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In page River Yare:

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The derivation of the name has been the subject of debate, M. R. James states that the Saxon Shore fort, Gariannonum (which he identified as Burgh Castle) takes its name from the original Common Celtic name for the Yare.[1] Ptolemy records a Latinized version of this name as Gariennos around 150 AD.[2] Eilert Ekwall gives another derivation, that the river name is a back-formation from Yarmouth.[citation needed]