Thanks to natural processes working at the surface that include weathering and a fermentation process that changes certain iron compounds to more magnetic forms, topsoils (and paleosols) are magnetically enhanced. This process is exacerbated by extended human occupations that introduce organic and fired materials, further enhancing soil magnetism within settlements.

Accumulations of topsoil, such as occur in mound or sod constructions, berms adjacent to excavated ditches, or when storage or other pit features are filled with topsoil after abandonment create local increases in the magnetic field. This is graphically illustrated here where an excavated profile across a prehistoric fortification ditch (dating to the mid-13th century) shows that the village inhabitants mounded the sod --magnetically enhanced A-horizon material -- along the inside of the ditch (right), causing a local magnetic increase in the corresponding magnetic profile (the largest peak is due to a small hearth).

Likewise, the removal of magnetically enriched topsoil during the construction of ditches, house pits, or other depressions, causes a local lowering of the magnetic field over these features. Because the A-horizon was removed from the ditch, as was a paleosol, the relative magnetic field is reduced in this area. With ditch in-filling with down-washed topsoil and other material subsequent to the occupation, a thick sediment occurs along its bottom that is somewhat magnetically enhanced, however, rasing the magnetic measurements near it center.

Forgetting the hearth, the result is an up-and-down pattern of magnetic values; from left-to-right: high, low, medium, low, very high. This result has an interesting effect in plan view, where "zebra-stripes" are produced. In the view below the slopes of the ditch are indicated in white (low values) that flank its bottom of intermediate values. The locus of the stacked sod is revealed by the subtle black line paralelling the ditch to the north.

The "zebra-stripe" magnetic effect that can occur in the vicinity of a prehistoric ditch. The location of the profiles shown above is indicated with the red line.

(Data source: Menoken Village, ND.)

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