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| SIGNAL BUTTE IN THE FALL. |
Traveling along county roads
west of Woodland Park, Colorado, travelers may spot a rather out-of-place
conical peak, Signal Butte (another name, according to the USGS GNIS is Cheops
Pyramid). The peak, not really a butte at all (butte = an isolated,
limited tableland capped with a horizontal and protective covering; Thornbury,
1962), is not readily visible from U.S. 24 nor from the lowlands of CO 67 north
of Woodland Park. Signal Butte is located at Lat. 39.05860 degrees N and
Lon. 105.2228 degrees W and is best accessed from Divide—north on Teller Co. 51
for 11 miles, right on FS 363 (Phantom Creek Rd.) for 0.6 miles until FS 362
(Signal Butte Rd.) for 3.3 miles. There is a small parking area and the
route to the summit seems obvious. One can also access the Butte from CO
67 near Westcreek; however, the roads are 4-wheel drive, several are washed
out, and most of the land is private. Whatever the route, the Butte is
obvious as a distinctly shaped conical peak at 9459 feet in elevation and
nearly a thousand feet above the adjacent creek.
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| AIR PHOTO OF SIGNAL BUTTE. FROM MAP CARD. |
From a geological point of
view, Signal Butte is interesting as it exposes middle Tertiary igneous rocks
(~1/4 sq. mile) almost in the center of the massive Pikes Peak Batholith (~1300
sq. miles, Tweto, 1979). The Precambrian Batholith (~1.1 Ga), with
exposures of pink granites and pegmatites, has been intruded (igneous magma
cooling below the surface and within the granite) by gray colored igneous rocks
“of felsic to intermediate composition with an age of 20-40 M.Y.” (Tweto,
1979). I have been unable to locate much additional geological
information about Signal Butte but will continue to search the
literature. The rocks that I collected from the Butte are dark gray in
color, and with a texture close to aphanitic (fine-grained) to porphyritic
(larger crystals within the groundmass) with visible lathes of feldspar
crystals. Wilson and Bryant (2006) stated the rocks at Signal Butte were a
“pyroxene-hornblende trachydacite” from a “plug”. Trachydacites
contain major amounts of sanidine feldspar with minor quartz. The Signal
Butte rocks contain the accessory minerals pyroxene [group] and
hornblende. If Wilson and Bryant (2006) are correct and the Butte is a
plug then the magma crystallized in a neck and/or volcanic vent. At a
later date, erosion removed the surrounding granite and the plug was left as a
major landform. Perhaps the rocks are hypabyssal and cooled in an intermediate
position between the surface and deep within the crust---the volcanic neck
never reached the surface. That might explain the somewhat larger
crystals (larger than basalt crystals) in the rocks that I
observed. Please understand that I am out of my comfort zone describing
the probable origin of Signal Butte without a more detailed study.
However, Tweto (1979) did show a fault extending north from the Butte; perhaps,
the magma came from a zone of weakness in the granite.
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| TRACHYDACITE CROPPING OUT AT SIGNAL BUTTE. |
Wilson and Bryant (2006) have
provided fission-track dates of 42.2 to 45.2 M.Y. for the trachydacites---an
Eocene event. That seems to be the approximate age of plugs near Boreas
Pass west of Como but suggesting any relationship to these rocks are beyond my
knowledge. The dates do seem to be in the middle between the major
igneous events of the Laramide Orogeny (~60 to ~80 M.Y.) and the outpourings of
volcanic rocks in the Oligocene (~25 to ~30 M.Y.).
Signal Butte is in the area
devastated by the Hayman Fire of 2002. This burn consumed forests and
buildings on nearly 140,000 acres of land and caused damages in the tens of
millions of dollars. I did notice that some new pine trees have reached
two feet in height and some aspens are now up to four feet so the forest is
slowly returning. However, it will take hundreds of years for the
streams, now clogged with sediment, to return to some semblance of local base
level and equilibrium. A fantastic photo of the fire burning on Signal
Butte may be observed on the Teller County website at: http://www.co.teller.co.us/HaymanFire/HaymanInfo.htm
Signal Butte has a fabulous
view from the summit, is an object of geological interest that is close to
Colorado Springs, and worth the easy trip to observe to some rather different
rocks. After all, how many of the readers have seen an example of pyroxene-hornblende
trachydacite?
There is a trail to the summit and the hike is well worth the view, and, the
rocks are quite interesting.
REFERENCES CITED
Thornbury, W.D., 1962,
Principles of Geomorphology: John wiley and Sons, New York.
Tweto, O., 1979, Geologic Map
of Colorado: U. S. Geological Survey, scale 1:500,000.
Wilson, A. B. and Bruce
Bryant, 2006, Isotopic Ages of Rocks in the Northern Front Range, Colorado: U.
S. Geological Survey Open-File Report 2006-1051.
mike
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