The Laramie Basin - Mountains and Plains: The Ecology of Wyoming Landscapes - page 295

the niobrara Formation in the Laramie Basin is exposed
on the west flank of Sheep Mountain and at scattered
localities on the basin floor, including Big Hollow.
though selenium is a requirement for animal health in
low amounts, it is poisonous if too much is eaten.
Another example is the distinctive way in which
elongated aspen woodlands grow along the contour
on the east flank of centennial Ridge, on the west side
of the basin (see fig. 10.3). in this case, the association
is between aspen and surface exposure of the contact
between permeable Paleozoic sedimentary rocks and
the impermeable Precambrian igneous and metamor-
phic rocks that form the main part of the ridge. Water
flowing from above mostly stays near the surface, just
under the thin soils, until it meets permeable sedimen-
tary strata of the Fountain Formation, where it infil-
trates the soil and bedrock. the whole mountainside
essentially funnels water to this contact zone, creating
favorable conditions for aspen.
in 1969, botanist William Myers studied the effects
of bedrock type on the abundance of native plants
in the basin. 23 With the assistance of noted geologist
Brainerd Mears, he was able to identify nine distinctive
sedimentary strata exposed on the west flank of Sheep
Mountain. Four of the strata were more resistant to ero-
sion and were fully exposed as hogback ridges, whereas
others were more easily eroded and formed low, soil-
covered troughs between the ridges. Some plants were
found only on the niobrara Formation, as might be
expected because of its high selenium content. they
included the selenium indicators mentioned previ-
ously along with greasewood, saltbush, and winterfat.
other plants were found only on the resistant rocks of
the Fountain Formation, probably because they ben-
efited from a more favorable water supply (for example,
aspen, shrubby cinquefoil, serviceberry, and common
juniper). Another group of plants was found on the hog-
back ridges some distance from the mountain (currant,
mountain-mahogany, limber pine, and ponderosa pine).
Myers found no plants that grew only in the troughs
between the ridges, probably because these low areas
had soils that were influenced as much by weathering
of the adjacent ridge rocks as by the underlying shales.
Myers's study illustrated that rather few species of
plants are restricted to one geologic formation. in his
study area, plant distribution is determined primarily

by the availability of water next to the mountain, soil
chemistry over the niobrara Shale and Forelle Limestone,
and a coarse soil texture (such as on the sandstones of
the Fountain, casper, Muddy, and cloverly formations).
Big sagebrush was found growing in relatively deep, well-
drained soils derived from all rock types.
Another informative approach to studying geo-
botanical relationships is to examine the concomitant
variation in topography, geologic surfaces, and land
cover along transects. Figures 17.15 and 17.16 illustrate
two profiles that were prepared for the Laramie Basin.
one is 17 miles long and extends southeastward from
the Little Laramie River to the Big Laramie River, pass-
ing across the Airport terrace and Big Hollow. the other
profile is 4 miles long, extending from the Big Laramie
River eastward to the Laramie Mountains, crossing a
subdued ridge formed on the chugwater Formation and
a prominent ridge formed from the erosion-resistant
Forelle Limestone, across the erodible shales of the
Satanka Formation, and then up the lower outcrop of
the casper Formation. the importance of shallow soils,
salinity, and inundation from flooding can be seen. For
both transects, grassland plants are most common, sug-
gesting that they tolerate a broad range of soil condi-
tions and topography.

Reflecting on the Past, Anticipating the Future

the Laramie Basin has experienced ecological changes
throughout the Holocene and for millions of years be-
fore that. tectonic activity, fires, flooding, and climate
change have been most influential. Recently, over
shorter periods, the changes can be attributable to
transitions in how native and euroAmerican cultures
used the land (see chapter 2). Beaver were used hardly
at all by the indians, but they were nearly extirpated
by euroAmerican trappers (see chapter 2). the people
who lived here for 10,000 years or more harvested only
a small number of trees, mostly small, in contrast to the
immigrants arriving about 150 years ago. they needed
railroad ties and lumber for construction. Unfenced
bison herds roamed the plains until the 1860s, when
cattle and sheep replaced them. Large numbers of
domestic livestock survived on the prairie vegetation.
Sheet-wash erosion left some slopes with little or no
soil, most likely because of heavy livestock grazing near