with a shift in the volcanic axis to the east, farther
from the Middle America Trench. Pleistocene (ca.
600,000 years ago) andesitic lavas and tuffs of the
Monteverde Formation lie discordantly on the rocks
of the Aguacate Group, mantling the upper slopes of
the Cordillera to depths of up to several hundred
meters. The volcanic foci that produced the Monte-
verde Formation have not been identified (Alvarado
1989), but several trench-parallel sets of peaks are
candidates for relict volcanic structures. For instance,
Cerros Chomogo, Amigo, Roble, and Cerro Ojo de
Agua lie on a single axis, while Cerro Frio and Cerro
la Mesa stand across the Penas Blancas valley from
each other, offset a similar distance (2-3 km) from the
prior axis. Seven kilometers farther northeast, Pico
1790, a relict volcanic structure, lies in a line paral-
lel with the peaks at the head of the Rio Aranjuez
valley. These are all of a scale similar to that of the
latest array of Volcan Arenal, Cerro Chato, and Cerro
los Perdidos (Borgia et al. 1988).
2.3. Modern Geography of the
Cordillera de Tilaran
The present Cordillera de Tilaran consists of a sinu-
ous main ridge, which runs southeast from Cerro
Nubes (1020 m) to Cerro Chomogo (1799 m) and Cerro
Amigo (1842 m) above Santa Elena and Monteverde,
across the Brillante saddle (ca. 1500 m) to Cerro Ojo
de Agua (ca. 1800 m; Fig. 2.8; Figs. 1.4,1.7). The main
ridge then turns east over a series of knobs, sepa-
rating the upper thirds of the Penas Blancas and
Aranjuez watersheds. The higher points on these
ridges are likely relict volcanic structures dating from
the Quaternary, the age of the Monteverde Formation.
Erosion has dramatically influenced the modern
landscape of the Cordillera. The Caribbean slope of
the Cordillera is drained by tributaries of the Rio San
Carlos. Rio Penas Blancas, the largest stream of the
range, drains the central Caribbean slope. The Pacific
slope of the Cordillera is drained on the northwest-
ern end by the Rios Cafias and Abangares, both of
which flow into the mouth of the Rio Tempisque at
the head of the Golfo de Nicoya, and the Rios Lagarto,
Guacimal, Aranjuez, and Barranca, which flow di-
rectly into the Golfo (Fig. 1.7).
Many of these rivers have parallel, very straight
central courses normal to the long axis of the horst.
These features suggest they follow faults in the
Aguacate Group (Bergoeing and Brenes 1977). Al-
though such fault-directed development of water-
courses and drainage patterns is generally obscured
by extensive erosional dissection of the landscape,
a fault conspicuously redirects the courses of three
adjacent streams flowing from the El Valle region into
the Rio Penas Blancas on the Atlantic slope (see Fig.
1.5). Faulting of the Aguacate Group core may have
dictated much of the erosional pattern and modern
configuration of the Cordillera.
Major streams and their tributaries descend rapidly
on both sides of the Continental Divide, often through
spectacular sets of rapids. The Rio Penas Blancas, for
example, descends 500 m in the 10 km between Dos
Aces and Poco Sol. Tributary ravines, termed que-
bradas, descend from the Continental Divide even
more dramatically; those in the upper Penas Blancas
valley drop 500-800 m in 2.5 km. These streams carve
spectacular gorges and often incise stream channels
into the bedrock, particularly as they traverse the
upper rocks of the Aguacate Group. Streams 2-5 m
wide descend via stepped waterfalls and rapids in the
bottom of channels 10-15 m deep in bedrock. The
Bajo Tigre trail (Fig. 1.8) offers views of the Rio
Guacimal cutting through the upper members of the
Aguacate Group. The Sendero del Rio of the preserve
offers views of the upper reaches of the same stream
cutting through the breccias of the Monteverde For-
mation (Fig. 1.5).
Above the cliffs of the Aguacate Group, the land-
scape is mantled by the less thoroughly dissected
lavas and tuffs of the Monteverde Formation (Chaves
and Saenz 1974, Castillo-Muiioz 1983, Alvarado
1989). Older relict volcanic structures (e.g., Cerros
Chomogo, Amigo, and Frio) are rugged, but younger
structures (e.g., Pico 1790 and the peak of Cerro los
Perdidos) present broad slopes, which, although
steep, are not yet carved into quebradas and ridges.
Saddles along the crest of the Cordillera (e.g., the
Brillante trail), broad summits (e.g., Cerro la Mesa),
and benches (e.g., Monteverde-Cerro Piano-Santa
Elena) present areas of conspicuously lower relief,
well dissected into small primary watersheds. Water-
sheds 10 ha in area along the Brillante trail, for ex-
ample, are drained by permanent*streams. The relief
within such watersheds, however, is generally less
than 50 m, and in some swampy areas much less.
These areas of lesser relief have been interpreted as
the development of nearly flat, eroded surfaces (pene-
plains) on the Aguacate formation (Castillo-Muiioz
1983). They are clearly related to the original topog-
raphy and geological structure of the Pleistocene
volcanic terrain of the Monteverde Formation. The
Monteverde—Cerro Piano—Santa Elena bench resembles
the area at the southwestern foot of Volcan Arenal in
terms of geomorphology. Road cuts in the Monteverde-
Santa Elena area reveal a heterogeneous mix of tuffs
and breccias, similar to those that have accumulated
around the base and on the lower slopes of Volcan
Arenal (Chaves and Saenz 1974, Alvarado 1989).22 The Physical Environment