GR translation: crossover point

[Mike Jing]

P14.40-15.8   At last, one proper Sherlock Holmes London evening, the
unmistakable smell of gas came to Pirate from a dark street lamp, and
out of the fog ahead materialized a giant, organlike form. Carefully,
black-shod step by step, Pirate approached the thing. It began to
slide forward to meet him, over the cobblestones slow as a snail,
leaving behind some slime brightness of street-wake that could not
have been from fog. In the space between them was a crossover point,
which Pirate, being a bit faster, reached first. He reeled back, in
horror, back past the point—but such recognitions are not reversible.
It was a giant Adenoid.

What exactly is the "crossover point" here?