Figure 7. 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 units, the primary texture (greater than 50 percent of the area) is given an uppercase letter and the secondary texture (less than 50 percent of the area) is given a lowercase letter. Only the uppercase letter is used for areas where more than 90 percent of the texture is represented by one basic sediment unit. Figure is modified from Barnhardt and others (1998).