Saturday, February 5, 2011



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.
 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. 
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:

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.

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.