Figure 6. Barnhardt and others (1998) bottom-type classification based on four basic sediment units: Rock (R), Gravel (G), Sand (S), and Mud (M). Twelve additional two-part units represent combinations of the four basic units. In the two-part (composite) units, the primary texture (greater than 50 percent of the area) is given an upper case letter and the secondary texture (less than 50 percent of the area) is given a lower case letter. Figure is modified from Barnhardt and others (1998).