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.
South Marches, Duke of the
Therefore, when Sir Marhaus arrived with the damsel "Été" and introduced himself - perhaps a trifle prematurely - as a knight of the Round Table, the duke gave him hospitality for the night but informed him regretfully that on the morn he would have to battle his host and his host's six sons.
Marhaus defeated them all and sent them to Arthur's court that Whitsuntide, where they were all reconciled to the King.
Sorgales, Sugales, Surgenale, Sutgales
In the chronicles, Merlin’s maternal grandfather (called Conan by Layamon) is the king of South Wales. In the Vulgate Merlin, the country is ruled by Belinant, and Marie de France names it as Tristan’s (Tristram) birthplace. Spenser names it as the kingdom ruled by Rions (Ryons).