Tectonic geomorphology of the Toroweap Fault, western Grand Canyon, Arizona: implications for transgression of faulting on the Colorado Plateau. The Toroweap fault is a major normal fault in Northwestern Arizona. Along its southern end are four displaced Quaternary surfaces, three of which have measurable displacements that are multiples of about 2.2 m. Soil carbonate analysis was carried out to estimate ages for the three surfaces. An extrapolated carbonate accumulation was used to estimate an age for the oldest surface of between 26 and 54 ka; the youngest displaced surface is between 4 and 11 ka. Oldest undisplaced surface is 2 ± 1 ka. Diffusion modelling determined the most recent surface rupture to be 3 ± 1 ka. An aid in determining degree of tectonic activity where displaced materials are not present is the escarpment sinuosity index (Es). Escarpment length was divided by fault length. Because cliff height varies from one area to another, the index was normalized by dividing by total cliff height. The index is inversely proportional to total displacement. Segments of the fault were distinguished using Es, presence and style of Quaternary displacements, and changes in fault orientation. There are five segments in the study area, the most active segment spanning the Grand Canyon. Report and two map sheets, map scale 1:24,000.