Isa 57:5

57:5 children. Child sacrifice was practiced in the worship of the Ammonite god Molech (v. 9 note; cf. 2 Kin. 23:10; Ps. 106:37, 38; Jer. 7:31; Ezek. 20:28, 31).

valleys . . . clefts. These are secret, hiding places (2:21; 7:19 note). The implication is that their practices are shameful.