This is the most documented work of Caedmon: It was written in Old English, Transliterated into Latin by St Bede, and variously translated.
Tolkien knew his poets, and knew the resonance of Middle-Earth: heaven above to attain to, and Hades below. If you want to know where he got his ideas, learn Old English, Icelandic and Finnish.
Now let me praise the keeper of Heaven’s kingdom,
The might of the Creator, and his thought,
The work of the Father of glory, how each of wonders
The Eternal Lord established in the beginning.
He first created for the sons of men
Heaven as a roof, the holy Creator,
Then Middle-earth the keeper of mankind,
The Eternal Lord, afterwards made,
The earth for men, the Almighty Lord.Caedmon