A geometric model for the excavation and modification of simple crater development is presented. Modification is modelled by considering the relative dimensions and geometries of the final crater and the transient cavity formed by excavation and displacement. The input parameters required are the rim crest diameter of the final crater, the depth to autochthonous basement, the rim crest height and the outer rim slope. The model calculates the diameter of the final crater at the original ground plane, the dimensions of the transient cavity and the rim crest volume difference between the final and transient cavities. This volume is considered to be the material which sllumped off the unstable, overheightened transient cavity wall during modification and corresponds to the observed volume of the breccia lens within the final crater. Excavation is approximated by a steady state Z model, with the diameter of excavation equivalent to the previously modelled diameter of the transient cavity at the original ground plane, the center of flow taken as 1 projectile diameter, and Z=2.5--3.0. The Z model predicts such parameters as the depth and volume of excavation, which can be compared with observational data. The applicability of the model has been tested with data from Meteor and Brent craters. In the case of Meteor, the modelled final crater diameter at the original ground plane is within 26 m of the observed value and the modelled breccia lens and the rim crest volumes of the final true crater are within 9.5% and 2.5%, respectively, of observed values. The results of the Z model for Meteor crater, with Z=2.8¿0.1, give results consistent with observational data on the depth and volume of excavation. The correcpondence for the more degraded Brent crater is less precise, as there is greater uncertainty in the input parameters. It is possible, however, to match the model parameters with the breccia lens volume and published estimates of original dimensions. The modelled excavated volumes for Meteor and Brent are 58% and 55%, respectively, of the transient cavity volume below the original ground plane. In the absnece of additional observational data for further testing, it is concluded that the model may provide first-order spatial information on the formation of simple craters.