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<br />Grand Junction Pipc <br />Gravel Pit. SE comer oC 23 and River Roads, Grand Junction. CO <br />November 12, 1997 Page 2 <br />Complex. Thc higher clcvations oCthe Colorado National Monument represent a vertical displacement in excess <br />' of 2000 feet. <br />Seismic events have occurred near, and possibly, in the Grand Valley area. These events were evaluated as <br />' hating Richter Magnitudes up to and including 4.4, with no reported damages. <br />This site is partially within the designated l00 Year Flood Plain, shown on the National Flood Insurance <br />Program Maps for Mcsa County. Colorado. Thc tract appears to be west of the Leach Creek Flood Plain and <br />is protected from flooding from the northeast by the elevated highway and railroad embanlancn[s. <br />' SITE GEOLOGY <br />Thc surface soils on this sift appear to be a mixture oC fine grained silty clay and clayey sill soils deposited by <br />Ancent Debris Flo+v activity originating in the Bookcliffs to the north northeast. This area appears to be near <br />the south. soulh+vcstem moss limits of the Leach Creek Debris Fan feature. The soils in the southern most <br />portion of the Debris Fan Ccature appear to be partially reworked by sporadic, ancient Good activity oC the <br />Colorado River. Sonic shin sand and occasional gravelly strata are found interbedded with the fine grained silty <br />cla_+s and clayey sills. These fine grained soils titre found to be 10 to I~ feet in thickness across this sift, based <br />on auger drill logs. It is anticipated these fine grained silty clays and clayey silts will be the overburden material <br />across this entire site. <br />r At a depth ranging from approximalch• 9 to I S feet below the presenL ground surface, a sandy gravel and gravel <br />and cobble alluvial deposit ++•as encountered. This coarse grained gravel and cobble deposit is the Ancient <br />' Tcrracc Deposit of the Colorado River. This gravel and cobble deposit is between 13 to 19 Ccct in thickness, <br />based upon auger boring logs. This is the gravel deposit yvhich is to be extracted and processed. In general, the <br />upper 3 to ~ feet of the gravel and cobble is somewhat sandier and may contain some Thin sand deposits. Thc <br />' maximum cobble silt is normally on the range of 7 to 9 inches (square screened size) but the lower 4 to G feet <br />of the deposit may contain occasional cobbles up to 12 l0 16 inches in diameter. <br />Thc ~+cathcrcd silk clays of the Mancos Shale Fornmtion was encountered at approximately 2A feet below the <br />prescnt ground surface, based upon the auger drilling logs. Thc Mancos Shale is considcrcd to be bedrock in <br />this portion of the Grand Valley. The Mancos Shale is generally described as a dark gray to gray black thin <br />bedded to laminated bedded shale with intabeds oCsiltstone and sandstone. The shale is believed to be less than <br />' 100 feet thick in this particular area, based upon exposures oC the geologic rock section in the river bluff on the <br />south side of the Colorado River. <br />' GROUND WATER <br />Based upon the shallow, auger exploration borings placed on this sift, the ground water clcvations arc <br />anticipated to be bct+vccn 13 to IG feet bcloav the prescnt ground surface. These ground water clcvations arc <br />largely controlled by the elevation oC the Colorado River and by irrigation practices to the north and east of this <br />silt This Bound +vatcr should be considcrcd a permanent feature of the site. De+vatering oC the open pit during <br />mining +vill be required <br /> <br />