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 Isa 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, etc. (2Ki 17:31; 2Ch 28:3; 2Ch 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; Jer 7:3).
under ... clifts - the gloom of caverns suiting their dark superstitions.