Riley's command was the first to pass the _pedregal_, when it occupied the road on the opposite side with Captain Simon Henry Drum's company of the Fourth Artillery. A detachment of Mexican lancers escorting a train was encountered and captured. Riley's command continued its advance, when a company of Mexican lancers was met and repulsed by Captain Silas Casey's company. A mounted force, under the Mexican General Frontera, consisting of two regiments, was met and repulsed by the Second Infantry under Captain Charles T. Morris and the Seventh Infantry under Lieutenant-Colonel Joseph Plympton. General Frontera was killed while leading a charge. Riley now withdrew to San Geronimo, which he found occupied by Cadwallader's and Smith's brigades, and a regiment of Pierce's brigade under command of Colonel George Washington Morgan. When General Valencia's advanced forces were driven in by Twiggs's division on the _pedregal_, Valencia announced (August 19th, 2 P.M.) to General Santa Anna at San Antonio that the enemy were approaching Padierna, the artillery had opened fire, and the battle had begun. General Santa Anna at once, on receipt of this information, sent an officer to Coyoacan with orders to General Perez to move at once to Padierna, and himself with two regiments and five pieces of artillery proceeded to join him. He arrived at Coyoacan just at the time when the command of Perez was moving, and he ordered it to move rapidly. On the evening of August 19th General P.F. Smith was in San Geronimo with three brigades of infantry, but without cavalry or artillery. His communications with the main army were cut off except through the _pedregal_. He determined to attack, however, the next morning at daylight, carry the enemy's works, and establish his communications with the main army. His disposition of troops was as follows for the night: Cadwallader's command in the outer edge of the village of San Geronimo, Riley's brigade parallel to it, the Rifles on the right, and the Third Infantry in the churchyard. In the night Captain R.E. Lee arrived, bearing a letter from General Scott asking to be informed of affairs beyond the _pedregal_. The information sought for was given, and Captain Lee was requested to inform General Scott of General Smith's intention to attack Valencia next morning, and asking that a diversion be made on Valencia's front. General Shields arrived at midnight, and was left to hold the village and cut off the enemy's retreat. In the meantime Colonel Ransom abandoned Padierna, which was soon afterward occupied by General Valencia's forces, but not without stout resistance by the small detachment left there.