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<br />2880 <br /> <br />In general, the unit consists of a sequence of grayish- <br />~ orange-pink (5YR7/2) to reddish-brown silty sandstone <br />interbedded with grayish-red (5R4/2), pale-red (5R6/2), <br />and moderate-brown (5YR3/4) siltstone or mudstone. TI1e <br /> <br />sandstone units are composed of very silty very fine- to <br /> <br /> <br />fine-grained subrounded, clear and frosted quartz. The <br /> <br /> <br />sorting is usually fair but sometimes is poor. The <br /> <br /> <br />usually thin-bedded sill:5tone units arc horizontally <br /> <br /> <br />laminated and con~only are crinkly or wavy. Bedding <br /> <br /> <br />ranges from a few inches in the siltstone and mudstone <br /> <br />to 3 or 4 feet in the sandstone beds. In the McElmo <br /> <br />. <br /> <br />canyon area the unit becomes more sandy toward the top, <br /> <br /> <br />and a sandstone bed near e1e top is 20 feet thick (Ekren <br /> <br /> <br />and Houser, 1958, p. 74). Here the thick bed forms a <br /> <br /> <br />prominent ledge, but in other places, the formation <br /> <br /> <br />generally forms a moderate slope with thin sandstone <br /> <br />ledges. <br /> <br /> <br />The Summerville Formation is approximately 140 feet <br /> <br /> <br />thick in McElmo Canyon. The thickness penetrated by wells <br /> <br /> <br />in the Towaoc area ranges from 130 to 150 feet. The <br /> <br /> <br />Summerville seems to maintain a fairly consistent <br /> <br /> <br />thickness throughout the reservation. The unit is 148 feet <br /> <br /> <br />thick at Red Mesa, 20 miles west of Four Corners and is <br /> <br /> <br />126 feet thick at Becabito Dome, 12 miles south of Four <br /> <br /> <br />Corners (Strobell, 1956). <br /> <br />. <br /> <br />t7tP <br />