The Lines of Nasca, or geoglyphs, are giant geometric shapes
(triangles, trapezoids, parallel lines) and biomorphs (birds,
plants, and mammals) delineated on the surface of the desert.
The geoglyphs were constructed by clearing the surface of small
stones darkened by desert varnish, exposing the lighter-colored
soil beneath. The majority of the geoglyphs have been dated to
the Nasca Culture which inhabited southwestern Peru 0 to 700 A.D.
In 1996, David Johnson proposed a new explanation for the function
of some of the geoglyphs based on his fieldwork and observations
in the Nasca River drainage. This new hypothesis is based on the
strong spatial correlation observed between faults crossing the
alluvial valleys, the positions of archaeological sites, the locations
of aqueducts, springs and high-yield wells and the geoglyphs,
particularly the geometric forms (Figure 1). Johnson argues that
some of the geoglyphs mark the path of aquifers that carry water
through geological faults. This is not an unreasonable explanation
given the geologic setting. This area of Peru is located in one
of the most active seismic zones of the world. Faults are common.
These faults are an integrated and interconnected network that
can collect water in one part of the region and conduct it across
the valleys to locations where it can be reached by digging puquios
or wells, or to locations where the water table is high enough
for springs or seepage to be present on the surface (Figure 2).
Due to insufficient surface water in the river system, the ancient
inhabitants of the drainage settled in locations adjacent to geological
faults because the springs and water resources associated with
these features provide a more reliable source of fresh water during
the dry season than the rivers. Thus, the ancients marked their
water supply distribution system with geoglyphs just as a modern
city delineates its underground utilities with maps.
The purpose of this research is to test Johnson's hypothesis.
This research will examine the relationship between the geoglyphs
and the limited water resources in this arid region of Peru.
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Development of the Hypothesis
The hypothesis proposed by Johnson has evolved over four years
of field work. During the summer of 1996, Johnson began his study
of the water resources of the Nasca Valley. The purpose of his
work was to help the local people find new sources of water to
augment existing supplies. Up to this point, the prevailing thought
concerning groundwater availability was that most of the groundwater
obtained from local wells or from the aqueducts (puquios) in the
region was derived from water flowing in the gravels of the river
valley a few meters below the ground surface (Schreiber and Lancho,
1995). They also argued this water was moving down valley parallel
to the rivers in an east-west direction. Schreiber and Lancho
(1995) have also suggested that many of the hills adjacent to
the valleys have promontories that jut out into valleys in the
subsurface. These subsurface promontories consist of impermeable
bedrock causing the subterranean water moving in the gravels beneath
the rivers to be become ponded or temporarily redirected. They
contend the ancients utilized this natural damming effect by constructing
aqueducts at these locations thereby creating infiltration galleries
to capture some of this ponded groundwater.
The fact that much of the groundwater moves parallel to the river
in the gravels is reasonable except Johnson found some inconsistencies
that could not be explained by this model. For example, the puquios
(age uncertain), which are located throughout the Nasca valley,
always flow continuously even when other wells which tap the subterranean
water in the river gravels have long since failed. Why would the
puquios continue to flow unless they were tapping an independent
source of water?
Johnson began mapping the location of all the puquios and the
position of all the high yield wells in the area. What he found
was a strong spatial correlation; where there were puquios there
were clusters of high-yield wells that were reliable sources of
water throughout the year. Wells having very low yields or that
dried up periodically were not associated with the puquios. This
led Johnson to speculate that there must be an alternative source
of groundwater entering the river valleys other than the subterranean
water moving down the valley in the river gravels. Water levels
in high yield wells located near the valley walls were substantially
higher than the water levels in wells constructed near the river.
This suggested that groundwater was moving north-south or perpendicular
to the river rather than east-west or parallel to the river and
indicated that some of the water entering the valley may be discharging
from the north or south through the bedrock.
The only way groundwater could be entering the valley from the
bedrock is through a fault or discontinuity that exhibits a high
degree of permeability and transmits water. Faults or fractures
are often very transmissive and can supply large quantities of
fresh water to local inhabitants. Field work in the foothills
adjacent to the valleys indicated evidence of fault activity including
striated fault plane surfaces, fractured rock and extensive mineralization.
The faults traversed the foothills in a north-south direction
until they intersected the river valleys. For example, one fault
was observed extending from the Socos river valley, across the
Aja and Tierras Blancas river valleys to Cerro Blanco (see Figure
1 for locations). At Coyungo water was observed discharging from
a fault into an aqueduct cut into the valley wall. So a second
pattern emerged; where there were puquios and reliable, high-yield
wells, there were faults entering the valley at those locations.
Soon another correlation began to emerge. Everywhere there were
faults, high-yield wells and/or puquios, Johnson found that the
faults and availability of fresh water were clearly marked by
geoglyphs. For example, trapezoids were found to lie directly
over the trace of faults and the width of the trapezoids defined
the width of the fault zone capable of transmitting groundwater
as concentrated flow. Triangles, referred to as pointers, pointed
to areas where the faults crossed the ridges or hilltops. Examination
of the bedrock exposures at these locations usually revealed evidence
of faulting. Numerous examples of geoglyphs marking the path of
faults and subterranean water can be found in the Río Grande
de Nasca drainage including Cantalloc, Aja, Orcona, Vista Alegre,
Usaca, and Cerro Colorado to mention a few (Figure 1).
The last correlation that Johnson noted was that there were always
archaeological sites affiliated with the geoglyphs, geologic faults,
puquios and wells. Over the course of four years, a systematic
survey of archaeological sites has been conducted in the vicinity
of the city of Nasca and from Usaca on the Río Trancas,
down the Río Nasca to its confluence with the Río
Grande, and then down the Río Grande, past Coyungo, to
the oasis of Maijo Grande (Figure 1). Over 128 archaeological
sites were recorded over this period, many of which were at or
adjacent to natural springs emanating from faults high above the
valley floor. The most important of these were at Usaca, Coyungo
and along the lower Nasca Valley near Agua Dulce.
Thus, the strong spatial correlation among archaeological sites,
aquifers, high yield wells/puquios, geologic faults and geoglyphs
was solidified into one unifying hypothesis: some of the geoglyphs
mark the location and path of subterranean water. In other words,
the ancients simply made a map of these features much like a municipality
in modern times assembles a utility map of its water supply distribution
system. In addition, the juxtaposition of faults, subterranean
water and geoglyphs is evident in every drainage of the Río
Grande basin in southwest Peru.
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For additional reading, see Bibliography