1918] ROBBINS—BOULDER PARK 509 
however, for the rim is also a feature of glacial lakes such as Red- 
rock (20) and Park, which are not genetically related to stream 
topography. In these it is no doubt formed by the sapping action 
ofice. Of course, stream action and ice action may be cooperative 
factors. At any rate, whatever its origin, once established the 
tim is maintained by vegetative building. At East Lake the rim 
about the edge has a mean height of about 16 inches. In texture 
it is a loose and spongy mass and consists of living and decayed 
Woter Jeve/ 
wenn - FLL 
d oke battom 
to.—Section of “rim” and sedge moor at East Lake: rim which overhangs 
water is loose and spongy texture and consists of living and decayed plant material, 
largely masses. 
plant material, largely mosses. Just back of the rim the soil is 
wetter, more compact, and the character of the vegetation some- 
What different. Being actually drier than the moor a foot or so 
landward, it supports an assemblage of plants, many of which 
would scarcely be expected to grow at the water’s edge. Salix 
chlorophylla and Dasiophora fruticosa are the important shrubs 
of the rim. The seeds lodge and germinate on the bare, more or 
less perpendicular, wall of the rim, and the young plants curve out- 
ward and upward over the water. Other characteristic rim plants 
are Betula glandulosa, Sedum rhodanthum, Dodecatheon radicatum, 
and Poa leptocoma Bong. The importance of moss in the building 
of the rim should be noted. As the rim slowly builds out over the 
