From the historical reference included: "Kemble, in his (appendix) list of "patronymical names," which he regards as "those of ancient Marks," has two references, from the "Codex Diplomaticus"...", what is meant by ancient Marks in this reference. The source in question doesn't include the term in its index and I have never heard of it before but it seems from the source title it might originate from England. Scholarly expertise is needed here I believe. Source: The Project Gutenberg EBook on Some Ancient Battle-fields in Lancashire, by Charles Hardwick, page 198
The document hints at your answer further on (at page 206):
The tumulus near to "Whittle Springs" is very similar to these in appearance, and may, on excavation, prove to be a sepulchral mound. Pickering, according to the method of interpretation adopted by John Mitchell Kemble, in his "Saxons in England," should indicate the "Mark" of a sept or clan bearing that name, like the Faringas as at Farington, Billingas as at Billington, and many others. But there is not the slightest reference by any writer of such a name ever holding property in the neighbourhood, and Mr. Kemble places the Pickering, in Yorkshire, only among the probable instances, as he had never met with any account of a Saxon family or mark answering to it.
From the context, I think it's safe to assume he's referring to a symbol of the sept or clan. Why it is capitalized in your quote but not consistently capitalized in the quote above, I don't know.