Deep within a forest northeast of Peterborough is an extraordinary collection of rock carvings, called petroglyphs. It is generally believed that Algonkian-speaking people carved this assortment of shapes and figures on an outcrop of gleaming white marble between 900 and 1400 AD. The 900 images depicting turtles, snakes, humans, and other symbols, are believed to express the teachings of the spirit world. With one of the largest concentrations of Aboriginal rock art in North America, the site is regarded as an archaeological treasure. Petroglyphs Provincial Park was established in 1976. In 1984, the glass enclosure was built, providing an environment free from the effects of acid rain, frost and algae.