The impression of depth in The Lord of the Rings is an aesthetic effect deliberately sought by its author, J. R. R. Tolkien. It was intended to give the reader the feeling that the work had "deep roots in the past", and hence that it was attractively authentic.
The effect was constructed on at least four factors, namely the enormous scale of The Lord of the Rings and the amount of background detail, including maps and genealogies; the apparently casual and incomplete mentions of this background; multiple inconsistent accounts, as in real history; and writing different texts in varying styles.
Scholars have noted some of Tolkien's medieval antecedents in the effect, such as Beowulf and Sir Gawain and the Green Knight. Fantasy authors such as Ursula K. Le Guin and J. K. Rowling have to an extent followed Tolkien in using the technique.
Effect
In an essay, Tolkien praised the 14th-century English chivalric romance Sir Gawain and the Green Knight for its "deep roots in the past, deeper even than its author was aware". In his opinion, this enabled it to survive even the severe test of being a set text for students; it deserved "close and detailed attention, and after that ... careful consideration, and re-consideration". In an aside, he went on to discuss what that meant:
In a letter, Tolkien provided at least part of his own view of the impression of depth in The Lord of the Rings, namely that
Tolkien noted further that this effect would be difficult to attain in the legendarium that lay behind The Lord of the Rings, "unless new unattainable vistas are again revealed". He added that "many of the older legends are purely 'mythological', and nearly all are grim and tragic".
The scholar of English literature Katarzyna Ferdynus comments that
Tom Shippey, a Tolkien scholar, writes that depth is "the one literary quality, to say no more, which most certainly distinguishes Tolkien from his many imitators" in fantasy, and that behind the visible text lay "a coherent, consistent, deeply fascinating world about which he had no time [then] to speak".
Precedents
This quality of depth, that Tolkien valued highly, he found especially in Beowulf, but also in other works that he admired, such as Virgil's Aeneid, Shakespeare's Macbeth, Sir Orfeo, and Grimms' Fairy Tales. Scholars such as Gergely Nagy have identified other texts well known to Tolkien that provide a strong impression of depth, including Thomas Malory's Le Morte d'Arthur and Geoffrey Chaucer's Troilus and Criseyde. Beowulf contains numerous digressions into other stories which have functions other than advancing the story, in Adrien Bonjour's words rendering "the background of the poem extraordinarily alive", and providing contrasts and examples that repeatedly illuminate the key points of the main story. Tolkien stated in The Monsters and the Critics that Beowulf
Factors
Scholars have identified four factors that help to build the impression of depth:
the enormous scale and detail of Tolkien's Middle-earth background
the apparently casual and incomplete mentions of this background in the text, as if it were something taken for granted the multiple accounts, containing gaps and inconsistencies, as if it were a real body of historical documents the multiple styles in which the various texts are written, as if by different hands.
Vast backcloths
Tolkien alluded to the first factor with the phrase "vast backcloths":
The Tolkien scholar Michael Drout, with colleagues, notes that the vastness was not an exaggeration, given that it encompassed The Silmarillion and much of the multi-volume legendarium edited by Tolkien's son Christopher, not to mention the many "further drafts, partially edited copies, riders, cancelled pages, and even lost texts" behind even those lengthy works. Shippey adds that Tolkien's maps, too, lend an "air of solidity and extent" to the work, providing "repeated implicit assurances of the existence of the things they label, and of course of their nature and history too".