Learn English – Away they went over stock and stone – What is stock and stone

Try Oxford Dictionary, adjective, definition 3:

3 With no addition of anything else; mere:
'the very thought of drink made him feel sick'

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Just seeing the creatures that Joel 2 describes is enough to be frightening. This does take into consideration anything else, such as what they might sound like, or do, which is probably cause destruction since this is apocalyptic literature.

So the very/mere sight of them produces fear.

Just seeing them, and nothing else, is frightening or scary. That is what Joel 2:6 means in this English translation.

a snaking line

looks and moves like a snake