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<br />~CRA NDE COUNTY
<br />light-colored, fine-textured, well-bedded tuff, which
<br />crops out on the gentle slopes just south of the Rio
<br />Grande and west of Pinos Creek. Overlying this tuff is
<br />a great bod,v of tuff breccia, which attains x thickness
<br />of more than 1,000 feet. Much of it is poorly bedded and
<br />sorted. The fragments are mainly subangular to angu-
<br />lar, rarely rounded, and for the most part are less than
<br />1 foot in diameter.
<br />Volcanic rock of probable Sheep Mountain age occurs
<br />only in a relatively small area in the vicinity of Del
<br />Norte. It has been included in the Sheep Mountain
<br />Quartz Latite because of general similarity to the
<br />Sheep Mountain rock occm•ring north of the survey
<br />area. The quartz lathe flows forming Observatory Hill,
<br />just south of Del Norte, overlie the Conejos rocks very
<br />irregularly'. The main flow is a purple-drab to dull-gray,
<br />banded, platy rock with alternate, highly vesicular
<br />bands.
<br />The volcanic rock of the Treasure Mountain, Albo-
<br />roto, and Piedra Rhyolite Formations generally is
<br />closely associated. The principal types are rhyolitic
<br />tuffs and welded tuffs formed from great explosive ash
<br />flows originating largely from the Creede caldera.
<br />The tuff is soft rock made up mostly of small frag-
<br />ments of glass, pumice, and the broken walls of gas
<br />cavities and carrying a few crystals of feldspar and
<br />biotite. It is generally pale pinkish, light gray, or cream
<br />colored. In many places it is associated and interbedded
<br />with gravel and sand. This fact shows some distribu-
<br />tion by water.
<br />Welded tuff representing all stages of duration
<br />makes up a large part of these formations. It is the re-
<br />sult of the cohesion of glass fragments in ash-flow
<br />material under certain temperature conditions prevail-
<br />ing at the time of deposition and subsequent cooling.
<br />The welded tuff is moderately hard to very hard, ranges
<br />from light brown to lavendar, and contains conspicuous
<br />phenocrysts of feldspar, biotite, and quartz. In many
<br />places it forms massive cliffs along stream valleys ar
<br />resistant cappings of mesas. In places it has a thin
<br />basal zone of obsidian m~ vitrophyre. The harder varie-
<br />ties of welded tuff have often been mistakenlc identi-
<br />fied as flow rock. Prnbabh' only a few of the rhyolitic
<br />rocks of this survey area are of the lava flow type.
<br />The Hinsdale Fm•mation, of Pliocene epoch, is mainly
<br />lava flow deposited on a flat erosion surfar•.e over Mio-
<br />cene volcanic material. The Hinsdale rock is mainly in
<br />the Greenie Mountain area south of Pock Creek. A few
<br />small remnants occur southwest of Sevenmile Plaza.
<br />The rock at the lower elevations in these areas has been
<br />classified as basalt. The lava capping Greenie Mountain
<br />is ]atite.
<br />The Quaternary, the latest and cun~ent period of
<br />geologic time, includes the Pleistocene and Holocene
<br />epochs. The Pleistocene epoch is marked by evidence of
<br />repeated worldwide glacial advances in the higher lati-
<br />tudes, fluctuations of sea level, and the appearance and
<br />migration of many existing species of plants and ani-
<br />mals. The Tlolncene epoch is roughly Che Inpse of time
<br />since the retreat of the last Pleistocene ire sheet.
<br />Deposits of the Quaternary perinrl consist mainly of
<br />uncmtsoliilated material weathered from Tertiary vol-
<br />canic rock and h~:m.cporterl and rerleposifed by water.
<br />I~ RE~COI,ORADO • III III'll'll' I
<br />These rleposifs consist of v:u•ving proportions of clay,
<br />silt, sand, gravel, cobbles, and boulders.
<br />The oldest Quaternary deposit in the survey area is
<br />sandy and gravelly nlltn~ial material, which mantles
<br />surfaces of the low, gently sloping terraces along both
<br />sides of the Rio Grande west of Del Norte and the
<br />mesas or fans along the mountain front south of Del
<br />Norte. In these areas at least two main sets of gravel-
<br />covered terraces can be observed at different heights
<br />above the modern strewn flood plain. From a point
<br />about 4 miles east of South Fork and extending east-
<br />ward toward the San Luis Valley, the two terrace levels
<br />converge rapidly and decline somewhat abruptly until
<br />in places they merge into the modern alluvium.
<br />Upstream toward the west along Rock Creek and
<br />Raton Creek, the higher mesa surface rises much more
<br />steeply than the channels of the streams. In sections
<br />20 and 32. T. 38 N., R. 7 E., the surface of the gravel
<br />mantle is less than 100 feet above the Berl of Rock
<br />Creek, whereas t..5 miles farther .vest it is more than
<br />-100 feet above the same stream.
<br />Tn some places west of 14onte Vista, the lower set
<br />of mesas or terraces blends into the younger alluvium
<br />of the flood plain. In places they are bounded on the
<br />east by escarpments °0 to 40 feet high, which descend
<br />abruptly from the mesa level to the flood plain. West-
<br />~~ard, the mesa surface rises more rapidly than the
<br />stream gradient, so that at the mountaimvard margins
<br />it is lii0 to °00 feet above the adjacent streambed.
<br />The two sets of sloping mesas or terraces mantled
<br />with gravel indicate up~~ard tilting of the mountains
<br />and downward tilting of the San Luis Valley during
<br />the Quaternary period. The gravelh~ alluvium mantling
<br />Che h~o sets of mesas a terraces probably can be cor-
<br />related with the two main intervals of Pleistocene gla-
<br />ciation in the San Juan Dountains that preceded the
<br />latest interval of Pleistocene glaciation.
<br />Along the Flood plains of the larger streams and on
<br />the Rio Grande aHucial fan, there are extensive alluvial
<br />deposits of latest Pleistocene (late Wisconsin) and
<br />Holocene epochs. A low glacial outwash terrace of prob-
<br />able late bViscmtsin age is a prominent feature on both
<br />sides of the valley of the Rio Grande in the vicinty of
<br />South Fork. Eastward its surface declines regularly
<br />with a steeper gradient than that of the Hood-plain
<br />alluvium. The terrace blends into the Recent alluvium
<br />of the flood plain in a few miles.
<br />Some soils in the mountarins have had reddish wind-
<br />blown silt and clay rlepnsiterl on them fi•om a source
<br />°00 or ,,°,00 miles to the southwest. In the eastern part
<br />of the survey area, some soils have been influenced by
<br />sandy material rewm'ked by wind, especially at the
<br />surface.
<br />fliwate `
<br />The F,io Grande County Area is in a high, ch y moun-
<br />tain valley. The climate is characterized by cold winters,
<br />cool summers, low precipitation, strong winds in spring,
<br />and much sunshine. "temperature and precipitation are
<br />shown in tables 11 ;uul 13 far the survev area.
<br />'~ TaLlc•s prepared with Ure asst sta nrr of .J. W. RF:RR\',
<br />clima[uln~;ist fm' Colo.. National R'cathcr Service, U.S. Depart
<br />went of Commerce.
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