A centuries-old grid of holes in the Andes may have been a ‘spreadsheet’ for accounting and exchange
- Written by Jacob L. Bongers, Tom Austen Brown Postdoctoral Research Associate, University of Sydney
In 1931, geologist Robert Shippee and US Navy Lieutenant George R. Johnson led one of the first aerial photography expeditions in South America. They captured stunning photographs of landscapes across the Andes, including some peculiar “pockmarks” in the Pisco Valley of southern Peru.
These “pockmarks” make up the mysterious archaeological site of Monte Sierpe. Stretching 1.5 kilometres and consisting of approximately 5,200 precisely aligned holes, Monte Sierpe translates as “serpent mountain” in Spanish, but the site is also known as the Band of Holes.
The purpose of Monte Sierpe has baffled researchers and members of the public alike, but new archaeological research carried out by an international team has uncovered evidence for a new interpretation.
As we explain in a new study, published today in Antiquity, Monte Sierpe, which dates to at least 700 years ago, may have functioned as an Indigenous system of accounting and exchange centuries before the European invasion.
Drone mapping and sediment analyses
We used a drone to map Monte Sierpe. Analyses of aerial imagery revealed that the site has a “segmented” organisation, composed of around 60 sections (or blocks) of holes, some of which are separated by empty spaces.
We also found numerical patterns in layout. For example, one section has nine consecutive rows with eight holes each. Another section has at least 12 rows that alternate between counts of seven and eight holes. These patterns hint at an underlying intention in the organisation of Monte Sierpe.
Our team analysed sediment samples collected from several holes across Monte Sierpe. We identified ancient pollens of maize (corn), one of the most important staple crops in the Andes, and of Typha (bulrush) and willow, which have been traditionally used to make baskets and mats.
These unexpected findings indicate that people deposited plants in the holes, perhaps using woven baskets or bundles for transport.
Parallels with a record-keeping device
To our surprise, we discovered that the organization of Monte Sierpe is similar to the structure of at least one Inca knotted-string device used for detailed record-keeping (khipu) that was recovered near Pisco.
Monte Sierpe’s 60 distinct sections of holes parallel the approximately 80 groups of cords documented in this local khipu.
This crucial discovery suggests Monte Sierpe may have played a significant role in Indigenous accounting practices.
A new model
Our results provide little support for alternative theories connecting Monte Sierpe to defence, water collection, or fog capture.
There is no evidence of fortification or weapons at Monte Sierpe.
The Band of Holes does look somewhat like the pits used for grape cultivation in Lanzarote, Canary Islands, but rainwater is effectively non-existent at these elevations (440–700 metres) in the Pisco Valley.
The site is located in an area of ephemeral fog oases (patches of dense fog that persist for up to a week), so it could have captured moisture. However, the resulting water would not be enough to grow crops, especially maize. Groundwater is not available on Monte Sierpe’s slope.
Instead, we hypothesise that Monte Sierpe initially served as a barter marketplace before being turned into a large-scale accounting device under the Inca Empire (1400–1532 AD) for collecting tribute from local communities.
The Andes is one of the few world regions where large-scale societies, such as the Inca Empire, developed, but there is no strong evidence of pre-Hispanic currencies or writing systems. How did Andean societies exchange resources and administer large numbers of people without currency or writing?
Monte Sierpe may offer answers.
Barter marketplace
Barter marketplaces are built around a shared understanding among participants about how the value of one type of good compares to the value of another.
One of the thousands of holes at Monte Sierpe, with a 20cm bar for scale.
Charles Stanish, CC BY
We envision the holes at Monte Sierpe as potential units of exchange. Depositing goods in the holes could have been a way of publicly displaying information about the quantity of goods available as well as the quantity of goods required for a fair exchange.
For example, a certain number of holes containing maize would have been equivalent to a certain number of holes containing another type of good, such as cotton or coca.
Accounting device
Across the Andes, the Inca reorganised communities into new groups that were expected to take “turns” paying a labour tax or its equivalent in tribute.
In our model, each section at Monte Sierpe could have been linked to a particular social group for the payment of tax. Numerical patterns in layout and variation in the number of holes across sections may reflect differences in tribute levels, or the number of taxpayers from specific villages and towns.
The thousands of holes at Monte Sierpe are divided into groups or blocks.
J. L. Bongers, CC BY
In a sense, Monte Sierpe may have functioned as a large “spreadsheet” for the Inca Empire.
While our model remains tentative, our research brings us closer to unravelling the purpose behind one of the most enigmatic archaeological sites in the Andes.
Authors: Jacob L. Bongers, Tom Austen Brown Postdoctoral Research Associate, University of Sydney
