An investigation was conducted to determine the maximum depth of scour produced by an impinging jet on a simulated rock mass. Fractured rock was simulated using 0.39 m by 0.20 m by 0.064 m lightweight concrete blocks. Three thousand six hundred blocks were placed in two layers at a 45 degree angle. Discharge was increased and tailwater depth was lowered until dislodgement of the blocks occurred at a unit discharge of 0.372 m2/s. Road base material was used in the tests for granular material. Scour tests were conducted at different tailwater conditions and different impingement angles. The tailwater depth and the angle of issuance proved to be major factors affecting the maximum depth of the scour hole in a cohesionless bed.