Craters were produced by firing spherical steel projectiles of 316- and 932−in. diam into sandstone and granite at velocities ranging from 300 to 6000 ft/sec. Impact angles of 30, 60, and 90 deg were used for the sandstone and 90 deg for the granite. The craters are formed by two mechanisms: (a) crushing of material in front of the projectile and (b) fracturing which takes place as fractures are initiated by a constant impulse in steplike fashion in front of the projectile and propagated along logarithmic spirals of maximum shear to the free surface of the rock. The volume of the material removed by crushing varies as the first power of the impact velocity and the volume removed by fracturing, as the second power of the impact velocity. Penetration varies linearly with the impact velocity and is inversely proportional to the specific acoustic resistance of the target material, the proportionality constant being dependent upon the shape of the projectile.