Present participle: the action of establishing clear boundaries or divisions.
From demarcate + -ing (present participle ending). Demarcate derives from de- + marca. The -ing form shows the action in progress.
When historians are demarcating the line between the 'Ancient' and 'Medieval' periods, they're actually making an argument—there's no magic moment where ancient became medieval, but historians need demarcating lines to tell coherent stories.
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