Because the Mount Cayley massif is rich in coarse proximal pyroclastic deposits, some of them hydrothermally altered, it is especially prone to slope failure and debris avalanches. At least three major debris avalanches have occurred from the western slope in the last 10,000 years, all of which blocked the Squamish River and formed temporary lakes upstream. The first and largest event about 4,800 years ago produced a debris fan exposed along the Squamish River. A thick sequence of silts, sands and pebbles interbedded in the debris fan suggests that it may be the product of two major, closely spaced, debris avalanches rather than a single event. Another large debris avalanche about 1,100 years ago deposited material immediately upstream of the mouth of Turbid Creek. The third event followed about 500 years ago with the deposition of two diamicton units along Turbid Creek and was the smallest of the three major prehistoric debris avalanches. A lack of organic and paleosol horizons between the two units implies that they most likely represent separate surges within the same debris avalanche event.