As Jackson's men marched down to take the train at Piedmont, Stuart gayly trotted past, having left Patterson still in ignorance that Johnston's force had gone. By four in the afternoon of the nineteenth Jackson was detraining at Manassas. But, as we shall presently see, it was nearly two whole days before the last of Johnston's brigades arrived, just in time for the crisis of the battle. When Johnston had joined Beauregard their united effective total was thirty thousand men. There had been a wastage of three thousand. McDowell also had no more than thirty thousand effectives present on the twenty-first; for he left one division at Centreville and lost the rest by straggling and by the way in which the battery and battalion already mentioned had "claimed their discharge" at Blackburn's Ford. Throughout the nineteenth and twentieth, while, sorely against his will, the Federals were having their "monster military picnic" at Centreville, he was reconnoitering his constantly increasing enemy under the greatest difficulties, with his ill-trained staff, bad maps, and lack of proper guides. Lee had chosen six miles of Bull Run as a good defensive position. But Beauregard intended to attack, hoping to profit by the Federal disjointedness. Consequently none of the eight fords were strongly defended except at Union Mills on the extreme right and the Stone Bridge on the extreme left, where the turnpike from Centreville to Warrenton crossed the Run. Bull Run itself was a considerable obstacle, having fairly high banks and running along the Confederate front like the ditch of a fortress. Three miles in rear stood Manassas Junction on a moderate plateau intersected by several creeks. The most important of these creeks, Young's Branch, joined Bull Run on the extreme left, near the Stone Bridge and Warrenton turnpike, after flowing through the little valley between the Henry Hill and Matthews Hill. Three miles in front, across Bull Run, stood Centreville, the Federal camp and field base during the battle.