Jamieson, Fausset and Brown on Isaiah 57:5

5. Enflaming yourselves--burning with lust towards idols [Gesenius]; or else (compare Margin), in the terebinth groves, which the Hebrew and the parallelism favor (see on Is 1:29) [Maurer].

under ... tree--(2Ki 17:10). The tree, as in the Assyrian sculptures, was probably made an idolatrous symbol of the heavenly hosts.

slaying ... children--as a sacrifice to Molech, &c. (2Ki 17:31; 2Ch 28:3; 33:6).

in ... valleys--the valley of the son of Hinnom. Fire was put within a hollow brazen statue, and the child was put in his heated arms; kettle drums (Hebrew, toph) were beaten to drown the child's cries; whence the valley was called Tophet (2Ch 33:6; Jr 7:3).

under ... clifts--the gloom of caverns suiting their dark superstitions.