580 
VAUGHAN MACCAUGHEY 
9. THE VALLEY WALLS OR LATERAL RIDGES 
The Manoa portal opens to the southwest and is bounded on east 
and west by the plainward terminations of its two irregularly sculp- 
tured lateral ridges. These terminal areas of the ridges may be 
designated as foothills; that on the east is the Manoa-Palolo foothill; 
that on the west is the Roundtop foothill. 
The ridges extend from the coastal plain up to the main summit- 
ridge of the range, which here has an average elevation of 2,300 feet. 
Like all the ridges which define the Hawaiian valleys, these are the 
remnants of an original volcanic dome. The lower or foothill ends of 
the ridges are sufficiently bare of vegetation to reveal the laminated 
series of basaltic lava flows, of which they are mainly composed. 
Each lateral ridge may be divided by vertical lines into three 
sections or areas: 
1. The terminal or outlier foothill, which fronts upon and rests 
upon the coastal plain. 
2. The transition or intermediate "knife-edge" region. 
3. The mountainward region, wherein the ridge connects with 
or springs from the main summit ridge. 
The Manoa-Palolo or East Foothill.- — ^This, viewed from above, is a 
fan-shaped mass, with the expanded portion abutting upon the coastal 
plain. The upper slope narrows to a high (1,200 feet) apical region. 
The seaward slope of the foothill has an angle of about 8°; the valley 
wall is abrupt, rising at about 40°. The origin and physiography of 
the foothill is due to the remarkably localized distribution of the 
rainfall, as has been referred to in a previous section of the paper. 
The rainfall on the foothill itself is comparatively slight. Therefore 
erosion has advanced much more in the mountainward districts, and 
has left the foothill as a more or less isolated and xerophytic outlier. 
In the Waianae district, on Oahu, are found the culminating stages 
in the isolation of the foothill from the main range. 
The Roundtop Foothill. — The lateral ridge which constitutes the 
western wall or boundary of Manoa Valley terminates in Roundtop 
(Uala-kaa). This whole ridge is distinguished by a series of ancient 
explosive volcanoes, of which Roundtop is the most seaward and 
Mount Tantalus {Puu Ohia) is the highest and most conspicuous. 
The highest points are Roundtop, 1,000 feet, Sugarloaf 1,400 feet, 
and Tantalus 2,013 feet. Tantalus has a well-defined crater; the 
craters of the other cones are either eroded away, or hidden under 
