Cfrtjryk

Both in the dunes and on the dunes are acceptable.

Another option, possibly better:

Walking among the dunes

To me, "in" means you're walking amongst and between the dunes, and "on" means actually climbing up and down the piles of sand. If there's a formal footpath designed to avoid most of the up-and-down, I'd probably go with "in" (or "among" as suggested by Zanna's answer). If you want to emphasize the climbing aspect (or maybe damage done to vegetation by straying from designated paths or boardwalks), use "on".

Per Google Ngram, both version see some usage in print with "on" being somewhat more common in recent years, but the n is so low that I'm not sure that's a true trend (and the data from 2000-2008 show "in" as more common). Disaggregating the data and looking at actual examples suggests some evidence for the distinction as I understand it (though I don't think it's a hard-and-fast rule by any means):

Walking in the dunes is discouraged because paths created by walking can widen into large openings between dunes.

—Scott Williams, Corpus Christi, 2009

" . . . We will walk in the dunes, play some games and I will read you a book." . . . The bikes were parked and they started to walk in search of a cozy dip in the dunes that would give some protection from the wind. They soon found the most marvelous spot made just for them. It was a bit grassy as well as sandy right against a hillside—a little dell—large enough for the family.

—Johanna Van Belle-Praamsma, Coram Deo, 2009

They decide to go for a walk in the dunes, with Bernie, too, of course. Amelie learns that dunes are hills of sand, covered in grass and bushes. There are paths leading through them.

—Eva Markert, Amelie Travels to the Seaside, Stories for Little Ones, 2017

With no experience of the coast, I thought it would be possible to walk on the dunes, looking down on the sea and whatever lay behind that tantalizing tangle of wire.

—John Hillaby, A Walk Through Europe, 1977 (snippet view)

During the day, Mark would often watch the tourists (whom the natives called "comers" and "goers") climbing the dunes. Some even walked up during the searing heat of the afternoon. Gramps said, "Anyone walking on the dunes in the blazing noonday sun is crazy. Between the burning sands and the broiling sun, you might as well be turning on a rotisserie."

—Anne Turner Coppola, Ghost Dunes, 2014

You could alternatively say "roam the dunes" - so you do not need to care about "in" or "on"

There is another caveat to your question it seems. To walk in the dunes would imply that the dunes are a location to visit, as you would be in the location where there are sand dunes and walking around. To walk on the dunes implies that there is some geometry or feature that is being walked on. In this context the word dunes is used interchangeably as a location to visit and an object or objects to be walked on the surface of. The word is used to describe a place and objects in that same place. You could make it clear which of the two meanings you want to use ahead of time.

Trying to pick the “correct” preposition in English is really frustrating for those learning the language. While it is easy to see that “Worms live under the ground” is correct while “Worms live above the ground” is incorrect, it is not so clear with the above “dunes” example. There are many examples of multiple prepositions being acceptable (correct) for some phrase and many examples where the acceptable (correct) preposition depends on the version of English being spoken as in “in back of the car” (US English) and “behind the car” (British English). Native speakers will generally be able to give good reasons why the prepositions that they use are correct while others are incorrect. So which preposition is correct? Let me answer it this way: if you were going to live in England, it would be preferable to learn English from a native UK English speaker rather than an American. Similarly, the “correct” dune preposition depends on where you are and with whom you are speaking. Even so, a native English speaker (Australian, British, ...., Zimbabwean) will have no trouble understanding you. Sorry about the long answer but this is one of those situations where trying to find the correct answer when there in no single correct answer is counterproductive.

both correct for sure.depending on the situation,in the case you mean it was walking for fun or leisure so you describe the totality of circumstances,and choose to use "in".But on the other hand,talking about the details of the route it was passing in it's way,"on"will fit best.

The word "on" means "on top of" (in this context), the word "in" means "within", or "inside". The use of "on" and "in" refer to the subject, or subjects, from a different frame of reference.

Derfi, you can:

• Walk in the mountains, on a flat road or on a mountain. [Walk within the area where there are mountains, possibly on the road and not on the mountains]. Similarly you can walk in or on the road, "on the road" means "on top" and "in the road" means "far (enough) from the edge".




• Walk on the mountain, do it twice and you are walking on the mountains, literally. It's a confusing and uncommon usage, even if it is accurate.




• Walk on the mountains, in the mountains. [Walk on mountains, within a mountainous area].

In order to "walk in a mountain" there needs to be a cave or tunnel, if you do it twice you are walking in the mountains. Accurate and confusing, without knowing the context.

Similarly for sand.

Walk near the edge of a sandy area and you are walking "on the sand", travel into the sandy area and you are walking "in the sand" (and "on the sand").

Because it's possible to sink into sand that is more than a few grains thick when you walk "on" sand you walk "in" sand. Again, this relies upon traveling a particular distance. Walking "on" sand near a pile of sand on concrete is not "walking in the sand". Stepping into the pile of sand, on concrete, and thus sinking into it is "walking in the sand" even if the pile is a couple of feet in diameter.

According to the Cambridge Dictionary grammar usage: Use "in" to generalize, and "on" when we describe the part. - "In the dunes, on the tallest one, at halfway to the top".

According to "Learning English - Everyday Grammar": Use "in" for the largest or most general places, use "on" for more specific places, but "at" for an exact location.

The Google Books NGram Viewer shows: "in" the mountains, but "on" the mountain (singular). See the question at ell.SE for "In vs. On".

See Merriam-Webster "Dichotomy and false dichotomy or polychotomies" to understand whether there is a division of something, to know which applies.

Lastly, there is our answer: When should I use "in" or "on"?.