The Athabasca Oil Sands are largely within the McMurray Formation, which is a transgressive blanket quartz sand of Lower Cretaceous age. Millennia of erosion have resulted in extensive exposures of oil sand along river valleys in the vicinity of Fort McMurray. Study of these slopes has contributed to understanding the nature and behavior of oil sands. Oil sand slopes with active toe erosion are characteristically high and steep (up to 70 m at slopes over 50°), have an indurated outer face and display a stress–relief exfoliation joint system that controls slope recession phenomena. Bitumen does not contribute mechanically to slope stability. The major agents affecting slope morphology are the lithology, the aspect and the basal stratigraphy. Ravelling along exfoliation fractures is the major failure mode, block falls are a minor failure mode and rotational landslides have not been observed. Remolded oil sand may flow viscously, but intact oil sand displays an unusually high strength.