The main cave at Roc de Marsal is part of a larger system of solution
cavities formed in a Cretaceous limestone that includes a number of lithological
beds distinguished, in part, by their texture, composition, and degree
of consolidation. The bedrock floor, as it existed just prior to initial
occupation, is a highly irregular and complex erosional surface that
had truncated some of these beds. At its lowest point, more or less in
the center of the cave, this floor is a basin-like de-pression, while
toward the back of the cave it rises abruptly approximately one meter
higher (see figure).
In the rear of the cave, some of the limestone beds include
irregularly shaped, sub-horizontal, erosional endokarstic tubes that
had been filled with sediments that accumulated prior to and perhaps
during occupation.
Overlying the bedrock, the deposits at Roc de Marsal consist generally
of mineralogenic sands and silts with abundant lithic and bone remains
and, in the lower levels, interspersed with stratigraphic units consisting
of intact combustion features. Seven major lithostratigraphic units
have been identified on the basis of color, texture, composition,
and rock content. They are separate from but largely overlap with
archaeological levels defined by Lafille.