STUDIES IN MESA AND FOOTHILL

VEGETATION, I

1. Geology and Physiography of the Mesas near Boulder

GIDEON S. DODDS

2. Climatology of the Mesas near Boulder

3.

FRANCIS RAMALEY

Distribution of Conifers on the Mesas

W. W. ROBBINS AND GIDEON S. Dodds

4. Distribution of Deciduous Trees and Shrubs on the Mesas

W. W. ROBBINS

I. GEOLOGY AND PHYSIOGRAPHY OF THE MESAS
BY GIDEON S. DODDS

In the following articles is given an account of some results of investigations carried on by the writers during the seasons of 1906, 1907 and 1908. The area studied is the mesa region, at the foot of the Rocky Mountains, just at the border of the plains. Observations have been made of this area for some distance along the foothills north and south of the city of Boulder, Colorado. A more detailed study has been made of two mesas, Horse Mesa and Long Mesa, situated about two miles south of Boulder, between Boulder Creek and Bear Canyon. Plate I.)

(See

The object of these studies has been to determine the conditions affecting plant distribution on the mesas. Studies have been made of physical and meteorological conditions. Many of the plant formations have been accurately mapped, and the physical characters of the different habitats determined. In articles to be published later, other plant formations will be described in relation to their habitats.

General Appearance of the Mesas.-The following account of the topography and structure of the mesas is taken verbatim from the report of Fenneman on this area.1

1905.

FENNEMAN, N. M., "The Geology of the Boulder District, Colorado," U. S. G. S. Bull. 265, pp. 1–98

East of the last foothill ridges the slope to the plains may be continuous or may be broken by a sharp step, leaving a high terrace adjacent to the foothills. This terrace is called a mesa, or locally "the mesa." It occurs in fragments of unequal height abutting against the most easterly foothill ridge or separated from it by a trough due to recent erosion. These fragments are commonly separated from one another by ravines. The mesas have a height of from one hundred to three hundred feet above the lower plains, and a west-east width in this vicinity varying from a fraction of a mile to three miles.

The eastward slope of the terraces may be as great as 10° on the foothill side, but it diminishes rapidly toward the plains. At one mile east from the foothills a slope of 3° or 4° is common, and at three miles the slope is 1°. An east-west cross section shows an even curve like the profile of a stream. In addition to the eastward slope there is generally a distinct northward or southward inclination toward a lower bench quite similar to the higher, but separated from it by a steep bluff, or more commonly by a ravine. In a general way the highest mesas are farthest from the larger streams and the height diminishes as these streams are approached.

Structure and Covering.-These mesas are essentially rock benches covered ten or twenty feet deep with unassorted rock waste. The rocks, chiefly Pierre shales, are steeply upturned, being affected by the mountain uplift. They have been smoothly planed off, not to a flat, but to an inclined surface. The débris covering may have a fairly uniform thickness of twenty feet on the highest benches. Generally speaking, the thickness is less as the mesa surface is lower. This thoroughly unassorted débris comprises fragments of all sizes from sand grains to boulders more than ten feet in diameter. The heaviest boulders are close to the foothills, but fragments a foot or more in diameter are found some miles to the east.

It is the opinion of Fenneman and most other recent writers that these mesas are, in their origin, river terraces.

HORSE MESA AND LONG MESA

Location and Surroundings of the Mesas.-As already stated, these two mesas are located between Boulder Creek and Bear Canyon, some two miles south of the city of Boulder. Here the Dakota "Hogback," commonly the outermost foothill ridge, is wanting, and the slope of the mesas extends unbroken with gradually increasing steepness, westward to the ridge formed by the Lyons and Fountain formations. Here the slope is very abrupt. In this locality the conglomerates of the Fountain formation are exposed in huge masses dipping at an angle of about 50° to the east. Locally these rocks are known as the "Flatirons" (Fig. 1). A mile to the west of the mesas, the summit of Green Mountain rises

to an elevation of 8,100 feet, 2,000 feet above the level of the mesas. South Boulder Peak, farther to the south, is somewhat higher (Plate I, Fig. 2). These two mesas extend from the foothills in a northeasterly direction. Their length is about three-fourths of a mile. Eastward the slope of the plains is gentle.

