DRM orientations of all samples from a site are then compared and their magnetic polarity is determined with Fisher statistics. Using Watson's criteria, the statistical significance of each sample site is evaluated. The latitudes of the Virtual Geomagnetic Poles from those sites determined to be statistically significant are plotted against the stratigraphic level at which they were collected. These data are then abstracted to the standard black and white magnetostratigraphic columns in which black indicates Normal polarity and white is Reversed polarity. Magnetostratigraphic sampling of the stratigraphic column on the left produced the results in the center.
Black circles are statistically significant sites.
White circles lack statistical significance but have the same polarity as an adjacent black circle. Sites plotting to the right of the centerline are normally polarized while those to the left are reversely polarized. The data are abstracted to the standard black and white column on the right in which black is Normal and white is Reversed. An isotopic age was obtained from a volcanic airfall deposit in the section to calibrate the magnetostratigraphic column. Because the polarity of a stratum can only be Normal or Reversed, variations in the rate at which the sediment accumulated can cause the thickness of a given polarity zone to vary from one area to another.