Tlalpam, on the road to Churubusco and the City of Mexico. The ground
on which San Antonio stands is completely in the valley, and the surface
of the land is only a little above the level of the lakes, and, except
to the south-west, it was cut up by deep ditches filled with water. To
the south-west is the Pedregal--the volcanic rock before spoken of--over
which cavalry or artillery could not be passed, and infantry would make
but poor progress if confronted by an enemy. From the position occupied
by Garland's brigade, therefore, no movement could be made against the
defences of San Antonio except to the front, and by a narrow causeway,
over perfectly level ground, every inch of which was commanded by the
enemy's artillery and infantry. If Contreras, some three miles west and
south, should fall into our hands, troops from there could move to the
right flank of all the positions held by the enemy between us and the
city. Under these circumstances General Scott directed the holding of
the front of the enemy without making an attack until further orders.

On the 18th of August, the day of reaching San Augustin Tlalpam,
Garland's brigade secured a position within easy range of the advanced
intrenchments of San Antonio, but where his troops were protected by an
artificial embankment that had been thrown up for some other purpose
than defense. General Scott at once set his engineers reconnoitring the
works about Contreras, and on the 19th movements were commenced to get
troops into positions from which an assault could be made upon the force
occupying that place. The Pedregal on the north and north-east, and the
mountain on the south, made the passage by either flank of the enemy's
defences difficult, for their work stood exactly between those natural
bulwarks; but a road was completed during the day and night of the 19th,
and troops were got to the north and west of the enemy.

This affair, like that of Cerro Gordo, was an engagement in which the