I can't answer your question about irregular stones moving. However, I would like to caution that you might get moss on the bluestone itself if it's in the shade and damp. I removed such a walkway after I slipped on it a couple of times.

thanks for the feedback ... i'll ask the stone mason about this ...
how shady and damp was your area? no sun at all?
the area I am considering faces north and there is tree there so for the most part it's shaing, but a little sun does find it's way there as the sun moves east to west.

thanks for the advice mich ....
we are getting close to a decision here ... we
are now considering either wet laid irregular bluestone
or wet laid brick pavers (although my fear here is the clay
brick possibly cracking). We like the look of brick pavers dry laid, but are fearful of the the maintenance (weeds) and possible movement, since this is a front walkway which will be used quite often. On the other hand we have heard that blustone can be slippery when wet? Any thoughts?

My walkway was on the north northwest side of the house - the portion that got no sun at any time was really bad. And of course, during the winter it was doubly bad.

njtea ... what material was the walkway ... really bad meaning slippery?

Gmilich, when I first moved into the place, the walkway was brick. I removed that because of the moss problems. Then I laid down bluestones. They were on the property in a big pilewhen I bought the place. I found the bluestone too got mossy and slippery where it never got any sun. I do believe, however, that even just a small bit of sun helped keep the moss off, unlike the bricks which seem to grow moss even when they get some sun, probably because of the irregular surface where moisture can hide.

Also because the bluestone was just laid down on top of whatever (I don't recall what the base was) it was difficult to shovel and I had problems with "black ice" where it got no sun - I think that might have been caused by the smooth surface of the stone.

So I pulled that up and now just have a mulch walkway. And of course I still have snow/ice problems where there is no sun, but at least the moss is no longer an issue.

Does any of this help?

Not sure what you mean by dry and wet. Does dry mean with concrete and wet with stone dust? If so I would recommend using concrete. We have an irregular bluestone pathway in our front yard (concrete) and an irregular bluestone patio (stone dust). My husband is constantly out on the patio spraying Roundup into the cracks between the stones. I think it would be a real headache for a front walk.Our Bluestone pathway is never slippery from the rain. (Can't advise on the moss issue because our path faces west!). Good luck

The advantage of wet laid (concrete base and concrete joints between stones) is that it doesn't shift if the pitch and drainage are right. A wet laid walk can heave and crack if with winter freeze/thaw/freeze cycles--but not always. Dry laid is generally set on a compacted QP (crushed quarry stone) base w/stone dust joints. It can shift a bit, and the stone dust can be a pain until it settles. It really depends on your site. Pavers are dry laid. Bricks can be either. Moss will be an issue if the area doesn't get any sun. High, open shade on the north side of a house should be fine since it's bright, but anything more dense than that will grow moss.

Gmilich, there's an article in today's garden Q&A in the NY Times that addresses this subject. Says the slipperiness is caused by algae, not moss, and the algae forms due to insufficient drainage.

You can find the article using the link below - registration required but it's free.

Here is a link that might be useful: Slippery walkways

Installing dry (with stone dust) is the way to go. I've installed plenty of them. Now, the ones that we have installed is with the oversized bluestone (2' x 3'). With 2-3" of stone dust on top of 3" quarry mix, we've rarely seen issues with weeds.

I don't recommend concrete as water will invariably seep into the areas between the slate and the concrete and it will loosen and begin to break up down the line.

The beauty of a dry job is that it can be readily pulled apart (if needed), yet if installed properly, will last for years (well beyond a wet job). Just my $.02

We had a dry laid brick paver walk professionally installed in the front of our house which faces northeast and is shaded by a tree for most of the day. It is never slippery unless, of course, there is snow or ice on it. We had the walk installed several years ago, and nothing has shifted. We do get a bit of moss growing between the pavers, but it is minimal and not slippery at all. In fact, we like the look of the moss and encourage it. Perhaps it depends on the type of paver you use which determines whether or not it will be slippery. Also, with dry laid, water can drain down between the pavers and we find that they dry relatively quickly, even in shade.