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Among the "Roluos Group" of temples there are some of the earliest permanent structures built by Khmer. They mark the beginning of classical period of Khmer civilization, dating from the late 9th century. Some were totally built with bricks, others partially with laterite or sandstone (the first large Angkorian temple built with sandstone was possibly Ta Keo)
Coedès, George (1968). Walter F. Vella (ed.). The Indianized States of Southeast Asia. trans.Susan Brown Cowing. University of Hawaii Press. ISBN978-0-8248-0368-1.