NORTHERN COLORADO PLANT COMMUNITIES 235 



feet below Tolland station, caused the formation of a lake which covered 

 all the western part of the park, and it is likely that other morainic lakes 

 of different sizes have existed here at different times. Perched boulders 

 on the steep hillsides both to the south and north show that the glacier 

 was at some time 300 to 400 feet above the present level of the park. 

 The soil of the park is made up largely of glacial drift for the most part 

 very slightly worked over by the action of water. 



Boulder Park is, like other broad valleys in Colorado, almost treeless 

 and the absence of trees must be due chiefly to soil conditions. The 

 soil is throughout less well drained and of somewhat finer texture than 

 that of the surrounding hillsides. Hence it is better adapted to the 

 grasses and sedges which now occupy it than it would be to conifers. 

 Again it must be remembered that in comparatively recent times the 

 level parts of the park were covered with ice, long after the adjacent 

 ridges were exposed and had become well clothed with timber. It is 

 possible also that inversion of temperature is a minor factor in shutting 

 out tree growth; for in the early spring, when tree seedlings would be 

 most susceptible to irregularities in the temperature, the nights are un- 

 doubtedly colder and the days warmer in the park than on the hillsides. 



The driest part of the park, with a soil of loose gravel and boulders, is 

 the ridge where the sage-brush scrub occurs north of Park Lake. Adja- 

 cent aspen groves in moister soil occupy depressions between different 

 ridges. Vegetation on the knolls east of the railway station and in the 

 shallow depressions, around East Lake and on the lower slopes of Balti- 

 more Ridge shows everywhere its dependence on physiographic features, 

 and these features are the work of glaciers which came down from 

 the Continental Divide. 



To those with some geological training I commend a study of vegeta- 

 tion in the park in relation to geologic history. Particularly interesting 

 are the old lake beds, the sage-brush ridge and the morainic lakes and 

 basins. Teller Lakes and the surrounding ridges offer much material 

 for study. 



Summary. — The plant associations of Boulder Park (Tolland, Colo.) 

 belong to the montane life zone. A sub-alpine forest of Engelmann 

 spruce is, however, established on north slopes just above the limit of the 

 park while the rock pine forest of the foothills occurs only a few miles 



