General Valencia, however, ordered a thorough reconnoissance by General Mendoza, an engineer officer, who reported “that Padierna was absolutely indefensible, and that it was believed best to retire for reasons expressed in his note.”  General Valencia ordered Colonel Barreiro to Zacatepetl to watch and report the movements of the enemy.  He further ordered Colonel Mendoza to occupy with his regiment the edge of the pedregal, having in his front a detachment of infantry under Captain Solos, and beyond him a detachment of cavalry.  To the left of Padierna was posted the corps of San Luis Potosi, to the right the brigade of Lieutenant-Colonel Cabrera, and on the ridge were the batteries and brigade of General Mejia.  The supporting line were three battalions.  The reserve at Anzaldo, a mixed company of infantry and cavalry, was the command of General Solos, supported on the right by two regiments of infantry.

Pillow’s and Twiggs’s divisions were observed by Colonel Barreiro to be moving over the mountain of Zacatepetl and the pedregal.  On an open ridge commanding the pedregal General Valencia had planted guns which commanded the pedregal in the direction of San Augustin.  On the morning of August 19th General Santa Anna ordered two battalions to move from Churubusco to San Antonio, Pillow’s division of the American army having moved out from San Augustin on the road to Padierna, which was to be covered by Twiggs’s division.  Twiggs moved, following Quitman, and passed beyond San Augustin.  General Alvarez closed on his rear.  A working party of five hundred men under engineer officers was detailed from Pillow’s division to make the road to Padierna practicable for artillery.  While work was progressing on this road General Scott notified General Pillow that Valencia was placing heavy guns in position, and ordered that the work be pushed forward as rapidly as possible.  Before the road was finished half the distance Twiggs’s division passed Pillow’s command, and its advance was fired upon by the Mexicans.  General Persifor F. Smith ordered the mounted rifle regiment under Major William Wing Loring, aided by a section of Magruder’s battery, to drive in the Mexican pickets.  Lieutenant George B. McClellan placed the artillery in position, but before it was ready for action it received a fire from the guns on the elevated ridge beyond Padierna.  The remainder of Smith’s brigade and the other section of Lieutenant John Bankhead Magruder’s battery were ordered forward, and the Mexicans were driven back.  General Bennet Riley’s brigade was ordered to the right, and to pass over the pedregal and take possession in the enemy’s rear.  General Cadwallader’s brigade was ordered to support Riley’s movement.  General Scott, perceiving that re-enforcements were approaching Valencia from the City of Mexico, ordered a regiment of General Franklin Pierce’s brigade to move forward and occupy San Geronimo, and General James Shields with