Adjective meaning very slow-moving

I usually use "glacial" to denote something as very slow-moving, but this time I am actually talking about a glacier so it can't be "the glacial glacier".

These are the synonyms of "glacial" (They all mean cold)

These are for "slow-moving", but none seem to actually imply the persistent slow movement of a glacier.

I have encountered creeping in the context of glaciers before.

creeping

advancing or developing gradually.

moving very slowly at a steady pace.

Another suitable adjective is sluggish.

You should have stated what register you were writing in. Journalistic (columnistic or newsy?), literary, academic etc. Helpful would've been a specification whether the adjective within your context should denote exclusively the rate of movement, or it can be multihued. Depending on how much you're allowed or inclined to wax poetic, you might decide to chose from among one of these (they're all "googlable"):

crawling

• These ever changing, ever moving crawling glaciers are very different from the high glacier plateau which they decend from.
• These had accumulated there over the last few millions of years deposited by the crawling bellies of glaciers long since gone.

torpid

• The arctics down whose voiceless valleys the torpid glaciers creep, the parched deserts of the tropics,...

slothful

• This side glacier, which comes in from the south, slid down its valley out over Sherman Glacier, and ended up almost at the terminal of Sherman, a slothful receding glacier.
• To the naked eye, glacial activity looks like anything but action. It may put up an appearance of functioning as a stagnant blockage of the water cycle; a stunt of the grunt, if you will, eternally stuck in a rut rather than accomplishing much for its mother to be proud of. In fact, it may be tempting to call a glacier slothful, reclusive, just a big hunk (unfortunately not the kind of hunks we've been looking for), and taking up good potential driving passage. We don't mind our Icefield.

languid

• down below in the land that forgot time we are eking forwards like a slow languid glacier
• Overhead, the yellow sun and the green sun circled each other with a languid incessant inevitability

scant movement

• Together with these deposits, and generally at a somewhat lower altitude, there exist deposits that are clearly morainic, either lateral moraines or rock glaciers, with very little fine fraction, due to the scant movement of the glaciers.
• The luscious green trees were scantily moving to the soft breeze, birds were chirping, it was an amazing scenery.

(variations on) slow

• Attempts have been made to explain these wide-ranging deposits as being laid down by a slowly meandering glacier.
• Humboldt, a very wide but slow-moving and slow-changing glacier, lies just to the west of Petermann Glacier
• It's a slow, grindingly slow, tortoise and the hare slow, glacier slide slow, birth of a star slow type of song.

May I submit relentless which dodges the question slightly; but, in my defence, I feel glaciers are not known for their startling pace, quite yet, anyway.

Closer to your request is restless to suggest movement, unrelieved.

Perhaps snail-like?

resembling a snail, esp in moving very slowly

Also consider inexorable

not able to be stopped or changed

while it does not, in itself, convey slow, it could be coupled with crawl, which does.

Inexorable crawl sounds like a glacier to me.

Relentless

Only by this word do we begin to approach the true being and essence of the glacier.

Perception of Time

Human perception grasps relations between things before a thing itself: to know what a thing is, we must first ask what it does. This question, put to a glacier, initially leaves us speechless.

A glacier does – a glacier is – what? Ice – yes, but this gets us no further. The attributive word slow hoves into view, on schedule, whenever we think of glaciers, but this is quite wrong: in a very important, originary sense, slow can never capture what a thing does, or what a thing is, essentially.

Slowness, it is not

Slow, when we consult its origin in the language, wants to say that its object somehow deviates from its true being or purpose. Its primary meaning relates to the defective working of a mind, as in someone who’s not up to proper mental speed. This defection, when pursued hard enough, overtakes the initial – instrumental or pragmatic – remissness to become a moral fault, as sloth, a later derivation from slow. But the notion of speed itself is deceptive: actually it’s foreign to the origin of this word.

From

• the Old English slaw (inactive, sluggish, dull) and
• its roots in the Old Saxon sleu and
• Old High German sleo (both meaning blunt, dull)

we have the primordial and real image of a metal utensil, whether knife or plough or, more ceremoniously, a sword, that has fallen from its proper use by the corrosion of a cutting edge.

