As soon as the infantry had replenished their ammunition, they wheeled to the left in echelon of brigades, and began to march towards Surgham ridge. The movements of a great force are slow. It was not desirable that the British division, which led the echelon, should remain in the low ground north of Surgham--where they were commanded, had no field of fire, and could see nothing--and accordingly both these brigades moved forward almost together to occupy the crest of the ridge. Thus two steps of the ladder were run into one, and Maxwell's brigade, which followed Wauchope's, was 600 yards further south than it would have been had the regular echelon been observed. In the zeriba MacDonald had been next to Maxwell. But a very significant change in the order was now made. General Hunter evidently conceived the rear of the echelon threatened from the direction of Kerreri. Had the earth swallowed all the thousands who had moved across the plain towards the hills? At any rate, he would have his best brigade and his most experienced general in the post of possible danger. He therefore ordered Lewis's brigade to follow Maxwell, and left MacDonald last of all, strengthening him with three batteries of artillery and eight Maxim guns. Collinson marched with the transport. MacDonald moved out westward into the desert to take his place in the echelon, and also to allow Lewis to pass him as ordered. Lewis hurried on after Maxwell, and, taking his distance from him, was thus also 600 yards further south than the regular echelon admitted. The step which had been absorbed when both British brigades moved off--advisedly--together, caused a double gap between MacDonald and the rest of the army. And this distance was further increased by the fact that while he was moving west, to assume his place in correct echelon, the other five brigades were drawing off to the southward. Hence MacDonald's isolation.