The Barrier of Shirakawa (sirakaFa no seki 白河の關) was the customs barrier at the entrance to the northern region of Michinoku 陸奥, and frequently used in poetry as it contained the word sira 白 ‘white’. In this case, however, Sanekata is referring to the reputed vigour with which the officials at the barrier manned it, making sure nothing unauthorised got in or out of the province.