Between the northern volcanic section and the more regular Talamanca range is the notable “Ochomogo” Pass, about twenty miles broad, and a little more than 5000 feet above the sea-level at the water parting.
To the eastward through this gap, and in a broad, deeply eroded valley, runs the tumultuous Reventazon River, and to the westward the Rio Grande de Pirris. On the south of this depression the Chirripo Grande mountain mass sends off east and west two immense flanking ranges. A part of the western range, lying between San Marcos and Santa Maria, for a length of about six miles, is known as the Dota ridge, to which former explorers gave great importance.
This lofty, transverse and precipitous mountain system almost forbids communication between the northern and southern halves of the Republic, and, as Colonel Church says, must at all times have had a marked influence on the movement of races in this part of Central America. Both the northern and Talamanca sections present mountains in masses instead of sierrated like many Andean chains of North America. Those of the Talamanca section are Rovalo (7050 feet), Pico Blanco (9650 feet). Chirripo Grande (11,850 feet) and Buena Vista (10,820 feet). There are no signs of recent volcanic activity in the Talamanca range. The Talamanca mountains have narrow crests and are very precipitous on the Atlantic side, with evidences of extensive denudations and erosions caused by the ceaseless rain-laden trade-winds.
Professor William M. Gabb, in his geological sketch of Talamanca, observes that the geological structure of the entire region is very simple. The greatest expanse is occupied by recent sedimentary rocks raised and nearly entirely metamorphosed by the action of volcanic masses.
At several points along the Atlantic coast, there are found masses of rocks of still later date. Professor Gabb maintains that the nucleus of the great Cordillera of the interior is formed by granites and syenites, which, like the sediment that covers them, are broken through here and there by dikes of volcanic origin identical with the eruptive material found on a greater scale in the northern part of Costa Rica. The syenites are intrusive and have their culminating point and greatest development in the Pico Blanco or Kamuk, a mountain of great altitude, unusual ruggedness and scarred with deep and precipitous cañons. All these dikes are of more modern formation and are porphyritic. Professor Gabb also notes a thick deposit of conglomerates and sandstones, schists and limestones, the schists being the most abundant; although the conglomerates, found all over the region, indicate the previous existence of an older sedimentary formation.
The pebbles which form the conglomerates are composed of metamorphic clay, having a character distinct from all the other rocks found in the country. The cement is also clay or sand. The absence of crystalline rocks in the conglomerates is irrefutable proof that, when these were deposited, the syenites and granites had not yet appeared from the interior of the earth. The limestone and sandstone represent a less developed geographical horizon of the sedimentary group, the latter appearing occasionally in layers, interstratified with conglomerates or more recent schists. In no place in Talamanca have fossils been found in these sandstones, although the same rocks are very fossiliferous near Zapote on the River Reventazon.
In regard to fossils, Professor Gabb saw at Las Lomas Station, about seven hundred feet above the sea, in the Bonilla Cliffs cutting, shark’s teeth, compact masses of sea shells, fish, etc., and at an elevation of 2500 feet large deposits of compact shell limestone.
The schists have a fine, leaf-like texture, and are easily decomposed and reduced to a black mud, if they have not been metamorphosed. In this rock fossils have been found which belong to a Miocene age.
Along the Talamanca coast calcareous deposits are found in horizontal layers, and are probably elevated coral reefs, a rock which Professor Gabb calls “antillite,” and which is developed in the entire Caribbean region. It belongs to the post-Pliocene formation, the last of the Tertiary series.
In the interior valleys a thick deposit of pebbles and clays of recent origin is observed. The limit between the syenites of the high mountains and the metamorphosed Miocene formation is found in proximity to the Depuk River. In the slopes of the hills the schists are usually decomposed and covered with red clay, a sub-soil above which is found a small cap of fertile vegetable mold. In the valley of Tsuku the schists are profoundly altered and transformed in a magnesic or semi-talcous rock. The schists are more silicified in coming near to the limits of the syenites.