A 14 year kinematic survey of Spruce Creek rock glacier, Colorado, USA, provides information on rates and controls of surface strain. Steel-tape measurements of differential movement yield data of cm-scale accuracy, sufficient to assess strain patterns over small portions of the very slowly deforming rock glacier. Flow rates are typically <10 cm a -1 , and measured strain rates range from 1.0 Â 10 -5 a -1 to 1.5 Â 10 -3 a -1 . The primary control on longitudinal strain is changing surface slope, with extending flow occurring in areas of down-valley increase in slope, and compressing flow in areas of down-valley slope decrease. Relatively high strain rates are associated with higher flow velocities and with the impingement of faster-flowing up-valley portions of the rock glacier on the slower-moving lower portion. Overall strain rates decreased through the study period, probably as a result of a slowing of the upper part of the rock glacier. Transverse ridges are associated with longitudinal shortening, caused by either slope changes or impingement of faster-moving sections of the rock glacier on slowermoving sections, but transverse ridges do not occur in every area of strong longitudinal shortening.
PREVIOUS WORK ON ROCK GLACIER KINEMATICSSince the pioneering work of Wahrhaftig and Cox (1959), many workers have examined rock glacier surface flow velocities, using field-survey (e.g. White