THE STUDY AREA
The study area includes most of the land used by Grand Teton and southern Yellowstone elk (Figure 1). It extends about 55 miles north of the south boundary of the National Elk Refuge near Jackson, Wyoming, through Grand Teton Park, and Teton National Forest lands into Yellowstone Park. Most of the area is on the west side of the Continental Divide in the headwaters system of the Snake River. A comprehensive account of the area's geology is given by Fryxell (1930), Love and Reed (1968). Physiographic features and soils show the effects of glacial action. Glaciers extended onto Grand Teton valley floor as recently as 9,000 years ago.
The main physiographic features on the study area and adjoining lands are shown on Figure 2. The map regions shown as Jackson Hole, the Mount Leidy and Pinyon Peak highlands, the Snake River drainage slopes of the Teton Mountain, Gros Ventre, and Hoback Ranges, and the east slope of the Snake River Range are called "the Jackson Hole area." The portion of the valley south of the Snake River outlet from Jackson Lake is called "the Jackson Hole valley."
A comparatively flat glacial outwash plain forms the floor of the Jackson Hole valley. Other features on the plain are glaciated ridges that project through the surrounding outwash deposits, terrace benches along the sides of past and present water courses, potholes formed from melting ice blocks, alluvial fans, and braided stream bottoms. Foothill ridges and slopes border the valley on the east. Glacial moraine deposits border the valley on the north and west and extend as projections onto the outwash plain.
A series of high ridges, mountains, and plateaus occur from the north end of the valley into southern Yellowstone Park. Elevations range from a minimum of about 6,200 feet on the refuge to maximums of over 13,700 feet on the Teton Range, and slightly above 10,000 feet for the highest mountain and plateau tops within southern Yellowstone Park.
The outwash plain soils on the valley floor are poorly developed and contain a high proportion of rounded cobblestones. Glacial moraine and alluvial fan soils contain relatively greater proportions of fine textured material in variable mixtures with angular rock and cobble. Ridge and foothill slopes have mostly fine textured residual soils, but coarse textures occur where resistant rock formations are exposed or overlying glacial deposits are present. Soils along water courses range from layer cobble with gravel and sand in interspaces to developed sand, silt, and clay loams. Mountain and plateau soils are extremely variable, ranging from deep residual loams to very coarse textures developed from rock talus or volcanic rhyolite.
The area has short, cool summers and cold winters. U. S. Department of Commerce records for Moran at the northern limits of the Jackson Hole valley show a 30-year (1936-1965) mean annual temperature of 34.8° F. (Houston, 1967). The mean temperature for July as the warmest month is 60° F. January as the coldest, 10.3° F. About three-fourths of annual precipitation of 22.2 inches for the period fell as snow.
West to east and north to south gradients in lessened snow depths were apparent. Midwinter snow depths on the west side and north of the Jackson Hole valley commonly ranged from 6 to 3 feet, as compared to 3 to less than 1 foot depths on the east side or in the southern portions of the valley.
Moran was at the northern limit of the area used by any appreciable numbers of wintering elk. December through March snow measurements at this station were used to reflect the relative magnitude of this environmental influence between 1961 and 1967 (Table 1).
The period from November 16 through April 15 was considered the winter season. April 16 through June 15 in valley areas, and May 16 through June 30 in the mountain areas, was spring. June 15 or 30 to September 15 was summer. Fall extended from September 16 through November 15.