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Geology
The Site
Northeast of the Diquís Valley are the mountains of Cordillera
de la Talamanca which are generally in a northwest-southeast direction;
south of the Terraba delta is the Osa Peninsula, which is one
of the locations with the highest bio-diversity in the world;
west of the delta is Terraba-Sierpe Mangrove Forest Reserve, which
is the estuary of both watersheds flowing into the Pacific Ocean.
Mineral
Distribution
By the diagram we can understand the whole Osa Peninsula and the
south end of the Terraba River alluvial plain which contain the
gold deposits. Golden artifacts were the symbols of power in ancient
local civilization, and also caught the attention of Spanish explorers.
Lateritic bauxite is another mineral found inside the hills (Fila
Costena and Fila Cruces) north of the Diquís Valley.
Geology
There are five major geology components around the Terraba-Sierpe
delta. We can understand the distribution by the geography diagram.
1. QTsc: Vocaniclastic sedimentation rocks (Eocene to Quatenary)-Grades
from Vocaniclastic breccia to sandstone, includes some marine
carbonate rock. This sedimentation is located in General Valley
(Vella del General).
2. TKsd: Deep-water sedimentary rocks (Cretaceous to Quaternary)-Limestone,
sandstone, shale, and turbidite. This sedimentation is located
on the hills northeast of the Diquís Valley (Vella del Diquís)
and the hills in the Osa Peninsula.
3. Ti: Intrusive rocks (Tertiary)-Gabbro to granite and
subvolcanic rhyolite. These rocks only appear sporadically in
where Terraba River cuts through the hills (Fila Costena and Fila
Cruces).
4. Qs: Alluvial and colluvial deposits (Quaternary)-Includes
landslide, fanglomerate, marsh and beach deposits. These are general
deposits moved or effected by natural factors like the river,
sea, wetland gravity or earthquakes. The locations are in the
Diquís Valley (Vella del Diquís) river bank of Terraba River,
and partial seashore of the Osa Peninsula.
5. Tjb: Sea-floor basalts (Jurassic to Eocene)-Theolitic
basalt, with intercalated pelagic sediment. This component is
located partially on the area of the Osa Peninsula and the north
seashore of the Dulce Gulf (Golfo Dulce). These areas have been
under sea level during the Jurassic to Eocene era.
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The Spheres
The ancient stone spheres were found throughout the entire watershed
of the Terraba River. Most of the stone spheres of Costa Rica
are composed of granodiorite, a hard igneous stone similar to
granite. Granodiorite can be found in outcrops in the foothills
of the nearby Talamanca range. A few of the spheres are made of
coquina, a hard material similar to limestone that is formed
from shell and sand in beach deposits. This type of stone can
be found in the mouth of the Térraba-Sierpe delta. Neither
of these materials can be found at the site of the spheres and
no partially carved spheres have been discovered.
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