=125. Surface.= The most important consideration with respect to surface is the presence or absence of cover, and the character of the latter. With the exception of snow, cover is, however, a question of vegetation, living and dead, and consequently is to be referred to the discussion of biotic factors. The surface of the soil itself often shows irregularities which must be taken into account. Such are the rocks of boulder and rock fields, the hummocks of meadows and bogs, the mounds of prairie dog towns, the innumerable minute gullies and ridges of bad lands, the raised tufts of sand-hills, etc. The influence of these is not profound, but they do have an appreciable effect upon the run-off, temperature, and wind. In many cases, this is distinctly measurable, but as a rule little more can be done than to indicate that the surface is even or uneven, and to describe the degree and kind of unevenness. =126. Record of physiographic factors.= Altitude, slope, exposure, and surface are essentially constant factors, and are determined once for all, after a few check readings have been made, except in those relatively rare habitats in which dynamic forces are very active. The form of record used is the following: ═══════╤═════════╤════════╤════════════╤════════╤═════╤════════╤═══════ DATE │FORMATION│STATION │ GROUP │ALTITUDE│SLOPE│EXPOSURE│SURFACE ───────┼─────────┼────────┼────────────┼────────┼─────┼────────┼─────── 10/7/02│Gravel │Golf │Eriogonare │2700 m. │ 23°│ N.N.W. │Even │slide │Links │ │ │ │ │ „ │Brook │Jack │Violare │2550 m. │ 5°│ E.N.E. │ „ │bank │Brook │ │ │ │ │ „ │Half │Hiawatha│Achilleare │2600 m. │ 14°│ E. │Uneven │gravel │ │ │ │ │ │ „ │Spruce │Milky │Opulasterare│2625 m. │ 12°│ N. │Even │ │Way │ │ │ │ │ ───────┴─────────┴────────┴────────────┴────────┴─────┴────────┴─────── =127. Topography.= As heretofore indicated, questions pertaining to the form and development of the land concern groups of habitats within which each habitat is the unit of investigation after the manner already laid down. A knowledge of topography is essential to the accurate mapping of a region, for which the simple methods of plane table and contour work are employed, while the geology of the surface is of primary importance in the study of successions. BIOTIC FACTORS