Speed, or lack of speed, dilatoriness, has therefore little to do with the word slow, unless it lingers behind the frustration of having to expend more time in cutting, striking, ploughing etc with the duller edge to achieve the utensil’s desired end.

Essence & Nature

So we must ask again: what does a glacier do? Certainly, it is ice. But what does ice do? Ice, unlike its other physical cognates – rain, snow – is not accompanied by a ready collection of doing-verbs. Rain falls and saturates the parched earth with its gift; snow falls and collects beautifully in drifts (“Covering earth in forgetful snow” – TS Eliot), the prime image of the winter eclipse of the land’s abundance; water runs and flows and streams and spills, everywhere giving life.

These are vital articulations of the earth’s lexicon of seasonal and shaping forces. But the most we can say of ice in this respect is that it melts. Curiously, slow recurs here – but indirectly, in its secondary, derived sense as a lack of speed – to lend its pallor to the melting process.

But melt itself is another occlusion: a word that is foreign to the being of ice. In its Danish, Old High German and Swedish origins, melting was a process undergone by bodies once they had exceeded their point of ripeness or ideal period of use or consumption: in the case of inanimate organic matter (pears, apples, harvested barley) this meant rotting; in the case of people, this meant corporeal decay and dissolution.

In melt we take the short step over the life-threshold to moult and then mould – where the evidence of decay and uselessness becomes manifest. So, despite appearances, melt is an odd fit as the essential doing-verb for ice, because ice would have had no real agency or use for men and women personally, or at a societal level, to fall away or decay from (it was only among Mediterranean cultures that ice was used as a food-preservative as in Northern Europe and Scandinavia the colder climate did the job: the Romans used to cart oysters from Colchester in Essex to Rome packed in ice and wrapped in parcels of hay). So ice, viewed in its larger forms – as a glacier for example – requires a verb of greater scope, unattached to localised human processes and activities.

Relentless origins

To this end, relentless begins to help. To relent is, effectively, to melt, to subside to the condition of slow, viscous or supple, contained in the Latin lentus. To refuse to do this, to willfully preserve this hard, inhuman solidity (relent was originally used in the context of the softening of a human heart: “The notion is probably of a hard heart melting with pity.”)

Such gets translated spatially into the idea of movement, of determined progress at all costs, becomes a kind of non-human automatic grind, nicely approaching what a glacier in fact does and is. But relent is still located in a human scale: and the reason it begins to resonate with glacier is the converse suggestion – of something nonhuman, larger-then-human, something almost godlike, in the familiar sense that all natural forces – sun, moon, stars, seasons, rainfall – were godlike, as in bathed with the divine aura and agency of deities, in earlier moments of historical-cultural development.

Godlike Relentlessness

To move further on this: glaciers are indeed godlike, in the vital sense that they create: their true doing, what they do, which relentless hints at while being merely an image at the crust of the real thing, is shape the land. In its origin (Old Saxon, Old German, Old Frisian, via Middle English) to shape was to create and fashion, and, mysteriously and beautifully, to draw water from the source.

This is what glaciers do: like gods, they command the elements in the service of real creation; they draw water from its source, the sky, and bodying this colossal force, they redraw the earth beneath us, bequeathing their legacy, centuries later, in the valleys and terrains we call our home. The reason then, that we have no name for what glaciers do and are, is perhaps because we have forgotten the awe due to them as creative titans possessed of godlike powers. We readily say what a hawk does: it hunts and catches food to feed its young; like us, it builds its nest and nurtures the coming generations. But glaciers are beyond this human scale, and our forgetfulness of what they do, and how they are, perhaps exhibits the curtailing and shrinking latent in our anthropocentric, anthropomorphising cognitive tendencies.

This is a very roundabout way of giving an answer to your question but perhaps it suggests a new path towards truer descriptions.

Glaciers are landforms, yes: but they are also landforming, landshaping, and relentless in their being of these things.