‘ And the Angel of God said to him, “Take the flesh, and the unleavened cakes, and lay them on this rock. and pour out the broth, and he did so.” '
“Pour out the broth”. This was an immediate indication that sacrifice was involved (compare Leviticus 17:13). The broth was probably poured over the whole.
Note the variation, ‘the angel of God '. The directions were given by ‘God', so that it might be emphasised that the sacrifice would be received by God under His covenant name, Yahweh. The change from ‘God' to Yahweh then emphasises the personal nature of the reception of the offering and is deliberate on the part of the writer.
Alternatively it has been suggested that the broth was poured into cup-like holes in the rock. Such were found in many places in rocks in Palestine and may have been part of earlier religious ritual. However the action in verse 21 suggests otherwise.