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PUEBLO AREA, COLORADO
<br />are resistant to weathering. Eolian silt or loess a fea•
<br />inches to several feet thick covers the plains in much
<br />of the Pueblo Area. In the foothills, loess is on east- or
<br />northeast-facing slopes. The `Viley soils were derived
<br />exclusively from loess, but the loess influence is also
<br />apparent in JVlanvel, Nunn, and Larkson soils.
<br />Sedimentary rocks of Cretaceous age include shale,
<br />limestone, and sandstone. The most extensive shale
<br />formation is the Pierre Shale. Outcroppings of it
<br />parallel Fountain Creek and the Arkansas River and
<br />extend about 10 miles south of Pueblo. The formation
<br />is divided into several distinct members that vary in
<br />reaction, color, content of gypsum, concretions, and
<br />content of fossils. Of particular interest is the Tepee
<br />zone member. It is characterized by' small, sharp-
<br />pointed hills shaped like inverted cones. The point of
<br />each cone has an irregular cap of light-gray or brown
<br />limestone that contains fossils. The topographic form
<br />of the buttes is the result of the resistance of their lime-
<br />stone caps to erosion. Razor and Llidway soils formed
<br />in Pierre Shale, and Limon soils formed in alluvium
<br />that n•as derived from Pierre Shale. Graneros and
<br />Carlile Shale formations also occur in the survey area,
<br />generally on steep slopes. Soils that formed in shale are
<br />slowly permeable; most of the rain that falls on them
<br />runs off. Consequently, little water leaches through
<br />the profile and soil horizons form slowly.
<br />Soft to hard limestone membet•s of the Niobrara
<br />Formation of Cretaceous age crop out in canyons and
<br />escarpments and underlie plains in much of the
<br />Pueblo Area. The Niobrara Formation includes thick
<br />marl and calcareous shale, the Smokey Hill member;
<br />and prominent limestone, the Fort Hays member, is
<br />at its base. Soils formed slowly in limestone and marl
<br />because of the abundance of lime. Soil-forming proc-
<br />esses proceed slowly until the lime has been leached out
<br />and the.soil material changes from alkaline to neutral
<br />or acid. The Niobrara Formation is the parent material
<br />in which Penrose, Shingle, Minnequa, IvIanvel, and
<br />LaPorte soils formed.
<br />Dakota Sandstone underlies the Niobrara Formation.
<br />It crops out in valleys and canyons, underlies plains in
<br />the southern part of the Pueblo Area, and lies as a
<br />hogback ridge in the western part. The Dakota Forma-
<br />tion consists of fine-grained, thin]}• bedded to massive
<br />sandstone and shale. Sandstone makes up a greater
<br />part of the formation; in places there are massive
<br />sandstone cliffs mote than 50 feet high. The sand-
<br />stone is generally noncalcareous, although soils that
<br />formed in material derived from it can have lime layers
<br />ft•om an accumulation of calcareous dust. Soils that
<br />formed in sandstone generally- have a reddish, fine-
<br />textured Bhorizon. Travessilla, Iiim, ~'amer, hlorten-
<br />son, Larkson, Pinata, and Stroupe soils formed in
<br />parent material that vas partly or wholly derived ft•om
<br />Dakota Sandstone.
<br />Precambrian Granite crops out on steep mountain
<br />slopes and is the parent material of the gravelly, shal-
<br />low Wetmore soils.
<br />Climate
<br />IVlost of the Pueblo Area is in the plains physio-
<br />graphic province. The plains have light rainfall, moder-
<br />ate to high ~e•inds and a wide range in temperature.
<br />83
<br />United States Weather Bureau records at the Pueblo
<br />Airport show an average annual precipitation of 11.9
<br />inches, an average annual temperature of 53.7° F, and
<br />an average summer (June, July, and August) tempera-
<br />ture of 74°. An important feature of the precipitation
<br />is that 70 to 80 percent falls in the period April to
<br />September, which is the growing season. Nevertheless,
<br />evapotranspiration exceeds the precipitation. Sum-
<br />mer precipitation is largel}• from thunderstorms, which
<br />sometimes are extremely- heavy and yield much runoff.
<br />Approaching the foothills and mountains in the µ'est-
<br />ern part from the plains in the eastet•n part of Pueblo
<br />County, there are a number of significant changes in
<br />the climate. The winds are less severe, temperature
<br />changes from day to day are not so great, summer tem-
<br />peratures are lower, and n•inter temperatures are
<br />higher. Precipitation increases significantl}~ with the in-
<br />creasing elevation of the foothills.
<br />An important factor in soil formation is the amount
<br />of water available for leaching during seasons when
<br />the soil is warm enough for plant growth and microbial
<br />activity. Water and temperature have a major role
<br />in the growth and acticit}• of organic life in and on
<br />the soil, in the physical translocation of substances
<br />in the soil solutions, and in controlling the rate and
<br />direction of chemical processes. For example, soils on
<br />the plains have undergone little leaching. The depth
<br />to soluble calcium compounds in soils that have distinct
<br />horizons ranges from a few to about 20 inches and
<br />represents the average maximum depth that moisture
<br />reaches. In the mountains of the Pueblo Area, soils
<br />have lost bases and generally are acid. The surface
<br />layer has a bleached color, and the subsoil is relatively
<br />deep.
<br />Biological factors
<br />Biological factors are active in soil formation. In
<br />the Pueblo Area, animals, insects, bacteria, and other
<br />organisms, including man, are important biological
<br />factors (8), but vegetation is the most important.
<br />The composition and density of plant growth differ
<br />between bt•oad geographic divisions in the survey area
<br />and, less obviously, among the different soils in a land-
<br />scape. The contrast between the vegetation of the plains
<br />and the woodland of the foothills and mountains is
<br />paramount.
<br />The vegetation of the plains includes short and
<br />midtall grasses. Plant growth varies widely from year
<br />to year, depending on rainfall, and is interrupted by
<br />winter. Periods of draught are common. The summer
<br />heat hastens decomposition of annual residue, which
<br />has usually decomposed before the next season's
<br />growth. Small leaflets and stems of dry plants are
<br />easily detached and blown about by the wind. Dlost of
<br />the organic matter in the plains soils comes from de-
<br />cayed roots; the organic-matter content in the upper
<br />8 to 10 inches of soil is 1 to 2 percent. In the foothills
<br />and mountains, where precipitation is higher and the
<br />temperature lower, plant residue is more abundant and
<br />decomposition is slower. The surface layer is darker
<br />colored under grass in these areas than on the plains.
<br />Also, it has 2 to 3 percent more organic matter.
<br />Under forest vegetation the forest litter, including
<br />fungi, produces acids and other substances of great
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