Ammianus Marcellinus Res Gestae 19.8.2

[2] For a long time the sanguinary battle remained undecided, and not a man anywhere through fear of death gave up his ardour for defence; and the contest had reached a point when the fate of both parties was governed by some unavoidable hap, when that mound of ours, the result of long toil, fell forward as if shattered by an earthquake. Thus the gulf which yawned between the wall and the heap built up outside was made a level plain, as if by a causeway or a bridge built across it, and opened to the enemy a passage blocked by no obstacles, while the greater part of the soldiers that were thrown down ceased fighting, being either crushed or worn out.