Isopach maps that show the thickness of sediments between two horizons are useful in studying structural growth. They are often prepared by overlaying maps of the two horizons and subtracting the contour values wherever the contours on one map cross the contours on the other. The differences are recorded on a blank map and then contoured. If the contours show a trend towards increased thickness in a certain direction, it may suggest that the region was tilted downwards in this direction during the period of deposition or that the source of the sediments is in this direction. Uniform thickness of a folded bed indicates that the folding came after the deposition whereas if the thickness increases away from the crest of an anticline, deposition probably was contemporaneous with the growth of the structure. Growth during deposition is usually more favourable for petroleum accumulation since it is more likely for reservoir sands to be deposited on the flanks of structures with even slight relief.