This time I will tell you about a story that began in the
Mid-Holocene (5000 years ago) and is set in the Bolivian Amazon. More precisely
in the south-eastern part of the Llanos de Moxos seasonally flooded savannah,
in what we call the Monumental Mounds Region MMR (Fig. 1). Here, between 400 and 1400 AD, pre-Columbians
built hundreds of monumental earth mounds, known locally as “lomas”. These earth
mounds are planned, complex buildings made by one or more pyramids built on top
of elevated platforms (Fig. 3-3). Monumental
mounds can be up to 20 meters high and can cover up to 30 hectares. In the MMR there
are more than 350 of these pre-Columbian buildings.
 |
box) and the paleo-courses of Río Grande (pink lines). |
During the early Holocene (between 11.000 and 5.000 years
ago) this portion of Amazonia was relatively dryer than today, inundations were
less frequent and rivers transported few sediments. During these stable
climatic conditions there was no deposition of fluvial sediments in the
savannahs and soils were forming all over the Llanos de Moxos.
Mid- Holocene: Río
Grande built the landscape
But things changed a lot during the Mid-Holocene, between 5
and 4Ky BP. The Rio Grande (pink lines in Fig. 1) entered in a period of
frequent avulsions and high sedimentation, probably triggered by a climate
change towards wetter conditions. As a result, in the South-eastern LM, it
formed a fluvial distributary system (FDS) (Fig. 2-1). Suddenly, the landscape
was transformed into a large swamp, dominated by something similar to an
interior delta. This FDS deposited several levees, crating relief at a local
scale (Fig. 2-2), and a sedimentary lobe, creating relief at a regional scale
(Fig. 2-3). The former soils were buried and the landscape became a mosaic of
patches of savannahs closely interwoven and sometimes enclosed by forested
paleo-levees.
 |
Late Holocene:
pre-Columbians transformed the landscape
In
the MMR, the lobe deposition favoured the development of a complex
pre-Columbian society by increasing the region’s agricultural potential.
Firstly, it created a convex-up topography, which greatly reduced its
susceptibility to flooding; secondly, the construction of the elevated fluvial
levees significantly improved drainage conditions at the local scale.
Furthermore, the Río Grande also provided relatively younger sediments derived
from its Andean catchment that are rich in nutrients. Thus, the Río Grande
removed the two biggest obstacles faced by tropical agriculture in the rest of
Amazonia: severe waterlogging and poor soils. But the Río Grande’s job was not
perfect: fluvial levees enclosed patches of floodplain, resulting in ponding
and pronounced waterlogging. Thus pre-Columbian people had to transform the
landscape through the construction of a drainage system in order to further
improve agricultural conditions (Fig. 3-1).
The network of canals had a significant impact on the
edaphology of the MMR: it pushed the forest-savannah boundary towards the
savannah, eventually increasing the area of well-drained, usable land. The inhabitants
of the MMR were very lucky because they also had several lakes placed on the
top of the sedimentary lobe. Building canals that transported the water from
the lakes to the agricultural fields (Fig. 4), they were able to perform
agriculture even during the dry season.
The spatial overlap (Fig. 3-2) between the monumental mounds and the area of
deposition of the sedimentary lobe created by the Río Grande during the mid- to
late Holocene suggests that good edaphic conditions favoured the
emergence of the monumental mounds culture (Fig. 3-3).
The
fluvial landscape created by Río Grande was probably an important factor
behind the emergence of the monumental mounds culture in the South-eastern LM,
as it provided favourable environmental preconditions in terms of soils,
nutrients and drainage characteristics. Pre-Columbians additionally modified
and improved their environment by building a network of drainage canals.
 |
Figure 4 (a) Drainage canal (A–B) and irrigation canal (C–D)
built
along topographic slopes. (b)The C–D canal in detail;
the irrigation canal linking
Lake San José to the Cotoca mound
joins pre-existing natural channels. Coring
in the middle of
the canal shows that the bed of the canal is only 50cm below
the present bed (which is about 50 cm below the modern savannah).
Therefore,
this canal was at most 1 meter deep but went through a slope
of 3 meters. The
only possible function of such a canal was irrigation.
|
You can find more on this story in my latest paper published
on-line in the journal “The Holocene”. It is entitled “Mid- to late-Holocene
fluvial activity behind pre-Columbian social complexity in the southwestern
Amazon basin” and can be found here.
Lombardo, U., & Prümers, H. (2010). Pre-Columbian human occupation patterns in the eastern plains of the Llanos de Moxos, Bolivian Amazonia Journal of Archaeological Science, 37 (8), 1875-1885 DOI: 10.1016/j.jas.2010.02.011
Umberto Lombardo, Jan-Hendrik May, & Heinz Veit (2012). Mid- to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin The Holocene
