Question about preposition -of-

Sentence:

A set of deer antlers lying on a flat stone just inside [of?] one cave reveals more recent visitors.

Should we use preposition -of- in that case?

I have checked Corpus of Contemporary American English and have found examples of both usages:

• ...to plant another 20 acres of vines, finish building the inside of an on-site cave, and work on building a retail tasting room (Fiction)


• However, opened and remained blue as the inside of an ice cave, paler than the shell of a robin's egg (Fiction)


• It looked like the wall on the inside of a cave: flat and kind of scale (Fiction)


• ...-To protect the inside of the cave. " // He stepped inside, leaving the gate open. (Fiction)


• Views outside and inside of Wind Cave in South Dakota (Magazine)


• Above, Black Hills grasslands and, right, boxwork inside of the cave (Magazine)


• You can see Bill Elliott's great photos of what you can see inside a cave. (Spoken)


• So, when I am asked, what was your feeling inside the cave? (Spoken)


• At some point, we found a small river running through the inside the cave (Spoken)


• We were gathered around a small fire inside the cave (Fiction)



• About 700 feet inside El Sidrn cave, a research team including Lalueza-Fox excavated 1,700 bones (Academy)

I still cannot come up with a clear pattern why we are using -of- in some cases but are not using it in other. Is it more a question of style rather then a grammar? Or it has something with how speaker/writer sees things around and perceives a reality? Does absence of -of- make things sound weird/wrong for native speaker? Thanks.

Answer

There's some free variation here. But I think there are some patterns emerging.

The key difference is whether "inside" is a preposition or a noun. If "inside" is a noun, you will need "of". In the some of the first examples it is the physical material of the cave's interior that is being discussed: The colour of the ice for example.

Alternatively, "inside" can be a preposition. "We gathered around a small fire inside the cave" is clearly a preposition showing location.

Now some of these examples don't follow this pattern: "... boxwork inside of the cave" uses "inside of" as a preposition; the "of" could be omitted. "A small river running through inside of the cave" could be improved adding "the" before inside. The speaker may have started out intending to use "inside" as a preposition, then changed their mind, the result is a bit uneven.

There is obvious overlap between its use as a noun "the inside of a cave" and as a preposition "inside a cave" and this leads to overlap in the grammar, with "inside of" being used as a preposition. In written language I would try to keep the preposition and noun distinct, and avoid "inside of" as a preposition.

This rather mirrors the development of the word "inside", which originally was "in side" a preposition followed by a noun. Originally this was only a noun, meaning the interior parts of the body. Use as a preposition came later.