The army of the Parthians was thus composed solely of cavalry, some barbed with iron, as well as their horses,[730] and armed with long and heavy lances; others furnished with powerful bows and arrows, which, while they carried much farther than those of the Romans, perforated defensive armour. After quitting the town of Carrhæ, the Roman army advanced towards the south, across the desert. The sand and heat made the march painful, while the enemy remained always invisible. At length, when they arrived on the banks, of a small river, the Balissus (_Belick_), which flows into the Euphrates, they perceived a few Parthian horsemen. Abgar, sent against them with a vanguard to reconnoitre, did not return. The traitor had betrayed Crassus to Surena. The proconsul, impatient and uneasy, then crosses the Balissus with his whole army, and, without allowing it to repose, pushes forward his cavalry, and obliges the infantry to follow it. A few soldiers soon arrive to inform Crassus that they are all who have been able to escape from an ambuscade into which his vanguard has fallen, and that the whole Parthian army is on its march to encounter him. At this intelligence, he, who believed that the enemy would not dare to wait him, becomes confused, and hastily forms his troops in array of battle on a long front, for fear of being surrounded. The cavalry is on the wings; the Osroenes form a last line. The Parthians first throw forward their light cavalry, which makes whirls in the plain, raising clouds of dust, and causing the air to ring with their savage cries and the noise of their drums,[731] and then retire as if in flight.[732] Crassus sends forward against them his light infantry; but, surrounded and overwhelmed with the more powerful missiles of the Parthians, it is obliged to take refuge behind the legions.