key:surface: stones, boulders, and rocky surface

I'd say rocky is appropriate for the 3rd and perhaps the 2nd example. However it's not for the first. Maybe I can propose a new value?

As for mtb:scale I think it describes different features of the pah, not necessarily related to its surface.

I don't think that it's necessarily helpful to "propose" a new value - I'd have a look at what other people are using for the tag, and if nothing fits, pick something better. Although the any tags you like page says "please document" in this case I suspect that added lots more entries to key:surface isn't going to help as that already has "User Defined" and a link to tagwatch at the bottom.

The problem is I'm not a native speaker and so I'm unsure about the right terms.

I see that fragments of rock are classified, according to size, in the following categories:

• Boulder: a detached rock mass, somewhat rounded or otherwise modified by abrasion in transport and larger than a cobble with a minimum size of 256 mm;
• Cobble: a rock fragment larger than a pebble and smaller than a boulder, rounded by the action of water wind or glacial ice. It is between 64 and 256 mm;
• Pebble: a rock fragment larger than a sand grain or granule and smaller than a cobble, which has been rounded by the action of water wind or glacial ice. It is between 4 mm and 64 mm.

But they all have in common a rounded shape caused by erosion, while in the mountains you may well have rock fragments which are much younger, and so not rounded at all!

Do you think these terms could determine confusion? See for instance my original first picture--would you call those fragments "cobbles"?

Are there more generic terms, not necessarily related to fragments modified by abrasion?

Otherwise, values like "pebbles", "cobbles", and "boulders" (in plural) would be ok for key "surface"?

Thanks again!

I guess that you're talking about something like this scale? The "Wentworth Scale" words are borrowed from common English words, but when used in a geological context refer to particular sizes only. In everyday English, "Boulder" might be interpreted as something larger than the Wentworth Scale value. Also, "cobble" is most frequently used in English to describe stones of about that size used to surface roads, and might be confusing if used to describe a rock-strewn path.

After looking at the taginfo values I'd probably use "mud", "sand", "gravel" and "rocky" in order of grain size. Historically (before looking at taginfo) I've used "rocks" for "larger than gravel" but taginfo definitely suggests "rocky" as the more popular options. I wouldn't try and subdivide surface grain size too much as people won't necessarily understand what you're trying to say. You can of course still used other words like maybe "boulders" if you think that's the best way of getting across what you need to.

In answer to the second part of your question, although the phi scale / Wentworth Class are often used to describe the size of grains carried by rivers etc., I don't think that they exclusively refer to stuff modified by abrasion (from what I can remember from geology classes at college a VERY long time ago, anyway).

Personnally, I wouldn't use "cobbles" because that's often used to describe a road surface (e.g. here)