(Hohokam petroglyph, age unknown)
A dark coating called varnish forms on rock outcrops in the Arizona desert. Wielding hard stones like chisels, the Hohokam Indians pecked away this coating to reveal the lighter rock beneath. Such petroglyphs can still be seen in many locations.
The Hohokam had no metal tools. They built their canals, adobe buildings, tools, and weapons using stone, wood, clay, and other materials that we would consider primitive. Yet their engineering skills were of such high quality that some of their constructions can still be seen in the landscape today, 700 years later.
