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David Johnson's Hypothesis

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

 

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