The City of Mexico lies west of Rio Frio mountain, on a plain backed by
another mountain six miles farther west, with others still nearer on the
north and south. Between the western base of Rio Frio and the City of
Mexico there are three lakes, Chalco and Xochimilco on the left and
Texcoco on the right, extending to the east end of the City of Mexico.
Chalco and Texcoco are divided by a narrow strip of land over which the
direct road to the city runs. Xochimilco is also to the left of the
road, but at a considerable distance south of it, and is connected with
Lake Chalco by a narrow channel. There is a high rocky mound, called El
Penon, on the right of the road, springing up from the low flat ground
dividing the lakes. This mound was strengthened by intrenchments at its
base and summit, and rendered a direct attack impracticable.

Scott's army was rapidly concentrated about Ayotla and other points near
the eastern end of Lake Chalco. Reconnoissances were made up to within
gun-shot of El Penon, while engineers were seeking a route by the south
side of Lake Chalco to flank the city, and come upon it from the south
and south-west. A way was found around the lake, and by the 18th of
August troops were in St. Augustin Tlalpam, a town about eleven miles
due south from the plaza of the capital. Between St. Augustin Tlalpam
and the city lie the hacienda of San Antonio and the village of
Churubusco, and south-west of them is Contreras. All these points,
except St. Augustin Tlalpam, were intrenched and strongly garrisoned.
Contreras is situated on the side of a mountain, near its base, where
volcanic rocks are piled in great confusion, reaching nearly to San
Antonio. This made the approach to the city from the south very
difficult.

The brigade to which I was attached--Garland's, of Worth's division--was
sent to confront San Antonio, two or three miles from St. Augustin