Topography of the Mesas.-The two mesas are separated from each other and from adjacent mesas by sharp ravines. The tops are very even and unbroken. The eastward slope on the part farthest from the foothills is about seven degrees (Plate I). This even top is from 150 to 250 feet above the level of the adjacent ravines. The slope from the crest

FIG. 1.-DIAGRAMMATIC REPRESENTATION OF A WEST-EAST SECTION AT THE EDGE OF THE FOOTHILL AREA.-At the west the igneous rocks of Archaean age form the mass of the foothills. To the east of the foothills in the mesa and plains region the edges of the sedimentary rocks are exposed. There are two prominent and continuous north and south ridges, one formed by the resistant strata of the Fountain conglomerate and the other by the Dakota sandstone. In places the Niobrara limestone also forms a low ridge. The diagram shows the relation and structure of the mesa. This, it will be seen, extends eastward from the Dakota "Hogback" and is underlain chiefly by Pierre shales. The rock waste of the mesa cap is derived chiefly from fragments of the Fountain and Dakota sandstones. This layer of rock waste is quite thick on the top of the mesa and much thinner on the steeply sloping sides. (After Fenneman.)

Erosion is going

of the mesa to the bottom of the ravines is abrupt. on actively on the sides where the easily eroded shales are not protected by the mesa cap. Thus the top remains flat and even and the sides steep. The mesa cap also serves to keep the upper part of the slope, just below the crest of the mesa, steeper than the rest of the slope (Fig. 2).

The sides of the mesas are cut by numerous gullies, mostly small and narrow and not very deep. The slope of the sides is broken by a number

of wider ravines, which, just below the edge of the mesa, widen out and assume an amphitheater-like form. These are of importance in relation to the distribution of moisture, as will be seen later. On Horse and Long Mesas these occur only on the south side. They are indicated on the topographic map by X (Fig. 3).

Gulches.-Bluebell and Pole Canyons head well up toward the summit of Green Mountain, while Cemetery Gulch is a younger, shorter gulch still working headward into the general surface of the mesa. In all of the gulches are intermittent streams. On account of several good springs in Bluebell Canyon it contains much more moisture than either of the other gulches and water flows in its channel during the greater part of the year. In Pole Canyon water flows during spring and early

FIG. 2.-DIAGRAMMATIC REPRESENTATION OF CROSS-SECTION OF A MESA.-This shows the underlying rock, usually Pierre shales, and the covering of rock débris. On Horse and Long Mesas the largest fragments are boulders several feet in diameter. This explains the extremely rocky zone at the edge of the mesa. The thin layer of rock waste on the steeply sloping sides is at places entirely wanting, exposing the underlying shales. Reference to the figure explains the seepage zone just below the crest of the mesa. Water percolates readily through the mesa cap to the impervious shales and at this level reaches the edge of the mesa.

summer, while Cemetery Gulch carries water only during wet weather.

Bluebell Canyon is a deep V-shaped gulch throughout the greater part of its length. Pole Canyon is deep and narrow at the foothill end of the mesa, but lower down in its course it widens out into a broader U-shaped valley. Cemetery Gulch is V-shaped with fairly steep sides.

From the above account it will be seen how great is the variety of slope and exposure on these two mesas, giving a wide diversity of conditions for plant growth. There is a difference of elevation of eight hundred feet between the highest and lowest parts of the area under study,

but it must be remembered that in this area differences of altitude are of little consequence in determining plant distribution. The contour lines

FIG. 3.-TOPOGRAPHIC MAP OF HORSE AND LONG MESAS.-Contour interval 50 feet. The numbers on the contour lines indicate feet above sea-level. The area shown is about 500 acres. The map shows clearly the even top of the mesas sloping gently toward the northeast. The limit of this even top is indicated by the broken line. Here the steep sides of the mesa begin. The sides are cut by numerous small ravines. The broad amphitheater-like ravines are indicated. Three of the more important of these are marked by X on the map.

on the map are not of so much importance in marking altitudes, as in showing the direction and steepness of slope.1

The altitude seems to be of importance in the distribution of a few shrubs. See Mr. Robbins' article on "Deciduous Trees and Shrubs."