GeoModeller runs an implicit geostatistical modelling engine to interpolate the geological interfaces of formations.

The topology of these interfaces is defined by the stratigraphic pile and the erode/onlap topological relations between the series of the stratigraphic pile. 

More specifically, Erode is a cross cutting relation while Onlap is a stops on relation.

The stratigraphic pile sets the sequence of the topological relationships, that is, in what order is each relationship considered and to which series is it applied.

Note that formations are always included in series and that the stratigraphic pile considers its element at the series level. 

Therefore, several formations included into the same series will necessarily follow the same topological rules in regard to formations in other series.

The most important aspect of the stratigraphic pile is that it follows a top to bottom influence rule. A series applies its topological relationship to every other series (and their included formations) situated below.

Building a valid stratigraphic pile is one of most crucial steps in being successful with GeoModeller, a great deal of thought must be put into its design to ensure consistency across the whole model. Moving a unit up and down the stratigraphic pile or switching its topological relation to the others will have stark impact on the geometries of the formations (see visuals below).

1. Green dyke crosses all other units if set to Erode relation and placed at the top of the stratigraphic pile:

2. Green dyke crosses all but the red intrusive formation when the latter is placed at the top of the stratigraphic pile: