Face mask or face guard in late roman period

[Nathan Ross]

I do not find the descriptions of Ammianus´ cataphracts exceptionally fancifull, and he had seen such cavalry as they were used by Julian during his German and Persian campaigns.

I agree. The description of the masked cavalry looking like statues concerns the adventus of Constantius at Rome in 357; Ammianus was not there at the time, but he doubtless witnessed other such imperial pageants. John Mathews (Roman Empire of Ammianus, p.11) suggests that the description of Constantius' parade might have been inspired by the entry of Theodosius into the city in 389, during Ammianus' later residence there - very similar, according to the panegyricist Pacatus.

Either way, Ammianus would have seen clibanarii and cataphracts, both Roman and Persian. He may not have got close enough to examine their armour in detail, but he was aware of the effect of their appearance. Neither he nor Julian would have described these men as 'masked' if they were not, I'd say.

Julian's panegyric clearly refers to 'your cavalry', addressing the emperor, so he was describing Roman and not Persian horsemen.

We can't be sure about what the masks looked like, but I don't think we can doubt that something of the sort was worn by these particular heavy cavalrymen. I suspect it was precisely this 'inhuman' masked appearance that inspired 'terror' and made them 'dreadful to behold'.