Capture of Rickett's Battery by Sidney King, 1964AD.
The Union attempt to break the Confederate lines along the Bull Run with a direct attack had failed a few days before at Blackburn's Ford, 18 July 1861, hence the flanking movement around the Confederate left that precipitated the battle of First Manassas. Here is a link to some images and information about that fight at Blackburn's Ford:
First Manassas (Bull Run) on the morning of 21 July 1861. [Map by Hal Jespersen, www.cwmaps.com]
The Union flank march was spotted in the early morning hours of 21 July 1861 from Signal Hill [see below], and the battle would unfold in three general phases: 10AM-11:30AM, Union victory on Matthews Hill; 1PM to 4PM, costly but inconclusive fighting on Henry Hill (though CS General Jackson earns his title of "Stonewall"); 4PM to dusk, the arrival of more Confederate brigades extending the fight to Chinn Ridge, and the Union army collapse.
The climax of the fight at First Manassas, afternoon of 21 July 1861. The arrival of more Confederate brigades would extend the fight to Chinn Ridge, and overtax the Union line. [Map by Hal Jespersen, www.cwmaps.com]
This video clip is attempting to show the moment in the battle when Stonewall Jackson's brigade arrives and stabilizes the Confederate army on Henry Hill after the initial setback on Matthews Hill that morning: