integral shaft serving as both the drive and driven shafts of the prior art for the sake of brevity features of the cutting head 40 which are similar to features of the prior art cutting head 10 are identified hereafter and in fig2 and 5 by the same numerals as are used in fig1 the cutting head 40 is shown in fig2 presenting a plurality of spirally arranged rock cutting and mixing picks 42 on both of its rotary drums 16 18 only the outline profiles 96 98 of the picks 42 are shown in fig3 the picks 42 are attached to each drum 16 18 through individual mounting brackets 43 with the pattern of picks 42 being loosely based on a two start helical spiral however a three four or other multiple start helical spiral may be used depending on the cutting requirements of the cutting head the receding direction of the spiral on the left side drum 16 as viewed from its exposed end as per arrow a is counterclockwise and the receding direction of the spiral on the right side drum 18 as viewed from its exposed end as per arrow b is clockwise it will be appreciated by skilled persons in the art that the directions of the spirals may be reversed in certain instances each pick 42 has in this instance a tungsten carbide tip 44 to retard the rate of wear of the pick 42 such as may otherwise result say from prolonged sandstone excavation the cutting drums.