As I understand it, expanded shale has a lot of air space in it like vermiculite. It would certainly help to lighten a clay soil.

Decomposed granite is just small pieces of granite, which is a very dense and solid rock. No void space in those particles. It won't do much to fluff your clay.

I live with clay (tan with some gray) here in central MO and I feel your pain. Take your eyes (and compost fork) off it for just a bit and it's clay again. There's hope, though, after enough years of compost additions. And if you can get any SILT or dare I say, sand (watch out, it can make concrete with clay), this will also help change the particle size distribution.

And keep up the compost.

Shale is composed of more than 50% silt- and clay-sized material. After traveling long enough to be mechanically weathered to that size range, much of the non-quartz portion of the original rock or rocks from which the sediments formed has been chemically weathered into clay minerals.

I don't know how friable expanded shale is, or what particular clay minerals are in what you are using, but perhaps it is the source of the rising levels of clay?

I could stop or taper-off mixing new amendments (mainly the e.shale) with existing, yet previously amended native soil, focus my labor/efforts into raising the height of the existing beds by a few inches (I enjoy the labor/mess/process), and proceed with layering organic material in my future efforts.

Good? Bad?

I really appreciate having a few different angles by which to look at this! I am a certified TX MG. HOWEVER - I do not possess a ag/hort ego at all and have no place for it in my practices; applying book learning and theory does not always result in successful efforts, so I continuously seek to learn from others.

weirdflowers has an interesting theory, and I confess to never having used expanded shale, so I have no idea what happens to it as it weathers. Entirely possible for it to turn to clay as far as I know.

So how would you raise the height of the beds? Unless your OM content is very low - which I doubt since you've been adding compost all these years - you'll have to have some kind of mineral based soil to substitute for the shale.

I have been to places where this shale is and have not seen anything growing in that shale unless there is an accumulation of organic matter there. Expanded shale, decomposed granite, or anything similar is no substitute for organic matter in your soil, especially since that stuff does not feed the Soil Food Web that will feed your plants.
Studies done by Texas A & M found that unless organic matter was added to the soil to which expanded shale was added plants did not grow any better then in other soils not amended with either. Plants growing in soils well amended with organic matter outgrew those in unamended soils or soils amended only with the shale.

Fine, but that's not the issue. TN is not trying to substitute expanded shale for organic matter, and has been adding compost all along. Also trying to change the mineral texture and is asking whether decomposed granite will do the same thing as expanded shale. That morphed into a discussion of whether the shale was decomposing into more clay, which defeats the purpose.

Raised beds are not going to stay raised adding nothing but compost, which will continue to sink as it decomposes. It will provide some permanent fluff to a heavy soil, but you can't fill a raised bed with nothing but compost. It's fine to advocate for organic matter, but it's off topic here.

What I meant by my question about how you would raise the beds was, what can be used instead of the shale to add permanent volume to the soil. I'm thinking a silty soil mixed with compost.

Thank you, ToxCrusadr! Your reminder of the fact that DG are solid, unchanged particles rather than the porous end product of expanded shale gave me my "DUH/I'm a moron" moment.

Regarding expanded shale research: I enjoyed a brief (grant-sponsored, part-time) employment at the Tex A&M Urban Solutions Center in Dallas a few recent years ago. I was very fortunate to spend a brief time involved in soil sciences. However, my experience with this discipline was limited to soil fertility. Having said that, I feel that some of their non-fertility amendment based research (corporation funded grants) and subsequent results are often an effort in ridiculous predictability. My own extreme and annoyingly sarcastic example would be "would an ice cube retain its crystalline structure in an oven or a freezer". Therefore, I hope that the general gardener would not decide to attempt a successful garden using expanded shale alone.

My understanding of the affects of various amendments to soil structure is limited to classroom lecture, persistent questions and personal experimentation. I am truly grateful for the chance to get advice from many varied angles.

I was wondering if there are other resources out there about this product and whether it breaks up into clay. I googled 'Expanded Shale Breakdown' and the first page of hits was pretty tantalizing. I'll let you do the reading and see what you find out.