Marius, the son of Arvirargus, succeeded him in the kingship. He was a man of great prudence and wisdom. A little later on in his reign a certain King of the Picts called Sodric came from Scythia with a large fleet and landed in the northern part of Britain which is called Albany. He began to ravage Marius's lands. Marius thereupon collected his men together and marched to meet Sodric. He fought a number of battles against him and finally killed him and won a great victory. In token of his triumph Marius set up a stone in the district, which was afterwards called Westmorland after him. The inscription carved on it records his memory down to this very day. [IV.7]

Tatlock suggests that Geoffrey got the idea for the triumphal stone in WESTMORLAND from William of Malmesbury's description, in the Gesta Pontificum (1125), of a vaulted hall in CARLISLE with the inscription "Marii Victoriae." He also points out that the general name Westmoringaland appears as early as 966 in the Anglo-Saxon Chronicle, and notes that there would of course have been many inscribed Roman stones in the general area (p. 20). Today, the Senhouse Roman Museum at Maryport displays a large collection of Roman altars and stones from the area.

Westmorland's only other appearance in the Historia is in the Roman section of the work:

Meanwhile Octavius, Duke of the Gewissei, led a revolt against a certain proconsul in whose hands, as a Roman dignitary, the government of the island had been left. The proconsul was killed and others with him; and Octavius seized the royal throne. This event was announced to Constantine and he sent Helen's uncle Trahern with three legions to restore the island to Roman sovereignty.... Octavius reassembled his men in companies, followed Trahern, and fought him in the province called Westmorland. This time Octavius was defeated, and fled. [V.8]