oxidization, or rusting, of the manganese and iron when mixed with oxygen in the air and water from rain; this varnish is dark, almost-black and glossy in appearance. Long ago, Native Americans, as well as Spanish settlers discovered that images can be created on the faces of the rocks by chipping away at this layer using rocks and other tools. Cultural history and significance The petroglyph images within the monument hold deep cultural significance to Pueblo peoples and neighboring Native peoples. This rock art has complex and varied meanings. Archaeologists have dated some carvings, primarily those in the Boca Negra Canyon area, as far back as 3,000 years ago. The relative age is determined based on the darkness of the image, its context, and its comparison to other works of the same age. It is estimated that about 90 percent of the petroglyphs were created during the period between AD 1300 until the end of the 17th century because of the "Southwestern Style" used. At this time, the Native population was increasing quickly and Pueblo adobe villages were being built along the Rio Grande River and at the base of the Sandia Mountains. Stupa controversy In 1989, at least a year prior to the National Monument's establishment, a Tibetan Buddhist stupa was built and consecrated on what was then private land owned by Harold Cohen and Ariane Emery. The National Park Service subsequently used eminent domain to seize this land and make it part of the Monument, over the owners' objections. The stupa was not removed, but all buildings on the land were razed. On June 10, 2010, the Superintendent of Petroglyph National Monument sent an email stating that "[w]hile soils are being stockpiled nearby for the future construction of an amphitheater, the National Park Service has no plans for the Stupa." The Monument website was also updated to describe the construction projects and clarify that the Stupa was not to be demolished. Other controversies Suburban development currently affects the Petroglyph National Monument site. The city of Albuquerque succeeded with their plans to build a 4 lane highway directly through the site itself. The issue was featured in the documentary, Reclaiming Their Voice: The Native American Vote in New Mexico & Beyond. Documents posted on June 6, 2012, by Public Employees for Environmental Responsibility (PEER) allege that although the Petroglyph National Monument is a valuable resource and location for the