In the mean time, as Alexander advanced toward Ecbatana, Darius and his forces retreated from it toward the eastward, through the great tract of country lying south of the Caspian Sea. There is a mountainous region here, with a defile traversing it, through which it would be necessary for Darius to pass. This defile was called the Caspian Gates,[G] the name referring to rocks on each side. The marching of an army through a narrow and dangerous defile like this always causes detention and delay, and Alexander hastened forward in hopes to overtake Darius before he should reach it. He advanced with such speed that only the strongest and most robust of his army could keep up. Thousands, worn out with exertion and toil, were left behind, and many of the horses sank down by the road side, exhausted with heat and fatigue, to die. Alexander pressed desperately on with all who were able to follow.