Littoral Sedimentary Processes — I nman, Gayman and Cox 
121 
Presumably, a difference in the degree of chemi- 
cal weathering accounts for the differences in 
mineralogy between the terrigenous sediments 
on the windward and leeward coasts. Plagioclase- 
rich sands occur near the Waimea River mouth, 
while on the north coast the same size grades 
of sand are plagioclase-poor and olivine-rich; 
however, there appears to be little difference 
in the average parent rock in the two source 
areas. It seems likely that a more rapid decom- 
position of feldspar, relative to olivine, takes 
place because the feldspar occurs in smaller min- 
eral grains. The plagioclase crystals, found only 
in sand-size rock fragments, are several orders 
of magnitude smaller than the olivine grains 
found in the beach sands. The plagioclase crys- 
tals are abundant in medium- and fine-grain 
beach sands only in the Waimea area. Appar- 
ently the easily decomposable volcanic rock frag- 
ments in these size ranges are brought to the 
shore in mass only where chemical processes are 
at a minimum. On Kauai a maximum in sedi- 
ment yield and a minimum in chemical weather- 
ing occur only in the arid Waimea drainage 
basin. 
Beach and Dune Rock 
The distribution of beach rock on Kauai is 
described by Emery and Cox (1956) in a com- 
prehensive study of beach rock in the Hawaiian 
Islands. They examined 72 miles of sandy 
beaches on Kauai and reported beach rock along 
21 miles, or about 13% of the entire coastline. 
The total occurrence of beach rock may be even 
greater, as the presence of beach rock is fre- 
quently obscured by a covering of loose clastic 
sand. During the present study, a large outcrop 
of beach rock that was surveyed and sampled 
(No. 18) at Oomana Point in 1955, was com- 
pletely covered with fresh sand when the site 
was revisited in 1959. Also, in the vicinity of 
samples 38 and 39, it was found that a p4-mile 
section of beach rock was covered with a uni- 
form layer of fresh sand, about 4 inches thick. 
Here the slope and general attitude of the beach 
rock was the same as that of the fresh beach face. 
Water seeping through the beach face was tasted 
and found to be fresh, indicating that its origin 
was the fresh water ponded behind the beach 
ridge. 
In fact, there is some correlation on the is- 
land of Kauai between beach rock outcrops and 
areas of present or past ponding of fresh water 
behind calcareous beach ridges. Although it is 
recognized (Emery and Cox, 1956) that there 
may be several methods of formation of beach 
rock, it appears that an important cementation 
process on Kauai is associated with the evapora- 
tion of fresh calcium carbonate-rich waters seep- 
ing through the beach face. This occurs most 
generally when fresh water is ponded in calcar- 
eous sediments by sand dunes or beach ridges. 
Presumably, the fresh or brackish water flowing 
through the porous calcareous sediments be- 
comes saturated with CaCOa. Evaporation of 
this water on or near the beach face causes ce- 
mentation, and rock is formed if the beach is 
sufficiently stable. This belief is strengthened 
by the presence of cemented dune sands, which 
are well above sea level and appear to have 
been formed in the same manner as the beach 
rock, with which these cemented sands are com- 
monly associated. 
Dune rock (eolianite) was found on the south- 
west coast of the island, northeast of Nohili 
Point. Near Nohili the dune rock was poorly 
cemented and appeared to be contemporaneous 
with the formation of the present dunes at Bark- 
ing Sands. In the vicinity of Kipu Kai (sample 
40 ) several rocky points, such as Alligator Head, 
are composed of well-cemented dune rock, which 
is now exposed to wave attack. The Kipu Kai 
dune rock is older than the present shoreline. 
Thin section studies of beach and dune rock 
samples (6, 18, 35, 40, 41) confirm the litho- 
logic and biologic similarity of the rock samples 
and the recent beach sand. The general structure, 
texture, and composition of the beach rocks sug- 
gest that they were formed either very recently, 
or under the same environmental conditions as 
those which are now dominant on Kauai 
beaches. Examination of thin sections and X-ray 
analysis suggest that the cement is calcitic, in 
agreement with the findings of Emery and Cox 
(1956). 
The only dune rock sample, 40, differed from 
the beach and beach rock samples in that the car- 
bonate fraction is composed entirely of magne- 
sium-poor calcite and the cement was consider- 
ably more abundant. It does not seem likely that 
the single carbonate mineral species now found 
