l6 UNIVERSITY OF COLORADO STUDIES 



Methods of Mapping. — It was found that the topographic sheets of 

 the United States Geological Survey did not show the mesas in sufficient 

 detail to be of use in mapping the different plant formations. This made 

 necessary a survey of the mesas in order to construct a satisfactory map 

 to work from. Long Mesa was mapped by first laying it off into ioo- 

 meter squares marked by stakes. These stakes served as a basis for 

 mapping streams and other features and for drawing in contour lines. 

 The aneroid barometer and hand level were used in mapping topography. 

 In using the aneroid barometer in sketching topography, great care 

 was exercised to avoid the numerous sources of error inherent in the 

 instrument. The contour interval used was fifty feet. This is sufficiently 

 close for the purposes of this study, and when drawn with care shows 

 the nature of the topography in sufficient detail. On the map thus 

 constructed, the plant formations were drawn in accurately with reference 

 to the stakes marking the ioo-meter squares. In mapping Horse Mesa 

 a somewhat different method was employed. Here a traverse plane 

 table with sight alidade was used. Mapping by this method is much 

 more rapid and accurate than by the method employed on Long Mesa. 

 By the use of the plane table a good map can be constructed with a 

 minimum of work, and no great amount of technical knowledge is 

 required for its manipulation. 



Soil of the Mesas. — Two kinds of soil may be recognized on the 

 mesas: (i) A soil of coarse texture composed of mesa cap material. 

 (2) A fine-grained clay soil, where the absence of the mesa cap exposes 

 the underlying shales or limestones. The texture of the mesa cap soil 

 is coarse throughout but it varies in the quantity of large rocks and 

 boulders mixed in it. A narrow zone just at the edge of the mesa 

 contains a large proportion of good-sized rocks (Fig. 2). Along the 

 gulches the soil is of somewhat finer texture than elsewhere, but there 

 is probably none which may be classed as alluvial. Along the streams 

 in the gulches there is a very small admixture of humus (Fig. 4). 



Soil Moisture. — Soil moisture appears to be the most important 

 physical factor affecting plant distribution on these mesas. During 

 the two seasons in which observations have been made, the rain- 

 fall has been much below the normal. Neither was its seasonal dis- 



