Although it shares many characteristics with other parts of the Amazon basin, the combination of strong seasonality in rainfall and the flooded savanna ecosystems makes Mojos a distinctive geographical region. Mojos is a shallow basin covering approximately 110,000 km2
Handbook of South American Archaeology, edited by Helaine Silverman and William H. Isbell.
Springer, New York, 2008
Figure 46.1. Location of the Llanos de Mojos (shaded area) within the Madeira River basin of South America. (John Walker)
Between the Andes to the west and south and the uplands of the Brazilian shield to the east and north (Hanagarth 1993; Langstroth 1996; Denevan 2001) (Figure 46.1). The soils derive primarily from Pleistocene lacustrine sediments, more than two kilometers deep near the Andes and a few hundred meters deep near the Brazilian shield. This thick cover makes stone usable for construction or tools very rare. Topographical relief is minimal, with elevations that only vary between 150 and 170 masl across most of the region.
Lying entirely within the tropics, Mojos has a climate characterized by high, stable average temperatures, high humidity and high rainfall. Aside from human activity, the determining ecological feature is the seasonal cycle of flood and drought, which affects soils, plants and animals. Mojos is therefore defined as a humid savanna, a grassland environment with a 2- to 7-month dry season and a total annual rainfall varying between 1,500 and 1,800 mm (Harris 1980). The dry season lasts from May through September, when weeks pass without precipitation. During these months the passage of southern cold fronts occasionally breaks the heat and humidity, dramatically lowering temperatures and sometimes bringing heavy rain.
The wet season lasts roughly from November through March. During these months widespread flooding changes the landscape in two different ways. Modern inhabitants call these two types of flooding “water from below” and “water from above.” The first kind of flooding results when tributaries of the Mamore River rise because of high water levels downstream. When the Mamore is high, these rivers “back up” and inundate large areas. Some rivers that are easily forded in the dry season are several hundred meters across in
Water level-January water level-July
Figure 46.2. Schematic diagram illustrating the patterning of local environments in Mojos. (John Walker)
The wet season. The second type of flooding is the result of heavy local rainfall. Rains can leave 20 or 30 cm of standing water on poorly drained terrain, even though that terrain may be at a higher elevation. During the dry season, this water drains more quickly, and these areas remain underwater for a short time. During the wet season, rainfall inundates large areas, making them much more difficult to cross on foot.
The interaction of these two processes with local topography creates a mosaic of environments. Low-lying gallery forests along rivers and streams contrast with higher river levees and savannas alongside. Topography and drainage influence the spatial arrangement of plant communities because soils vary with drainage and differences in elevation. Broadly speaking, four types of environments are present today: flooded forests, dry forests, dry savannas and wet savannas (Figure 46.2).
Flooded forests are often found in areas of lower elevation along rivers and can be underwater for as much as half of the year. Trees that are submerged for months tower overhead in the dry season. Many flooded forests are located in “galleries” along the rivers, on the sloping banks rather than the adjacent levees. Dry forests are elevated above floodwaters, and are well drained so that rainwater does not accumulate. These forests are often located on levees along the rivers, and in forest “islands” along smaller creeks and abandoned river channels. Dry savannas are also elevated and well drained. Many dry savannas are located on river levees. There are many transitional zones between dry forest and savanna, and it is difficult to separate the two categories clearly. Finally, wet savannas are found both in areas that are elevated but poorly drained, and in lower elevation back slopes. Some soils in the wet savannas accumulate sediment and organic material from annual floods. Older rivers have also cut channels across the savanna, leaving behind levees and back slopes.
Differences in earthworks have prompted archaeologists to distinguish four “divisions” within the Mojos region (Denevan 1966, 2001; Erickson 2006) (Figure 46.3). Each of these four divisions has a distinct form of landscape modification:
• North. Large raised fields comprise the most imposing element of the built environment. Occupation debris suggests that villages with 1,000-2,000 inhabitants could have been present (Walker 2004) (Figure 46.4).
• South. The built environment is composed of large mounds, some of which were burial mounds. This is where most archaeological work in Mojos has taken place (Nordenskiold 1913, 1920, 1924; Bennett 1934; Dougherty and Calandra 1981, 1981-82, 1984; Erickson and Balee 2006; Prumers 2004). Many of the mounds contain urn burials, and some of them contain several dozen burials. The largest mounds are close to 20 m tall, and some of them are hundreds of meters across.
Figure 46.3. Map illustrating the regional diversity of anthropogenic landscapes in Mojos. (Adapted from Denevan 1966: fig. 4)
East. Earthworks include long causeways, zig-zag causeways and circular ditches (Erickson 1996; Dougherty and Calandra 1985). The circular ditches are distinct from the landscapes modified by the zig-zag causeways, which Erickson interprets as weirs for the harvesting of fish (Figure 46.5).
West. Here are perhaps the most complex built environments, where canals, causeways, mounds and several kinds of raised and ditched fields were combined into integrated landscapes (Erickson 1980; Erickson and Walker ms.).
Many raised fields seem well suited to the production of root crops, such as manioc or New World taro. Because raised fields reduce the effects of flooding, they help protect root crops from rotting. Botanical remains from a raised field excavation show that Xantho-soma, Annatto and Ilex were being grown: a root crop, a dye, and a stimulant, respectively (Erickson 1995). This roster suggests that multi-cropping was a cultivation strategy, and
Figure 46.4. Reprojected aerial photograph of large raised fields and settlement north of Santa Ana del Yacuma, along the Iruyanez River. (John Walker)
Figure 46.5. Long causeways near Baures. a. Aerial photograph; b. Diagram (adapted from Erickson 2000: fig. 4). (John Walker)
That industrial products were as important as food crops. Mojos is within the southern Amazon basin, which has been proposed as the “hearth” of both manioc and peanut cultivation (Olsen and Schaal 2001; Jarvis et al. 2002). Deposits of anthrosols indicate large areas of human occupation along the large tributaries and main course of the Amazon River. Appearing as a distinct soil type on national surveys, anthrosols are nuanced evidence both for settlement and for intentional modification of soils for agricultural uses (Lehman et al. 2003; Glaser and Woods 2004).
The human geography of Mojos is also complex. According to the Summer Institute of Linguistics, thirteen languages are present in Mojos today, including groups classified as Arawak (Baure, and Mojo, which is split into Ignaciano and Trinitario), Tupi (Guarayu and Siriono), Tacanan (Ese Ejja and Cavinena), Panoan (Chacobo), and isolated languages (Canichana, Cayuvava, Itonama, Movima and T’simane) (Gordon 2005). Several of these are extinct or nearly extinct (Baure, Canichana, Cayuvava and Itonama). This situation suggests that before the European conquest a variety of languages were present, as in other regions within the upper Amazon. A recent study of mitochondrial DNA among modern populations suggests that considerable genetic diversity is represented (Bert et al. 2004). The study is a significant step and continued research will provide an independent line of evidence to compare with linguistic and archaeological data about population history.
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