1918] ROBBINS—BOULDER PARK 497 
end of the retreating glacier. Typically, there is little accumula- 
tion of humus upon them. The vegetation is a “dry grassland.” 
The terminal moraine below Tolland undoubtedly acted as a 
dam to the stream coming from the glacial front, and for a long 
time the Park was the site of a lake. Subsequent to the period of 
deposition, the stream was lightened of its load and immediately 
began to cut into the terminal dam, which being of easily eroded 
material was quickly cut through. This resulted in a rapid 
drainage of the lake and the formation of a high terrace on each 
side of the stream issuing from the glacial front. The middle 
terraces are of stream origin. 
ORIGIN OF LAKES AND PONDS.—With the exception of Park and 
Filled Lakes, all the natural ponds in the Park are of oxbow origin. 
Park and Filled Lakes are the largest and deepest. Their depth 
alone shows that they are not of oxbow origin. In the center of 
Filled Lake the peat is over 10 ft. deep. At no other point in the 
Park is there such a deep peat deposit. The relation these two 
lakes bear to the higher level shows that they were not formed 
during the deposition of the material composing this level. On the 
retreat of the first glacier two large ice cores were left on the valley 
oor. Hence, when the wash from the later terminal was brought 
down the valley it was deposited about the edges of these débris 
covered bodies of ice. The ice melted later, leaving the two lakes, 
Park and Filled. 
Climatic factors 
There are no extended climatic records for Boulder Park. 
Rosptns (22) has shown the following average temperature and 
precipitation relations to exist in the “lodgepole pine forest zone’”’ 
of Colorado, and from these a notion may be gained of climatic 
conditions in Boulder Park. 
meaty annual temperatite. . 6. 6G. 34-9" F. 
Mean winter temperatite.. 6... oe. 18.4" 
Mean spring temperature. ....... Ceseel ee eyaes 33-4 
Mean summer temperature...............00555 5 3.6" 
Maeen tall tetnnetatite os ois ons cee es 38.4 
Average length of frostless season. ............-. 67 days 
Average date of last spring frost..............-- une 20 
Average date of first Bs SOOM oo ice eu s September 9 
Absolute annual range of temperature.........-. 104 da 
Mean siaea} men ye ON eee ee 
PAean MUA) BROWIN 3 6. is va ve es eee 
