Aboriginal rock art is the oldest continuous art tradition in the world, dating as far back as 60,000 years. From the Gwion Gwion and Wondjina imagery in the Kimberley to the Sydney rock engravings, it is spread across hundreds of thousands of sites, making Australia the richest continent in terms of prehistoric art. 19th-century Indigenous activist William Barak painted ceremonial scenes, such as corroborees. The Hermannsburg School, led by Albert Namatjira, received national fame in the 1950s for their desert watercolors. Leading critic Robert Hughes saw contemporary Indigenous art as "the last great art movement of the 20th century". Key exponents such as Emily Kame Kngwarreye, Rover Thomas and the Papunya Tula group use acrylic paints on canvas to depict dreamings set in a symbolic topography. Clifford Possum Tjapaltjarri's "Warlugulong" (1977) typifies this style, popularly known as "dot painting". Art is important both culturally and economically to Indigenous society; central Australian Indigenous communities have "the highest per capita concentrations of artists anywhere in the world". Issues of race and identity are raised in the works of many 'urban' Indigenous artists, including Gordon Bennett and Tracey Moffatt.