The patterns of stone lines located between Nasca and Palpa, Peru (Figure 1) form geometric and biomorphic geoglyphs spread over an area of several hundred square kilometres of desert surface. The geometric examples occur as long straight lines, triangles, rectangles, trapezoids, spirals and zigzag forms, while the biomorphic features exhibit humanoid, animal and plant forms. They were reported by early Spanish explorers and then appeared in the archaeological literature as early as 1927 (Mejia Xesspe 1927). The lifelong work of the mathematician Maria Reiche made them known in public and initiated their broad scientific exploration (Reiche 1969). Most recent studies use modern methods of mapping and photogrammetry (Grun et al. 2000; Reindel et al. 2001, 2002). Theories about their purpose and use are still under discussion (Morrison 1978; Aveni 1990; Silverman & Proulx 2002). Proposed explanations of their purpose include astronomical/calendrical objects, indicators for underground water, agricultural use, artistic expression, ceremonial sites, religious pathways and guideposts for desert travel.
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Our research demonstrates that their origin can be ascribed to a two-stage process. First, humans built elongated stone heaps leaving behind areas free from larger stones, and then natural geological processes infilled the stone heaps with finer-grained sediment. A colour contrast can now be seen in aerial views, with the darker zones being the in-filled stone lines and undisturbed desert pavement, while the cleared areas show up as lighter areas.
Previous attempts to determine the age of the geoglyphs have not been conclusive. Wooden posts at the intersections of two different sets of lines have given uncalibrated [sup.14]C dates of AD 525 [+ or -] 80 and 490 [+ or -] 80 (Bray 1992). Pottery fragments found around the lines are numerous and encompass many local styles, but their use as geo-chronological markers is problematic. This is because the lines can be either older or younger than the pottery itself depending upon the circumstances at the find locations(Hawkins 1967). One early approach was made through a comparison of the designs found on pottery of known age in the region with the designs of the geoglyphs themselves. The best match was suggested to be the designs on Nasca Style 3 and 4 pottery, which confined the age to the Early Intermediate Period dated to between 100 BC and AD 350 (Hawkins 1967), equivalent to 2100-1650 years ago. More recent finds of pottery by Johny Isla of the Nasca-Palpa project associated with the geometric geoglyphs at Sacramento and San Ignacio near Palpa are attributed to styles of Nasca ceramics with a time range of AD 200-650, using correlation with [14.sup.C] ages of nearby contexts containing those styles (J. Isla pers. comm.).
Other attempts to directly date the geoglyphs using the age of diagnostic pottery fragments found on cleared surfaces lines have suggested a range of times that different geoglyphs were produced. In the seminal study using this approach (Silverman & Browne 1991), the areas near Palpa and those further south in the Ingenio River valley were studied. In both areas the great majority of diagnostic pottery fragments that were closely associated with geoglyphs were classified as Nasca ceramics from the Early Intermediate Period (200 BC to AD 600). However, a small number of sites in each area had fragments of much earlier Early Horizon 10 pottery (800-200 BC) or much later Late Intermediate Period (AD 1000-1476) style ceramics. In that study they assumed that the presence of a particular period's pottery indicated use of the geoglyph in that period, and that the earliest pottery on a site indicates the time at which, or before which, the geoglyph was first elaborated. Finally, AMS [sup.14]C ages were obtained on organic matter (lichens, cyanobacteria or fungi) beneath rock varnish …