Pond Boss Forums POND CONSTRUCTION Soil questions Thin gray layer

During the expansion an odd layer of material was found about 3ft deep in one spot. About 6in thick. Oil guy says it looks like drilling mud but doesn't smell like it. No smell at all. Thought maybe ash but there's no charcoal bits. I dissolved a chunk in water and then tested with pool test strips. Slightly higher PH than tap and pond water but otherwise about the same. Vinegar caused it to foam up quite a bit.