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VAUGHAN MACCAUGHEY 
mediate region is very narrow, in many places being only 2 or 3 feet 
in width. The valley walls of the foothill are relatively smooth and 
unfurrowed; the walls of the transition ridge are deeply fluted, with 
numerous alcoves. 
The Transition Region marks the area intermediate, in ecologic 
features, between the high, humid ridges of the rain-forest proper, 
and the low, arid foothills with their covering of xerophytic and 
I semi-xerophytic vegetation. It marks with considerable accuracy 
the usual seaward limit of the summit-ridge cloud-cap. 
On the west ridge there is a marked discrepancy between the 
situations of the topographic transition region and the vegetational 
transition region. These two do not coincide; the topographic 
transition region lies two miles mountainward of the vegetational 
transition region. This difference is due to the presence of the 
Tantalus series of volcanic craters along the west ridge; these have 
pushed the topographic region much further mountainward than it 
otherwise would have occurred. 
On the east ridge practically none of the normal vegetation of the 
lower or middle forest zones occurs seaward of the Transition Region. 
On the west ridge Mount Tantalus rises to a height of 2,000 feet on 
the seaward side of the topographic Transition Region, and supports 
a luxuriant lower- and middle-forest flora. 
The east transition ridge is but 1,200 feet high, at its lowest point, 
whereas the west transition ridge is about 1,700 feet high. The rain- 
forest, which on the east ridge does not extend beyond the Transition 
Region, on the west ridge covers, not only the "transition" region, 
but also the mountainward half of the Tantalus mass. This condi- 
tion clearly illustrates that rainfall and not topography determines 
the lower limits of the montane forest. 
12. THE VALLEY HEAD 
The head of Manoa Valley is an expanded amphitheater of erosion, 
rimmed by abrupt and deeply dissected walls. From the standpoint 
of plant life it is an ecologic complex, comprising the following elements: 
1. The upper valley floor, already described. 
2. A zone of broad, gentle, grassy slopes, \ying above the valley floor 
and below the kukui zone. Many of these ridges are knife-edged 
and precipitous in their upper courses, and separate deep, narrow 
ravines (700-1 ,400-ft. contours). 
