The linked discussion was about the use of natural=cliff drawn around rock cracks of various sizes which does not really represent the reality on the ground. There are many areas where natural rock surfaces show such cracks and I wonder why we do not have a simple tag for those, as we do have for gullies for instance. Large gullies can best be mapped with natural=earth_bank on both sides whereas for small gullies wie have natural=gully.
Large gorges can be mapped by using natural=gorge and addtionally natural=cliff on both sides (which is not a must anyhow). I would like to have a separate tag for small cracks which in reality no one would call a gorge. A few examples of such cracks, the biggest ones up to a max. of 10 m wide, 15 m deep and 50 m long, the smalles ones 0.5 m wide, 2 m deep and 1-2 m long:
I would still tag these as natural=gorge. Personally I would call the canyons but Tag:natural=gorge - OpenStreetMap Wiki is documented that natural=gorge includes both canyons and gorges.
The wiki says not to use it on an area, but I think that’s wrong, and it’s best to map it as an area, and therefore the size/width can be determined from the geometry.
The sides are bare rock (limestone) and they have been formed primarily by ground subsidence according to my knowledge. Of course erosion by water supports the process but that is the case with nearly every gorge es well. The cracks are located close to a a sheer limestone cliff and sooner or later the lower side will break away and tumble down into the valley where lots of scree has already accumulated. Would you call these cracks gullies? I’d say no but I am not absolutely sure.
Red line indicates the cliff edge, left side the cracks are visible. The major crack direction is parallel to the cliff edge and the lower sides (between crack and cliff) break away step by step.
I would not call those gorges nor canyons. A minor crack of a single or 2 meters width does not qualify as gorge imo, and again for other natural features we also have dedicated tags distinguishing between a large and a small object of the same kind.
It might be due to perspective, lighting and leaves in the foreground, but… isn’t that just the shadow cast by that boulder? There may be a depression in the ground, but it doesn’t look very noteworthy at all. Maybe 10cm wide, not even a metre long, and who know how deep.
As for your other examples, I think that the two estableshed alternatives, natural=gully and natural=gorge. After reading the wiki about gorge and gully, I would tag your other two examples as natural=gorge.
No, it is a crack of approx. 50 cm width and 100 cm lenght and it continues to the right for another meter or so which are just covered by twigs and foliage. The depth varies from 1 to 2.5 meters which is not that much but enough to fall into and break an ankle with some bad luck. There are more of these minor cracks in that area.
Maybe the best approach would be to draw a line around the whole area and tag it with hazard=holes?
No, these cracks are not caused by hollows in the underground, but by the movement of the top limestone layer, slowly sliding sidewards down the slope.
Mammi71
(One feature, Six mappers and still More ways to map it)
12
@Map_HeRo
I’ve seen cracks like that too, in Bad Urach and Dettingen (Baden-Württemberg), right on the Albtrauf escarpment (green line; to the right of it is the plateau, and to the left it drops down into the valley).
Mammi71
(One feature, Six mappers and still More ways to map it)
14
No, a karst aven has a different geological origin, as it is formed by water erosion. The cracks here are not caused by erosion, but because the harder overlying rock rests on softer rock and slowly slides down until it fractures at the surface.
In the given case these are fractures caused by tensile stress, but I did not find a dedicated geological term for them, although I am quite sure there is.
These cracks, crevices or fractures are definitely no gorges nor gullies and the sides (walls) of those cracks are not cliffs (which describes a different geological formation) but we all know that OSM likes to pamper it’s own terms and definitions instead of caring to use the correct ones …
Ok, as there are no further comments I’d like to propose a separate tag for these kind of cracks. What would fit better: natural=crevice or geological=fracture? Or even anything else?
I think I will use geological=fracture for those cracks which neutral in view of the crack origing (natural or man made). Again it allows to add other tags like natural=gorge if one wants to tag it as such.
