Nazca Lines: The Mechanics ... How They Did It?

Two Methods were used to create geoglyphs in the coastal plains of Peru and Chile.

The typical method, the lines were made by removing pebbles which cover the surface of the Nazca desert. When the gravel (desert pavement) is removed, they contrast with the light-colored earth underneath.

The other approach is to pile stones or gravel to form the symbol or line.  There are also geoglyphs that employ a combination of the two.

There are several thousand simple lines and geometric patterns on the Nazca plateau and surrounding regions, as well as over seventy curvilinear animal, insect, and human figures in the Nazca area alone, with more in other areas. The area encompassing the Nazca lines is nearly 500 square kilometers (200 square miles), and the largest figures can be nearly 900 feet (270 meters) long.

The geoglyphs persist due to the extremely dry, windless, and constant climate of the Nazca and coastal Peruvian regions: the Nazca desert is one of the driest on Earth and maintains a temperature around 25°C (77°F) year round, and the lack of wind has helped keep the lines uncovered to the present day.  Other geoglyph areas, such as the Atacama desert of Chile are even drier!

What is Desert Pavement?
Nazca Lines The Mechanics3Large, flat areas devoid of vegetation and covered by a layer of tightly packed small stones are conspicuous features of extremely arid landscapes. These desert pavements are rare or absent in the moister parts of the world's deserta, but become increasingly pronounced in the driest parts (see the photo below).

Nazca Lines The Mechanics5Some of the most extensive and well-developed areas of desert pavements occur on stony alluvial fan deposits flanking the rugged, low mountains in the extremely arid Pacific coastal deserts of South America. Geologically young deposits (Holocene-aged, less than 11,000 years) lack the flat-surfaced pavements. Surfaces of these young deposits are typically cluttered with large stones and rocks irregularly piled in elevated bars; these low bars are separated by intervening swales. This stony jumble of bars and swales is the topographic imprint of the surface’s creation by the powerful tumult of moving water laden with rocky debris. Over time, though, this imprint disappears as the vertical relief of these coarse, rocky deposits is leveled out, eventually forming the flat pavement of small stones. The best-developed pavements are those that have formed over the passage of several tens of thousands to a few hundreds of thousands of years.

Research conducted within the last fifteen years in the U.S. Mohave Desert by a team of soil scientists and geologists (L.D. McFadden, S.G. Wells, and M.J. Jercinovich, "Geomorphology of Desert Environments") provides a detailed picture of how desert pavements form on rocky parent materials such as these. Physical weathering of the large rocks on the surface produces the smaller stones that eventually form the pavement surface. These smaller stones tend to accumulate in topographic lows on the original, uneven surface. Special soil characteristics found directly beneath pavements provide clues to an additional process involved in creation of pavements. If you carefully remove the layer of stones from a pavement surface, you will find a distinct, fine-grained soil horizon called a vesicular A (or Av) horizon (frequently lighter in color than the covering pavement) (see the photograph above).

The name “vesicular” refers to the many vesicles or large pores found throughout the horizon (geologic term for layer). “A” denotes its position as the topmost mineral layer of soil. The Av horizon is typically a few centimeters (about an inch) thick, and contains mostly silts and clays, or in the case of Coastal South America: sand; it lacks coarse materials, even though small stones of the pavement cover the Av horizon and rocky materials can occur in the soil underlying it.

The origin of the fine-grained Av horizon is an important key to understanding how the overlying flat-topped pavement develops. Examination of the minerals contained in the Av horizon at one site in California demonstrated that the materials in this horizon did not originate from the weathering of the rocky parent materials. Instead, dust deposited on the stony surface is the source of the silts and clays of the Av horizon. These fine-grained materials accumulate beneath a layer of surface stones, separating these stones from the rest of the underlying rocky materials. Over time, the further accumulation of fine-textured materials in the Av horizon literally lifts the mono-layer (top layer) of stones of the pavement and levels the surface (see the illustration below).

Prior to this work, it was commonly believed that most desert pavements originated through the selective erosion of fine materials from the surface by either wind or water, a process called deXation. However, such a process cannot explain the development of pavements in stony parent materials that initially lacked fine-grained materials (such as many coarse-grained alluvial fan deposits or areas of exposed bedrock, including basalt flows). Nor can it explain the presence of the fine-grained Av horizon that separates the surface pavement from underlying rocky materials. In coarse parent materials, atmospheric additions of fine materials (rather than their selective removal) and incorporation of these materials into the Av horizon below a layer of stones are responsible for creation of the pavement surface.

More rarely, however, stone pavements can also be created by a process of deflation in certain environments where the original parent material consisted of small stones and pebbles mixed with abundant fine-grained sands, such as in beach deposits that ring ancient lake beds in some parts of the region. In such cases, the selective removal of the sand by wind and/or water leaves behind a lag of pebbles that resists further deflation. Once the surface lag stabilizes the fine-grained deposit, the pebbles act as a dust trap and airborne materials accumulate to eventually form a silt- and clay-rich Av horizon beneath the pebble pavement in the same manner as that which occurs on rocky parent materials. These kinds of pavements exist in a limited number of locations in the Sonoran Desert region, but are less common by far than pavements that developed on coarse, rocky parent materials as described previously.