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PACIFIC SCIENCE, Vol. XXII, October 1968 ! 
abundant in the area between Ofu Village and 
Nu’utele Islet. A "lithothamnium ridge” oc- 
curs along the reef front near Nu’usilaelae Islet 
on the south and Tauga Point on the north. 
Millepora tenera Baschma, a stinging coral, was 
found only on Ofu but, according to the natives, 
it is present on Ta’u also. 
Both large, deep channels and shallower, 
narrow channels or surge channels are cut into 
the reef front. The large channels have sand 
floors, extend out to depths of about 60 feet, 
and are often as wide as 25 feet. Strong cur- 
rents flow out through the larger channels such 
as those off Ofu Village between Nu’utele Islet 
and Tauga Point and between Nu’usilaelae Is- 
let and Nu’upule Rock, as well as off Olosega 
Village. The surge channels extend out to about 
30-foot depths, have overhanging sides, and 
usually do not have sediment covering their 
floors, although a boulder-sized coral shingle 
sometimes occurs. Generally they do not cut 
completely through the reef into the reef -flat, 
as do the larger channels. Surge channel widths 
vary from 3 to 10 feet. 
tered with large talus blocks in front of areas 
where cliffs are high enough for falling rocks 
to build up sufficient momentum to roll across 
the reef. A Samoan legend relates how a young 
girl was killed by a large block that rolled 
across the reef at Sili, where she was fishing. 
The constructional bench 12-15 feet above 
sea level that was found on Ta’u is also well 
developed on Ofu and Olosega, although here 
it is nearly 20 feet above sea level in some 
places. Sili, Olosega, and Ofu villages are all 
built on these sandy areas. Other examples of 
this constructional bench are found at Va’oto, 
To’aga, and Mafafa on Ofu and at Oge on 
Olosega. The bench is about 900 feet wide at 
Va’oto and over 1,000 feet wide at Olosega 
Village, but the average is about 300 feet. 
Usually a 9-10° foreslope rises steadily to the 
crest of the berm 15-20 feet above sea level; 
sometimes there is an intermediate berm. At 
Va’oto and Olosega Village, swamps have 
formed between the berm crest and the talus 
slope at the base of the cliff. 
On the seaward side of Nu’utele Islet a bench 
is present at the same level as that in the tuff 
complex at Faleasao on Ta’u. The bench is 5 
feet high, has a maximum width of 30 feet, and, 
as on Ta’u, was developed only in tuff. No evi- 
dence was found to indicate how the bench was 
formed, whether by a Recent higher stand of 
the sea — of such short duration that only the 
easily eroded tuff was affected — or by lithifica- 
tion of the tuff at sea level due to contact with 
sea water. 
Beachrock is commonly exposed above the 
high tide line along beaches, indicating that it 
is out of equilibrium with present sea level. 
If beachrock forms at a shallow depth beneath 
the beach surface, then its exposure in Manu’a 
may indicate formation during a slightly higher 
stand of sea level. 
Like Ta’u, Ofu and Olosega are completely 
encircled by a narrow fringing reef (Fig. 6, 
top). The reef front is only 0.3 mile offshore 
at its widest point, Ofu Village. The reef 
there has a more prolific coral growth than in 
any other area in Manu’a. Most of the coral 
genera recognized on Ta’u were also found on 
Ofu and Olosega. In addition, Orbicella and 
Zoanthus were found only on Ofu. The latter 
is a soft Alconarian coral that is particularly 
Geologic History 
Estimating the probable lengths of time re- 
quired for the various events in the formation 
of Ofu and Olosega islands, it seems most 
likely that at least five cones built during Plio- 
cene time were buried by lavas from the A’ofa 
and Sili shields during Pleistocene time. After 
summit collapse of the shields, volcanism de- 
creased so that a sea cliff about 300 feet high 
was cut around the islands. At approximately 
the same time, gravity collapse similar to that 
suggested for Ta’u may have caused the embay- 
ment of southeastern Ofu and southwestern 
Olosega. Faulting and further collapse also may 
have occurred north of the islands in the area 
of Sili caldera. 
Recent volcanism on southwestern Ofu built 
the tuff cone on Nu’utele Islet. A few thick aa 
flows in this area may have poured down old 
valleys from post-caldera cinder cones. As on 
Ta’u, the 5 -foot bench in the tuff could be ex- 
plained by a consolidation due to contact with 
sea water, but a Recent shift in sea level is 
indicated by the beachrock now being eroded in 
many places around the islands. Hurricanes and 
storm waves may be responsible for the 15- to 
