Maj. Jed. Hotchkiss, Confederate Military History, a library of Confederate States Military History: Volume 3, Virginia
Clement Anselm Evans, Ed.

While providing for and awaiting the general attack, Beauregard was, on the 19th, urged by Adjutant-General Cooper to withdraw his call upon Johnston for assistance if the enemy in front of him had abandoned an immediate attack. As this was not an order, Beauregard paid no attention to it, and continued his efforts to secure the early arrival of Johnston's forces, intending, with their help, to take the offensive. McDowell spent the 19th and 20th reconnoitering the Confederate front and waiting for rations. During these two days, 8,340 of Johnston's men with twenty guns, and 1,265 of Holmes', with six guns, arrived upon Beauregard's left and right; the larger number of them in the afternoon of the 20th. Most of these were ordered to the Confederate left-center and left, at the instance of General Johnston, as Beauregard had placed the most of his own army on his right-center and right, expecting, from McDowell's demonstrations of the 18th, that his main effort would be to turn the Confederate right by marching southward to Union Mills.

From Centreville, in the rear of which McDowell had established his headquarters, and around which he had massed his troops, seven public roads diverge to the principal points of the compass, and from each of these, at no great distance from that village, other roads diverge to intermediate points, until not less than a dozen roads lead from that village, crossing Bull run at nearly as many fords, making it an extremely difficult matter to watch the movements of an army there concentrated and having for its objective the southwestern side of Bull run. A circle with a radius of 3 miles from Centreville will pass through or near ten of these fords, from Mc-Lean's on the southeast to Poplar on the northwest. Bull run, in this interval of 6 miles of arc, nearly follows the three-mile circle drawn around Centreville. A circle with 7 miles of radius, drawn around the same center, crosses Bull run to the south of Centreville, near Union Mills and the bridge of the Orange & Alexandria railroad; about 9 1/2 miles away to the northwest it crosses the Sudley ford of Bull run; and from that ford, back toward the beginning, in a distance of 3 miles, it passes directly through the field of the 21st.