MESA VERDE Environment of Mesa Verde, Colorado Wetherill Mesa Studies

SOUTHWEST EXPOSURE CANYON-SLOPE SITE, C—1

This site was 125 feet above the canyon floor, at an elevation of 6,500 feet. It was halfway up a southwest facing talus slope of 35° (fig. 12). The vegetation is a juniper-pinyon community with scattered big sagebrush. The trees are small and widely spaced (fig. 13), the average distance between them being more than 20 feet, which is considerably greater than in the other stands (table 12). Most of the local shrubs, with the exception of oak and fendlerbush, are represented in the community. Sagebrush is confined to deep, well-drained soils at other sites on the mesa, but on talus slopes it is common on the heavy soils associated with shale outcroppings. Since there are lenses of shale throughout the slope, sagebrush is the characteristic shrub of the stand. The other species present are listed in table 2.

Fig. 12 C—1, on west-southwest-facing talus slope in Navajo Canyon, with instrument shelter visible in lower center, between sandstone outcroppings, about 125 feet above the canyon floor.

Fig. 13 C—1 instrument shelter, at about 6,500-foot elevation. The soil here is a Sandstone Outcrop—Stonyland Complex, and the vegetation is an open juniper-pinyon/big sagebrush community.

TABLE 12.—TREE DATA AT C—1

The largest juniper and pinyon were 24 and 14 inches in diameter, respectively. The oldest of 10 pines sampled was 240 years, and the average for the 10 was 160 years. This is the only stand type sampled in which juniper is more abundant than pine.

The soils on the steep canyon slopes are formed partly from coarse colluvial-alluvial materials and partly from the underlying bedrock. Because several different soils could be recognized and because of the steepness of the slope, the general soil type suggested for the southwest facing talus on which C—1 is situated is the Mughouse Rock Outcrop Complex. The soil at the site is classified as a shallow phase of Mughouse stony loam. It is shallower, less developed, and somewhat more brown than normal, but it fits the series fairly well.

Developing on interbedded shales and sandstones of the Menefee formation, the soil is quite stony, with most of its coarse material having come from the more resistant Cliff House sandstone above. The depth of the soil is variable; below the cliffs it is often less than 12 inches deep. Many shallow outcroppings of shales and sandstones occur on this broad talus slope. The profile description in table 13 is taken from the open area just to the right of the shelter in figure 13.

TABLE 13.—SOIL PROFILE UNDERLYING C—1 JUNIPER-PINYON/BIG SAGEBRUSH STAND*

*Soil classified as Mughouse stony loam.

A contact zone between the Cliff House sandstone and the underlying Menefee formation (primarily shales) is replete with seeps and springs. Below this point of contact, moisture from lateral flow of ground water seems negligible, at least over much of the canyon slopes. Thus the primary source of water for the slope vegetation is precipitation, but retention of moisture varies greatly where soils are so variable. Where coarse colluvium occurs, the interstices between the rocks and boulders trap and retain much of the runoff to the advantage of the deeper rooted plants. But where the soils are fine textured along the shale lenses, porosity and water permeability are low, even though the water-holding capacity (as shown in table 5) is quite high.

The C—1 site represents a segment of the Mesa Verde terrain whose climate is controlled by the local topography or, more precisely, by exposure. Facing west-southwest, the slope receives more insolation, on an annual basis, than any of the other sites. This is especially pronounced during the winter months. (See air temperature data in the appendix). As a result, daytime temperatures exert the greatest influence on the environment. Whereas M—1 represents the most arid mesa-top environment, C—1, indicative of the more exposed canyon slopes, is in the more arid part of the canyon landscape.

At C—1, moderate soil temperatures during the winter and the southerly exposure favor early spring development of the vegetation. Snow rarely blankets the slope for any extended period of time. During the winters observed, snow persisted from the latter part of December through late January; otherwise, cover was generally spotty. All these factors support the conclusions drawn from our phenological information: that plant growth commences earliest at this site. In 1963, the flowering, of Phlox hoodii began the third week in March and was abundant by the end of the month. This was delayed about a week the previous year. Flowering of the shrub species, beginning with Amelanchier, also occurred first at the C—1 site.

In brief, the environment of the southwest-facing canyon slopes is comparable to that along the southern part of Mesa Verde, and plant growth is least favorable at these locations. Data on the atmospheric factors are given in table 11 and the appendix.