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  • The western Taylor Park area covers about 65 square miles on the western flank of the Sawatch Range. It is underlain chiefly by Precambrian rocks, partially covered on the east by glacial debris and overlapped on the west and south by Paleozoic sedimentary rocks. The Precambrian rocks include a group of metasedimentary rocks of an undetermined age greater than 1700 m.y. The metasedimentary rocks include five mineralogically distinct units, quartzite, quartz-muscovite schist, quartz-epidote schist, quartz-plagioclase-biotite schist, and quartz-biotite-microcline-sillimanite schist. The higher grade sillimanite-bearing biotite schists are restricted to the western half of the map area and closely associated with the Forest Hill granitic rocks. The metasedimentary rocks were intruded twice during Precambrian time. The oldest intrusive event resulted in syntectonic emplacement of granodiorite and biotite tonalite rocks. These rocks correlate with the 1700 m.y. old Denny Creek granodiorite gneiss and Kroenke granodiorite which are mapped and informally named in the adjoining Mount Harvard quadrangle. The second, post-tectonic, igneous event produced granitic rocks that are dated at 1030 m.y. by Rb-Sr methods. These rocks include two comagmatic granitic phases, the Forest Hill porphyritic granite and the Forest Hill cataclastic granite. A third phase, the Forest Hill hornblende granodiorite, intrudes the porphyritic granite and, therefore, is younger than the granitic phases. The 1030 m.y. age is very significant because the only other 1000 m.y. old intrusive rocks in Colorado (the Pikes Peak batholith) are restricted to the Front Range. Paleozoic sedimentary rocks are exposed along the western and southern margins of the map area. They range in age from Cambrian through Permian. Tertiary igneous rocks, occurring as plugs, dikes and flows, crop out locally. Two major episodes of Tertiary activity occurred in the region, one during the Laramide orogenic event and the other during Oligocene time. The Tertiary rocks in the map area were formed during the later igneous event. The principle structural features of the area include: (1) northeast-trending Precambrian shear zones and faults, (2) northwest-trending folding and faulting, (3) east-west-trending tear faults in the Paleozoic rocks along the western margin of the map area. Although the northeast and northwest-trending structural trends originated during the Precambrian, reactivation along these zones of weakness probably occurred into mid-Tertiary time.
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