A few of the streams of the Pacific slopes rise in the Guatemalan Highlands, and force a way through the Sierra Madre at the bottom of deep ravines.

The Central American pine–oak forests ecoregion covers the Sierra's high peaks and northern slopes.

The direction of the great volcanic cones, which rise in an irregular line above it, is not identical with the main axis of the Sierra itself, except near the Mexican frontier, but has a more southerly trend, especially towards El Salvador.

It is, however, impossible to subdivide the Sierra Madre into a northern and a volcanic chain; for the volcanoes are isolated by stretches of comparatively low country; at least thirteen considerable streams flow down through them, from the main watershed to the sea.

Viewed from the coast, the volcanic cones seem to rise directly from the central heights of the Sierra Madre, above which they tower; but in reality their bases are, as a rule, farther south.