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PACIFIC SCIENCE, Vol. XXII, October 1968 
the lava flows strike about N 5° E and dip east- 
ward, indicating that the caldera boundary 
swings from east-west toward north in that 
area. A few of the thinner vertical dikes ex- 
posed in the cliffs behind Sili and Olosega 
villages are radial dikes which are sometimes 
cut by later thick dikes concentric to the caldera. 
About six vertical dikes, varying from 0.5 to 
6 feet in thickness are exposed in the cliff be- 
hind Samo’i beach on Ofu, and are nearly paral- 
lel to the western boundary of A’ of a caldera. 
One of these is a multiple dike approximately 1 
foot thick which is intruded concordantly by 
several small dikes 3-6 inches thick. A piece of 
dike rock containing dunite xenoliths was also 
found on the beach there. Several other dikes 
occur in widely scattered places along the cliffs 
of northern and southeastern Ofu. Usually they 
are less than 4 feet thick, vertical, and approxi- 
mately parallel to the cliff face. An ankaramite 
dike more than 40 feet thick crops out at the 
top of the cliff at Muli’olo and Tumu, and is 
probably the source of nearly horizontal ankara- 
mite flows on Tumu. It may be related to the 
collapse of the A’ofa caldera. 
NONCALCAREOUS SEDIMENTARY DEPOSITS: 
The talus and alluvial deposits are very similar 
to those described on Ta’u. Rock waste is always 
present at the base of the cliff encircling the 
islands. Several fan-shaped landslide deposits 
can be seen at the base of the cliff along the 
coast of southern Ofu, between Va’oto and 
To’aga. 
Many of the streams on southwestern Ofu 
and southeastern Olosega cut through deeply 
lateritized thin-bedded pahoehoe(P) flows and 
have beds of reddish-brown, sticky silt. There- 
fore, the stream beds are of a somewhat differ- 
ent character than deposits along other streams 
in that few large boulders are found. Parts of 
Tala’isina, Tope’a, Etemuli, and Si’umalae 
streams on southeastern Olosega and the upper 
portions of Alei, Saumolia, Tufu, Matasina, and 
other small streams on southwestern Ofu also 
have silty stream beds. Large basalt boulders 
are much less common in the upper portions 
of Tafe and Sinapoto streams on northern Ofu, 
where they traverse the relatively level floor 
of the A’ofa caldera, than in the typical boulder 
beds downstream. 
CALCAREOUS SEDIMENTARY DEPOSITS: Most 
of the beaches are about 50 feet wide and rarely 
exceed 100 feet. They have a constant foreslope 
of about 10° from sea level up to the vegetation 
along the backshore. The beaches are composed 
of sand, pebbles, and cobbles of coralline 
algae and coral. On a few beaches some volcanic 
fragments of basalt, ankaramite, tuff, olivine, 
and augite occur, but only as minor components 
in the predominantly calcareous sands. The 
median grain size, as on Ta’u, is usually coarse 
sand to gravel. Samples collected at sea level 
from most of the beaches around the islands 
and offshore at Ofu and Olosega villages are 
well sorted (only 1 of 35 analyzed samples had 
(*0 > 1.3). Beachrock is exposed in the inter- 
tidal zone of most beaches on Ofu and Olo- 
sega, and is more than 6 feet thick at Olosega 
Village. Usually the beachrock has approxi- 
mately the same foreslope as the present beaches. 
Major Structures 
The cones of the Asaga Formation generally 
have slopes of 20-35° because they are com- 
posed mostly of pyroclastic material. These 
cones were all subsequently buried by aa and 
pahoehoe flows dipping 10-20° away from the 
two shields which were centered at A’ofa and 
to the northwest of Sili Village. The nearly 
horizontal flows forming the highlands of Ofu 
must have been near the original summit of the 
A’ofa shield. 
A slight break in slope forms a benchlike 
feature at Papausi on southeastern Olosega. 
Lateritized thin flows exposed on the surface 
in this area dip 5-9° NE, somewhat less than 
the present ground surface. Their approximate 
strike (N 30-40° W) can be seen in the forma- 
tion of "steps” 1-2 feet high that presumably 
represent different lateritized thin flows. A cross 
section drawn from the high cliff at Sili through 
Le’ala Point (Fig. 8) shows that these beds 
most likely do not represent the normal slope 
of the Sili shield. There are no indications of 
any vents in this area. These lavas appear to 
have come from near the summit of the shield, 
but apparently their dips have been flattened by 
some kind of an obstruction. The thick aa flow 
of picrite-basalt that forms Le’ala Point dips 
about 14° SE and probably also came from the 
Sili shield. It suddenly increases in thickness 
