Just wondered what had caused this slug to leave such an unusual trail behind it or whether it is completely normal and just something I've never noticed before.
No identification made yet.
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But this is a fascinating post! If you change it (My Spot - Observations, click the Edit tab) to "Invertebrates", then you may find a better response (the Clever People tend to use the site filters to only look at what interests them most).
I would guess it's just looking for a crevice to hide away in, so it's following the crack and casting about to see if it can find one of a suitable size.
Lat/Lng: 53.5, -0.7
OS grid ref: SE8907