On August 18, 1961, at the head of May-flower Gulch in the Tenmile Range of central Colorado, after a 24-hour rainfall of 24.5 cm, a series of alpine mudflows was observed. The flows occurred as a series of pulses which moved at a maximum observed velocity of 980m/min (53 feet/sec) over and through saturated talus on slopes as steep as 41 degrees. Samples of the matrix of the moving debris had a density of 2.53 g/cm3, 11 per cent silt and clay, and less than 10 per cent water. Viscosity of this material calculated from flow rates is roughly 3 × 104 poises. Four successive ancient mudflow deposits were recognized at the site of the largest mud-flow. Lichenometric analysis indicates that they occurred about once every 150–400 years.