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interdigitating (interfingering, interlocking) Applied to a facies boundary where the line of lithological change between one sedimentary rock type and the next laterally adjacent rock type is itself broken down into a series of wedge-shaped zig-zags or tongues. Where such interfingering occurs it is a record of the fluctuations in the local depositional environment, implying that both types of sediment were being laid down at the same time.