1. In a system having a computer having a tangible storage device that contains instructions for automatically detecting a ravine between a first saddle point and a second saddle point in a virtual three-dimensional dentition model in a dental arch; wherein said model is in the form of a single mesh comprising points interconnected by triangles, the improvement comprising: said instructions program the computer for executing the following steps; a) store said model in a memory accessible to said programmed computer; and b) operate on said model in the following respects: 1) perform a tree search starting from said first saddle point along paths traversing edges of said triangles; 2) determine a quality for each of said edges along said paths; and 3) select a path which reaches said second saddle point, wherein said selected path has the highest quality at said second saddle point; wherein the quality of a path of the search tree is obtained by adding the qualities of all edges from said first saddle point to the end of the edge multiplied by their lengths and dividing them by the total length of the path; wherein said selected path is the ravine between said first saddle point and said second saddle point.