Index map
Nature is said to abhor a straight line, and geology is no exception. Why, then, the several north-south linear contacts between different Late Cretaceous ages in North Dakota, Minnesota, and Iowa? The reasons are complex and reflect vagaries of the geologic-mapping process rather than the geology itself: in this case, the bedrock formations defining time periods are so thin and discontinuous (and also overlain by glacial deposits), that their true contacts have yet to be located and thus were drawn arbitrarily - along nearby state, county, or even map-sheet boundaries, which often are straight lines. |
