138 UNIVERSITY OF COLORADO STUDIES 



(about 2 feet) high. (See Fig. 8.) This rim is made up of loose, spongy 

 soil composed largely of organic material. Farther back from the 

 lake in the outer part of the sedge zone and in the shrub zone, the soil 

 becomes more compact, freer of organic material and assumes a clayey 

 character. On the south side of the lake, a thick growth of moss in 

 the shrub zone makes a spongy surface layer. The forest zone is 

 restricted to the morainal ridges. These are made up of a coarse material 

 and are very thinly covered with drift. This shallowness of soil is 

 due to rapid run-off and partially accounts for the number of uprooted 

 trees and for the openings in the forest where trees have fallen. 



Comparison with Other Lakes. — In level areas, such as are found 

 on the plains of Colorado or prairies of the middle states, the zones about 

 the lakes are much wider than they are about mountain lakes. In the 

 case of level-country lakes the general run-off is slow, consequently a 

 fine soil can accumulate along the shore. Seepage water from the lake 

 works its way out through the soil of the shore, is held by the fine soil 

 and thus forms a wide marshy area. On the other hand, in the case of 

 Redrock Lake and many other bodies of water in mountain districts, 

 the general run-off about the lake is rapid. This condition prevents 

 the accumulation of a fine soil about the lake and so the seepage water 

 in the soil at the lake margin is quickly carried away and does not pro- 

 duce an extended marshy area. It will be seen (see Fig. 1) that the 

 plant zones on the north side of the lake are particularly narrow. Here 

 the lake is bounded by a low moraine, which slopes gradually toward the 

 shore (see Fig. 3), so that there is but little wash material carried to 

 the lake edge. 



II. Climatology 



General Statement. — No weather records are available for Redrock 

 Lake except those made by the present writers during the summer of 

 1908. A Draper thermograph was kept running during June, July and 

 August, trips being made weekly from Boulder, thirty miles away, to 

 change the records. The instrument was housed in a suitable shelter 

 (see Fig. 12) with slat sides and was located about 100 meters (320 ft.) 

 east of the lake on srround two meters above the level of the lake. A 



