Dear Mav: I found out what the "marc" is, from a site that gives the name of each step in the process of making wine:
"2b. Breaking - Foulage
A horizontal press squeezes the broken grapes, separating the fresh juice (must) from the skins (marc). For most red wines, this process is used to breaking up caked grapes, but the marc goes into the fermation vats with the juice.
Note - Breaking the grapes starts the fermentation process."