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 two regiments (New York, and Palmetto, South Carolina) was ordered forward as a support. General Persifor F. Smith now moved to the front across the _pedregal_, having left detachments as supports to the artillery of Magruder and Callender, which were ordered to open fire on the beginning of General Smith's movement. This movement of General Persifor F. Smith was led and conducted by Lieutenant Gustavus W. Smith. When this force reached the village or town of San Geronimo a large force of the enemy came in sight. Pierce's brigade was at once ordered to the front, and was met by a heavy fire. General Pierce having been disabled, Colonel Robert Ransom, of the Ninth Infantry, was in command of the forces, which were conducted by Lieutenant Isaac Ingles Stevens, and moved to the right and front of Magruder's battery. Ransom, uniting with the detachment left by General Smith, took possession of Padierna, driving the Mexican General Mendoza.