 The Army of the Potomac 's disposition in the lead @-@ up to the battle was more orderly than Lee 's Army of Northern Virginia ; all of McClellan 's forces would be concentrated in one place , save for several divisions from Maj. Gen. Erasmus Keyes 's outfit and Keyes himself , who were posted across the James River at Haxall 's Landing . A Confederate scout observed Union soldiers resting in position , and moving about the hill unworried , whilst the disposition of the cannons around the hill 's slope gave him the impression that the position was " almost impregnable " . McClellan 's army was on the hill in force . 
