The Duke of the South Marches hated Arthur and his knights because Gawain had killed some of his sons. Marhaus came upon the duke’s castle and fought him and his remaining sons.
The South Marches is almost certainly the marches of Arthurian Cornwall. Putting them east of Avilion would give Cornwall proper more territory and help fill in more of the central region. Or they might simply be the area south of Avilion to the sea, forming a sort of pass from Logres to Cornwall.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470