The Johnson Hypothesis argues that many of the geoglyphs mark
sources of subsurface water. The hypothesis relies extensively
on the fact that faults and structures in the bedrock transmit
groundwater to the alluvial valleys.
In order to test this idea scientifically five questions must
1) Do the faults exist?
2) Do the faults intersect the valleys?
3) Is there evidence that the faults transmit water?
4) Are the faults marked by geoglyphs?
5) Are there geoglyphs associated with the faults?
Accordingly, we have developed a working hypothesis and a research
plan that will address these critical questions. We hypothesize
that there are two independent pathways that move water from the
source area on the Altiplano to lower elevations where it is used
for irrigation and domestic needs. One pathway is water that moves
as surface flow and as groundwater flow in the alluvium along
the main river channels (hereafter referred to as the river pathway).
The other pathway utilizes an interconnected system of fractures,
faults, and other transmissive discontinuities that convey water
on a regional scale across basins until it discharges from the
bedrock along the valley sidewalls into the adjacent alluvium/colluvium
(bedrock pathway). The groundwater derived from the bedrock pathway
discharges at springs and subsurface seeps along the main river
Preliminary evidence collected over the last three years suggests
that water-bearing conduits in the bedrock supply a reliable,
high quality source of fresh water to the east-west river valleys.
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We have developed a multi-faceted research plan to test the Johnson
Hypothesis. The plan employs standard geological and hydrogeological
techniques in conjunction with customary archaeological methods.
A brief description of the research plan follows.
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Remote Sensing and Geologic Mapping -
Faults are discontinuities in the bedrock that arise from tectonic
forces causing the rocks in the earth's crust to shear and move
displacing one side of the fault relative to the other. Once the
rocks have been broken, the fault plane becomes a permanent zone
of weakness that often enhances the permeability of the surrounding
host rock by providing fractures through which water can move.
Depending on the type of rock involved and the nature of the fluids
moving along the fault plane, some faults can become mineralized
and act as a barrier to flow forcing groundwater to seep out at
the ground surface whereas other faults remain open allowing water
to flow freely along the fault plane. Faults can be a few millimeters
to several kilometers in width and range from a single fault plane
to a wide zone of highly fractured rock that contains an anastomosing,
interwoven network of permeable, interconnected fractures.
There are several clues that provide evidence for faulting in
the field. These include offset between rock types on opposite
sides of the fault plane, folding of the host rock due to drag
along the fault, brecciated rock within the fault zone and striated
fault plane surfaces, highly fractured rock adjacent to the fault,
and extensive mineralization suggesting past or recent fluid movement.
For this study, we employed remote sensing analysis and geologic
mapping to assess the propensity of the bedrock to transmit water.
1. Satellite images and aerial photographs were acquired to look
for evidence of faulting. Because faults represent weaknesses
in the rock, they are more prone to weathering. As such, they
can be more easily eroded than the surrounding country rock and,
therefore, often appear as topographic depressions or linear scars
across the landscape. These features can often be detected on
topographic maps, aerial photographs, and satellite imagery particularly
in arid to semi-arid environments where vegetative cover is sparse.
These features are referred to as lineaments and very often represent
faults. Field checks were made to verify the existence of faults
from the lineament analysis.
2. All published geologic maps were gathered for the region and
3. As a final step, traverses were made at specific sites by geologists
to verify the published geologic maps and to confirm faults observed
on the satellite images and aerial photographs. Any unmapped faults
were located and characterized.
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Geophysical Analysis - In order
to verify that the faults observed in bedrock exposures and on
satellite imagery intersect the major river drainages and lie
beneath the surficial materials deposited within the valley, geophysical
tools were employed, where feasible, to locate the faults in the
subsurface. In addition to locating faults, geophysical tools
were also utilized to determine the depth to bedrock below the
unconsolidated surficial deposits and the depth to the water table.
Although several geophysical methods have been used over the last
three years of field work, two were successful and include: 1)
seismic refraction to locate the depth to bedrock beneath the
surficial soil material and to determine the depth to the water
table; and, 2) electromagnetic induction (EM-34) to map the position
of buried faults.
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Water Quality Sampling Program - Water
samples were collected from springs, seeps, and wells, where feasible,
and analyzed for major inorganic constituents and hydrogen and
oxygen isotopes. The chemistry of groundwater inherits properties
of the material through which it travels and will vary depending
on the flow path and length of time the water has resided in the
ground. Accordingly, inferences can often be made on the source
of the groundwater from a close examination of the geochemistry
of the water. In this study, temperature, conductivity, dissolved
oxygen, alkalinity and pH were measured in the field. The remaining
samples were collected, preserved, refrigerated and shipped back
to the U.S. for analysis of other constituents (calcium, magnesium,
sodium, potassium, fluoride, chloride, nitrite, bromide, nitrate,
phosphate, sulfate, boron, barium, chromium, iron, manganese,
strontium, titanium, zinc, silicon and hydrogen and oxygen isotopes).
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Archaeological Analysis - The age and
cultural affiliation of archaeological sites was determined by
means of differences in the associated ceramic, textile, and tomb
styles and in architectural patterns. The function of the component
parts of each site was determined by artifact clustering and by
differences in the size, composition and distribution of the architectural
features. Published archaeological reports on the Río Grande
de Nasca drainage were examined as well as unpublished documents.
The location, azimuth, length and character of geoglyphs were
also noted. Maps were prepared of the geoglyphs along with the
location of the nearest water sources and faults.
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