Using images taken by the Cassini spacecraft, we have identified a longitudinally confined arc of material located at the G ring's inner edge that is roughly five times brighter than other parts of the G ring. Images taken over the last two years show that the orbital mean motion of the arc is 445.475 +/- 0.007 degrees/day, corresponding to a semi-major axis of 167,495.6 +/- 1.3 km, and extremely close to the 7:6 corotation eccentricity resonance with Mimas at 167,493.4 km. Thus perturbations from Mimas are almost certainly responsible for confining the material in this arc (see also abstract by Tiscareno et al.). We propose that the rest of the visible G ring is composed of small grains escaping the arc and evolving outward under the influence of non-gravitational drag forces. The ring's brightness then decays exponentially with distance from the arc because the particles are steadily eroded as they move outward. We have also discovered subtle longitudinal brightness variations in the outskirts of the G ring near the 8:7 Inner Lindblad Resonance with Mimas at 169,815 km.