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.~ R. J. Irish Consulting Engineering Geologist, Inc. mouth of its valley, Trout Creek has eroded a narrow floored, several hundred-foot deep gorge flanked by rounded to rugged, castellated and pinnacled, rock-faced slopes. The granodiorite across the planned quarry site is a strong, hard rock that ranges from fine to medium grained to fine to coarse grained and porphyritic, and is partly slightly gneissic. It includes phases of very coarse-grained granodiorite porphyry, and dikes and irregularly shaped bodies of quartz-feldspar-biotite pegmatite. This rock mass is complexly jointed and erratically fractured, but does not appear to be faulted. Vertical to steeply dipping joints of 7 joint sets and near-horizontal to shallow dipping joints of 3 sets break the rock into giant, near-vertically standing slabs and polygonal blocks. Joint spacing typically ranges from several feet to 10 feet or so, but a few 5-foot to 10-foot wide bands of closely jointed rock are interspersed through this otherwise "massive, moderately jointed" rock (Terzaghi classification). Generally its Rock Mass Rating (RMR), we estimate, falls in the range of 85 to 95 ("good" to "very good" quality). The intergranular unconfined compressive (Uc) strength of the granodiorite, we estimate, ranges from about 18,000 psi to about 25,000 psi generally, with the Uc strengths of some of the finer grained parts of the rack mass as high as 30,000 psi. It weighs about 170 lbs./cu. ft. The Uc strengths of the porphyry and the pegmatite probably are on the order of 15,000 psi. The CERCHAR abrasivity of the granodiorite, we estimate, is on the order of 4, thus it is a potentially "very abrasive" rock. Its Moh's Scale hardness probably falls in the range of 6 to 7. The strikes of the near-vertical to steeply dipping joints of 7 joint sets almost "box the compass": N. 75° to 85° W.; N. 5° to 10° E.; N. 85° E.; N. 10° to 20° W; N. 25° W.; N. 65° W.; and N. 35° E. Their dips range from about 75° on one side of the strike to about 75° degrees on the other. The low angle joints of 3 joint sets have strikes and dips of N. 10° to 20° E., 20° to 30° NNW; N. 65° W., 20° SW; and N. 75° to 85° W., 20° NNE. Many of the joints have been etched-out several inches wide by erosion, but that weathering appears to be superficial. Below that depth of weathering-induced etching--a few inches to 5 feet or so--the joints and associated fractures appear to be tight. They are rough-surfaced, due mainly to the angularity, strength and hardness of the minerals, mainly plagioclase (feldspar), quartz, hornblende, and biotite mica, that form the rock. Exfoliation is rare; hydrothermal alteration resulting in "punky" rock is essentially absent; and, for the most part, the rock mass is only slightly weathered and incipiently disaggregated to a depth of less than an inch to 2 inches. Many large and small blocks of rock have broken away from the in-place rock and commonly lie in multi-block jumbled masses on the surface. Wind-deposited and slopewash soils, mainly silty to clayey sands as much as a foot or two thick, blanket the bedrock across scattered small patches; but across the floor and side slopes of the ravine that topographically divides the 2 ridges at this site the soils there are estimated to be as much as 5 feet thick. Seismic activity could impact the planned quarry site owing to the proximity of that site to the Rio Grande Rift. Faults bordering the graben formed within the rift zone initially were activated during the early Miocene Period about 20 to 25 million years ago, and some researchers consider that fault reactivations that generated sizable earthquakes have continued into modern time. Field evidence suggests that the Sangre de Cristo Fault, sited along the eastern side of the San Luis Valley about 50 to 60 miles south of the planned quarry site, may have generated strong earthquakes (with magnitudes on the order of 7.5) about 2000 to 4700 years ago. The recurrence rate for that fault has been suggested to be 3000 to 5000 years, but may be as long as 15,000 years or longer. 2