The stratigraphic column of North Dakota represents the relationship between episodes of rock formation and continuous time. Geologic time is subdivided into age, era and period in decreasing lengths of time involved. Estimates of geologic time measured in millions of years before present are assigned on the basis of generally accepted time-unit boundaries.
The sequence, rock unit and pictorial rock columns represent the discontinuous geologic record contained in the preserved sedimentary rocks of North Dakota. Rocks found at the surface in North Dakota are represented by the etched column. The relative resistance of the various rock layers are represented by the relief contained along the right hand side of the column.