The standard type of heading blast consists of a straight in or main drive, at right angles to the rock face, and a back drive at right angles to the main drive and parallel to the face. The main drive is normally driven at quarry floor level to a depth of 0.6 times the height of rock above the back drive. Apart from exceptional circumstances, the maximum depth of the main drive should be 50 ft (15 m), so that with faces over 85 ft (26 m) high the 0.6 ratio should not be used, but the main drive limited to 50 ft.