natural liner

The clay in the soil formed a natural liner.

Additional shadowing of color can be added for a soft natural liner or a more bold, definite line can be achieved.

Shell stated that the channel was protected by a layer of lower permeability till beneath the tailings disposal area, which would act as a natural liner.

They may form from slow permeability of soils, rises in the groundwater table, or from the creation of a natural liner of impermeable or slowly permeable clays or soils in the depression.