What was depicted on roman triumphal arches?

The vault was ornamented with coffers. Some triumphal arches were surmounted by a statue or a currus triumphalis, a group of statues depicting the emperor or general in a quadriga. The inscriptions on Roman triumphal arches were works of art in themselves, with very finely cut, sometimes gilded letters.

Answer gutzon borglum was both a gifted painter and sculpture;

This would be tesserae.