Palaeolithic artists were faced with the difficulty of representing three-dimensional space. The rendering of relief supposes that the artist has an exact idea of the procedures that will allow him or her to recreate the illusion of depth.
The creation of depth is done on several different levels. We find it in the smallest anatomical details, in the grasp of the subject as a whole, and also in the distribution, within the same surface, of different animal subjects that are part of a panel's composition.
The Panel of the Crossed Bison in the Nave speaks to such a process. This diptych brings together two male aurochs. They are symmetrically moving away from each other. Their separation is not total, however, because the images are superimposed at the level of the hindquarters.
The reserve technique involves leaving an uncoloured space between two anatomical segments that are normally joined or superimposed. The idea is to optically dissociate two planes that are found at two different depths. The procedure is used above all in the creation of hindlimbs that are in the background of the animal figure. The use of this technique can also be seen in other segments. Superimposing fields of colour at the level of the hindquarters would chromatically muddy the animal's forms, which are both black. To overcome this difficulty, the artist has created a slightly lighter border to mark the boundaries of the two surfaces. In addition, we note that the hooves in the foreground are more accomplished – their cleft mature is visible – than those in the background, for which the drawing is limited to an outline. We note the simplification of lines based on the relative distance of the viewer from the various elements in the scene.
The choice of the location of this panel contributes to the illusion of an explosion of the diptych's elements. To heighten the effect, the artists chose a wall with a very obtuse angle, and painted a bovine on each plane. Additionally, we can see that the wall is not vertical but leans forward. The corbel created by this position strengthens the illusion that the image is falling towards the viewer below. At the same time, it gives the impression of accelerated movement, a reflection of the power and characteristics of these two animals.