We have used Mars Orbiter Camera (MOC) images and Mars Observer Laser Altimeter (MOLA) data to correlate specific layers within the upper ~500 m of the Martian north polar layered deposits in many troughs. Using these correlation results, we derive relative accumulation rates across the polar layered deposits and through time. We identify two major layer sequences, one lying stratigraphically above the other, each consistent with an overall curving downward layer structure with shallow slopes but with significant localized variations in layer height. Major disruptions of stratigraphy (such as angular unconformities) do not appear within the correlated layer sequences. On local and broad scales, each layer sequence exhibits a different relative accumulation rate pattern, indicating that this rate has not remained constant through time and has been affected by localized processes. The overall pattern of the lower layer sequence is similar to that of a classic terrestrial ice sheet, with a decreasing relative accumulation rate away from the center of the polar layered deposits southward. The upper layer sequence exhibits no obvious overall trends. We also compare our correlation results to predictions of the effects of simple ice flow on layer structure, finding that mass balance patterns have overprinted any effects of large-scale ice flow. Flow has not been significant, compared to mass balance, in forming the overall structure of the correlated layers.