Figure 41. Generalized cross-section, Copper River Basin, showing position of overpressured compartment, from which fluidized rock material, mainly shale, and high- pressured water with minor hydrocarbons are being ejected and venting at the surface to form a mud volcano. The rising, high-pressured mixture may pressure-up any shallow, permeable beds encountered, thereby locally complicating recognition of the layered arrangement of hydraulic systems.