Patriarchal beards

The law set no limit on the right of owners to administer corporal punishment to their serfs; disobedience, laziness, drunkenness, stealing, fighting, and resisting authority were cause for being beaten with whips, cudgels, and the knout. The only limit on a nobleman’s power was that he was not permitted to execute a serf; he was, however, allowed to inflict punishment likely to cause death. A French traveler in Russia wrote, “What has disgusted me is to see men with grey hair and patriarchal beards, lying on their faces with their trousers down and flogged like children. Still more horrible—I blush to write it—there are masters who sometimes force the son to inflict this punishment on his father.”

what does 'patriarchal beards mean? Does that refer to a person is old enough to be grandpa?

Is the expression ususal? Thanks!