Galileo stereo images covering about 1500 km2 of Uruk Sulcus on Ganymede have revealed two scales of ridges; (1) large-scale ridges and troughs spaced ~6 km apart, corresponding to the grooves seen in Voyager images, and (2) small-scale ridges spaced hundreds of meters apart superimposed on the large-scale ridges. We interpret the small-scale ridges to be the result of tilt-block normal faulting of the surface brittle layer, while the large-scale ridges may be due to necking of the brittle layer over a ductile substrate. The geometry 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-1 during groove formation. ¿ 1998 American Geophysical Union