made up of even older strata:
Precambrian (500 or more mya) granitic and metamorphic rocks.
WHY
You might be asking yourself why it is that older rocks should
appear above newer ones. Geology should work like a closet floor
– you store some stuff in boxes... but after a while you
realize that you have layers and layers of boxes, the most recent
on top and the old stuff way at the bottom – right? Well, in
general, yes. It sort of works that way. But this area is unique in
its deviation from the standard rules.
To wrap our heads around the “why”, we must turn the
clock waaaay back....
From the beginning of the so-called Paleozoic Era (550-600 mya) to
the Pennsylvanian (290-320 mya), most of Colorado lay beneath a
shallow sea. The continental plate was at that time drifting
northward and rotating into its current orientation, with Colorado
in the equatorial region. During the Pennsylvanian, two great
mountain ranges were uplifted in central and western Colorado.
These are known as the Ancestral Rockies because of their
geo-spacial relation to the present-day Rocky Mountains. These
ranges, however, with their 10,000+ ft. peaks eroded down to
“low hills” within the next 100 million years. The
mountains were reduced to fine sands, silts, and mud, which was
deposited at the sea floor. This deposition, under the sea’s
pressure, became a reddish sandstone and conglomerate known today
as the Fountain formation.
During the Triassic (200-250 mya) and Jurassic (150-200 mya) Eras,
Colorado’s shores were repeatedly lapped and covered by the
periodic advance and receding of the sea. In the early part of the
Jurassic, sand dunes “were slowly covered by river-borne
sand, then by varicolored shales and sands of the Morrison
formation, laid down in streams, marshy areas, and lakes. Dinosaurs
flourished in this moister climate, and their fossils and
footprints are numerous in rocks of late Jurassic age.”
This pattern repeated itself, with subtle variations, giving us the
Dakota formation, the Pierre, and the Niobrara – all of which
can be found in the environs of Boulder.
The Fountain formation, along with these younger layers, were
“dragged and faulted upward by the rebirth of the Rockies
during the Laramide Orogeny, 72-40 million years ago.” The
diagram on the info panel illustrates how, just as pulling the rug
out from under a stack of boxes will turn things on end (so to
speak), so too this faulting and uplifting has reoriented these
rock layers.
To claim credit for this EarthCache, study the above description,
the diagrams on site, and the view in front of you. Send the
answers to the following questions to me via my geocaching
profile.
- According to the sign found at this site, what formation are
the Flatirons part of?
- How does the age of this formation compare to the formation of:
- the ancestral Rocky Mountains
- the current day Rocky Mountains
- Were there ever dinasuars in this area?
- Describe in your own words the difference between the Flatirons
feature and the other ridges that don't have a feature like the
Flatirons. Why do you think the Flatirons might have become such a
prominent feature in this particular location?
Additionally, please photograph yourself with your favorite angle
of the view (not the info panels!) in the background. Attach this
photo to your log. Please also include the number of people in your
team and whether this is your first EarthCache find.
References:
Roadside Geology of Colorado, 2nd Ed.. Halka Chronic and Felicie
Williams. Mountain Press Publishing Co., 2008.
Boulder, A Sight to Behold; Guidebook. Donald D. Runnells. Estey
Printing Co, 1976.
A special thanks to the Colorado Historical Society
for maintaining this location and many others, and for granting
permission for this EarthCache.
Additional Hints
(No hints available.)
Find...
Geologic map of Boulder
credit Boulder, A Sight to Behold
Geologic map of Boulder KEY
credit Boulder, A Sight to Behold
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Current Time:
Last Updated: on 1/29/2012 8:16:23 AM (UTC-08:00) Pacific Time (US & Canada) (4:16 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum
