Arrian

“ Though they fought on horseback, it seemed more like an infantry than a cavalry battle; for they struggled for the mastery, horses being jammed with horses and men with men, the Macedonians striving to drive the Persians entirely away from the bank and to force them into the plain, and the Persians striving to obstruct their landing and to push them back again into the river. ”

“ Memnon himself, as well as his sons, were running every risk with these; and the Macedonians who first came into conflict with the Persians, though they showed great valour, were cut down, except those who retreated to Alexander, who was now approaching. For the king was already near, leading with him the right wing. He made his first assault upon the Persians at the place where the whole mass of their horse and the leaders themselves were posted ”