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toplap A discordant relationship in which the upper boundary of a depositional sequence is marked by the termination of initially inclined beds (e.g. those formed in the foresets of deltas). Toplap results mainly from nondeposition, possibly with minor erosion. Coastal toplap occurs during a stillstand of sea level. Each unit of strata dips seaward with its upper, terminal edge wedging back towards the land. Successive units of strata build out laterally towards the sea. Compare BASELAP.