Galileo stereo images covering about 1500 km 2 of Uruk Sulcus on Ganymede have revealed two scales of ridges; (1) large-scale ridges and troughs spaced -6 km apart, corre- sponding to the "grooves" seen in Voyager images, and (2) small-scale ridges spaced hundreds of meters apart superim- posed on the large-scale ridges. We interpret the small-scale ridges to be the result of tilt-block normal faulting of the sur- face brittle layer, while the large-scale ridges may be due to necking of the brittle layer over a ductile substrate. The ge- ometry of the tilt blocks revealed in Galileo images leads to a minimum estimation of 51% to 58% extensional strain in the area. The strain estimate, when incorporated into a model for the formation of grooved terrain by necking of a brittle layer undergoing extension, leads us to estimate a thermal gradient of--20 K/km and a strain rate of "'10 '14 S 'l during groove forma- tion.