Among the Uranian satellites, Ariel appears to have the youngest surface with the fewest impact craters and Umbriel's the oldest. Miranda has fault canyons deep, terraced layers, and a chaotic variation in surface ages and features. Miranda's past geologic activity is thought to have been driven by tidal heating at a time when its orbit was more eccentric than currently, probably as a result of a former 3:1 orbital resonance with Umbriel. Extensional processes associated with upwelling diapirs are the likely origin of Miranda's 'racetrack'-like coronae. Ariel is thought to have once been held in a 4:1 resonance with Titania.