But then, I may have been overexposed to Hesiod.

Nope, it's all through Sophokles too—the Theban plays, at any rate. Land as grave as mother. Which makes Oedipus' whole thing about lying with his mother a lot mythologically weirder, if you ask me.

I don't think that κόλπος here is very much about wombs, but the usage is attested: to lie in the lap of the earth is to be dead, or unborn.