Graben

	TOURING COLORADO GEOLOGY
	Sphinx Park Domes	Back to Roadcuts
1		Along the road between Shaffers Crossing and Pine in the town of Sphinx Park. Exfoliation domes of Pikes Peak Granite, Jefferson County, Colorado.. Latitude: 39º 25' 23.5" Longitude: 105º 18' 54.2" Elevation: about 7000' (2134 m) Along Elk Creek which flows through the small town of Sphinx Park are some spectacular granite domes. The rock is Pikes Peak Granite, approximately 1.09 billion years old. This outcrop of granite is near the northern limit of the Pikes Peak batholith exposure. Just north of Sphinx Park, the granites give way to Idaho Springs metamorphics which outcrop northward up to Shaffers Crossing along Highway 285. The domes are the product of mechanical weathering where joints form parallel to the surface of the granite mass due to pressure release with the removal of overburden by erosion. These "plates" of rock which have separated away from the rest of the granite eventually spall or slough off thus forming the dome surfaces. The domes can be seen on both sides of the narrow dirt road that follows Elk Creek and are definitely something to see.
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4		Another geological feature of interest in these granite rocks are aplite dikes as shown in the photo to the left. The term "aplite" refers to the texture of the rock, not to its composition. Compositionally these dikes are essentially the same (?) as the granite they cut through. Texturally, they are phaneritic, anhedral, equant grains of quartz and feldspar, sometimes referred to as xenomorphic-granular or allotriomorphic-granular. The origin of these dikes is not well understood, some geologists suggesting that it is a late phase crystallization of a volatile-poor melt (Hyndman, 1972), others a water-saturated silicate melt (Ehlers and Blatt, 1982)
	References: Ehlers, E. G. and Blatt, H., 1982, Petrology: Igneous, Sedimentary, and Metamorphic: W. H. Freeman & Company. Hyndman, D. W., 1972, Petrology of Igneous and Metamorphic Rocks: McGraw-Hill.