Geology

The spine of the peninsula is formed by the Lomas de Taraco - the Taraco hills. These are low, rolling hills whose peaks rarely exceed 4000 m.a.s.l. Geologically, the Taraco Hills are formed by the Taraco Formation.

The Taraco Formation ... consists of conglomerates approximately 200 meters thick lying in discordance over the Middle Miocene Kollu Kollu Formation. Gravels predominate in clasts with diameters up to 20 centimeters embedded in a sandy-clayish matrix ... The clasts are formed of Devonian mudstones, quartzites, vein quartz, Permian calcareous rocks, and ganodiorites. ... [I]t has been assumed that these deposits belong to the Pliocene, dated in other locations to 5.4 million years. ([Argollo et al. 1996]: 69)

The elevation of the Taraco Hills tends to decrease from south to north, and for this reason most watercourses run in this direction. The interior of the peninsula is therefore dissected by deep gullies or quebradas running from south to north, interspersed with high ridges (see contour map in Figure 2.2).

Figure 2.2: Map of the Taraco Peninsula

The conglomerates of the Taraco Formation, at least where they are exposed on the surface, are composed predominantly of quartzite cobbles. The ground surface over most of the peninsula is littered with quartzite cobbles of all sizes. However, the formation also contains large quantities of chert pebbles and cobbles. These are smaller than the quartzite cobbles, rarely exceeding 8 cm or so. These chert nodules were frequently worked using a hammer and anvil bipolar technique to extract short small flakes. I have also collected samples of chalcedony from the hills near Chiripa. All of these materials were used in an expedient manner by the prehistoric inhabitants for the production of stone tools, predominantly informal flake tools. This kind of extensive quarrying activity has been practiced since the first human occupation of the region. As a result the entire surface of the peninsula is a low-density lithic scatter. Within this scatter it is very difficult to discern any true concentrations, and this long-term quarrying has no doubt obscured some more concentrated activity. This is possibly one of the reasons why no preceramic sites have been reported from the peninsula, despite finds of Archaic Period projectile points in the Chiripa excavations and in other regional surveys ([Albarracín-Jordan and Mathews 1990]: 51).

Finally, the quartzites of the Taraco Formation not uncommonly contain fossils, frequently trilobites. There can be no doubt that the prehistoric inhabitants of the peninsula remarked the existence of these fossils and accorded them some importance. Fossils are found with some frequency on the surface and in the deposits of sites, indicating that they were collected in prehistory. The significance which the inhabitants of the peninsula attributed to these objects cannot be known with certainty.

The Kollu Kollu Formation is located below the Taraco Formation.

It is composed of conglomerates, red sandstones, clayish mudflows, and alluvial clays intercalated with acid tuffs. Swanson et. al. ... assigned [middle Miocene] radiometric dates of 18.3 to 16.6 million years to this formation. ([Argollo et al. 1996]: 69)

This formation was used in several ways by the prehistoric inhabitants of the Taraco Peninsula. Most importantly, it was the source of all the clays which they used to manufacture their ceramics. The red, yellow, and white pigments used on ceramics of the Chiripa culture were slip paints, and so were extracted from the Kollu Kollu Formation, which contains clays of all these colors. These clays were also used to produce colored plaster used to decorate buildings and to lay prepared clay floors. In short, the presence of red, yellow, and white clays in the Kollu Kollu Formation determined the color palette of Chiripa Culture architectural and ceramic decorative art. Less importantly, the tuffs encountered in the Kollu Kollu Formation were used locally in the Late Formative and Tiwanaku periods for the manufacture of an enigmatic class of stone tools known to archaeologists as trompos (see [Bermann 1994]: 61 for illustrations). The function of these small, conical artifacts remains uncertain, though it is debated wherever Titicaca Basin archaeologists gather.

The interface between the Taraco and Kollu Kollu Formations is generally located slightly above the modern lake level. I have not made a formal survey, but the Kollu Kollu deposits frequently appear in the bottom of quebradas near the base of the hills on the northern side of the peninsula. It is also exposed on the surface over portions of the western half of the peninsula, particularly in the modern communities of Nachoca, Ñacoca, and Santa Rosa. My informal observations indicate that the elevation of this geological boundary is typically between 3820 and 3840 m.a.s.l. A published geological section from somewhere in the western Taraco Peninsula - probably close to the 508200 E meridian - shows that the boundary in fact slopes down from south to north, with the Taraco/Kollu Kollu interface at approximately 3850 m.a.s.l. on the southern side of the hills, and approximately 3820 m.a.s.l. on the northern side ([Argollo et al. 1996]: 75).

This geological boundary has one important implication for human occupation of the Taraco Peninsula. The underlying Kollu Kollu Formation, being compact and composed primarily of fine sediments and of clays in particular, is much less porous than are the Taraco Formation conglomerates. This creates a situation in which the groundwater trapped in the natural aquifer of the Taraco Formation is forced to the surface where the Taraco/Kollu Kollu Formation interface intersects the modern ground surface. This effect creates a zone of springs and water sources in the lower slopes of the Taraco Hills and in quebradas. On the northern side of the peninsula, this zone is located at approximately 3820-3830 m.a.s.l., and is somewhat higher on the southern side. Surface water on the Taraco Peninsula is for this reason very abundant. At no point within this elevation range is one more than a kilometer from a spring anywhere on the peninsula. It should come as no surprise that this distribution of surface water is a significant determining factor in Taraco Peninsula settlement systems. The majority of the population lives in the 3810-3850 m.a.s.l. elevation range today, and has for the peninsula's entire history.

A final note on springs: since the volume of the Taraco Formation deposits on the peninsula is actually relatively small as aquifers go, it can discharge its water content fairly rapidly. I have observed spring flow over an annual cycle, and it is clear that discharge rates begin to drop dramatically toward the end of the dry season, and some springs even dry up. Typically, the aquifer is recharged by the annual rainy season. However, given prolonged drought conditions it is possible that the aquifer could be depleted altogether and most of the springs could be dry for most of the year. This would present difficulties as far as continued occupation of the Taraco Peninsula is concerned.