1918] ROBBINS—BOULDER PARK 501 
As is quite commonly the rule on flood plains, the first woody 
plants to gain a footing are Salix species. In this case the invaders 
are S. chlorophylla and S. padophylia (figs. 4,5). After a few years 
a willow thicket is formed; at the present time extensive willow 
thickets prevail along the whole course of the stream on ground 
not far removed from the water level. The willow thicket has a 
peat deposit from a few inches to over 3 ft. deep. As has been 
indicated, the water level is near the surface at all times, and the 
association may undergo flooding in the early season. 
In the series of successions starting with gravelly and stony 
stream banks, willow thicket is replaced by a meadow scrub. The 
presence of tall willows about a terrace lake in the Park has come 
to be looked upon as evidence of its oxbow origin. The dying out 
of tall willows in the drier portions of the willow thicket, the quite 
common presence of relicts of willow thicket throughout the 
meadow scrub, and the occurrence of small patches of meadow 
scrub throughout the willow thicket, are all evidences that willow 
thicket is being succeeded by meadow scrub. Such relicts are 
usually represented by a few tall Salix padophylla and S. chlorophylla 
shrubs, and in almost all instances such individuals possess many 
dead branches. 
Meadow scrub attains its typical structure on the middle 
stream terrace. The characteristic shrubs are Salix glaucops and 
Dasiophora fruticosa, both of which are low forms as compared 
with those shrubs dominating the willow thicket. The herbs are 
those found in the herbaceous meadow of the Park. Meadow 
scrub commonly has a striking hummocky character. This is due 
to herbs building up about the shrubs. In places Dasiophora 
dominates the association. This shrub stands about 18 inches 
high, and the individuals usually 2 or 3 ft. apart. It has a con- 
siderable habitat range. In the progressive drying of the meadow 
scrub, it lags behind asa relict. It is, on the other hand, a common 
invader of the sedge moor. 
The fact that meadow scrub on the middle terrace is laid on 
gravel indicates that the stream must have moved laterally rapidly 
on that level, thus giving little opportunity for the development of 
peat. At the present time the stream is swinging back and forth 
