Glacial Till Rock Garden

[Gerdk]

Thanks for information Cohan, found your answer only now!

Gerd

[Lori S.]

Just a point on geological terminology... "Glacial till" does not make a good substrate for alpine gardening (or for any other purpose that relies on permeability such as for drilling water wells), since it refers to a  completely unsorted and unstratified mix of (primarily) clay with boulders and sand.  If anyone recalls the previous discussion on permeability ("sharp" sand versus rounded and well-sorted sand), till has very low perm because the fine material fills the pore spaces between coarser particles (i.e. the sand grains and the boulders).
Anyway, a more suitable term for what you are planning to create from glacially-transported boulders would likely be a "glacial moraine".  

[cohan]

thanks for the clarification lori;
i wasnt so much thinking of till as the final product, but more the raw materials, though i will admit i didnt find really great working definitions...lol--most of the references i found online were snippets of books which i would probably need to actually read to get a better grasp of the terminology and some of the final processes involved in local soil/substrate creation

clearly the basis of the landscape here is glacial, though apart from gravel deposits (none on my place, unfortunately) i'm not clear on where the line is between what was dropped off, scoured away, and what was here before the glaciers-the point i forget about most--that there could have been soil here before glaciation which remained after? was it all removed and dumped back?

anyway, yes, i was thinking that moraines are closest to what i was thinking of as a finished product
would you say that till is what plants are growing in in places such as at the foot of glaciers? those that i saw near columbia icefield were all in more or less soggy soil, i think, not sure if they ever get dried out in mid summer--i was there in july, i rather doubt at that altitude there is much time between melt and new snow, and as you say, what 'soil' there is among the stones is clay, so not much drainage other than on slopes or pockets that are mainly stone (such as the moraines a bit farther back, which i think were drier than those nearer the glacier which probably still have seepage)..

[Roma]

Cohan, thought you might be interested in these pics of the rock garden at Tilliepronie House in Aberdeenshire.  The rocks are massive and would have needed heavy machinery to move.

[Lori S.]

clearly the basis of the landscape here is glacial, though apart from gravel deposits (none on my place, unfortunately) i'm not clear on where the line is between what was dropped off, scoured away, and what was here before the glaciers-the point i forget about most--that there could have been soil here before glaciation which remained after? was it all removed and dumped back?

Bedrock that had been exposed on surface long enough to be broken down into soil would certainly have been the easiest to be bulldozed off by moving ice, so chances are that much of it was removed, but I couldn't guarantee that.   On the other hand, in all the wells I've drilled in the areas I've worked, I've never seen any reason to think there was a preserved Cretaceous (for example) soil horizon under the glacial drift - it tends to be a clean surface.  So from a limited dataset and with much broad assumption applied, I'd venture that pre-existing soils were largely planed off in this area.

anyway, yes, i was thinking that moraines are closest to what i was thinking of as a finished product
would you say that till is what plants are growing in in places such as at the foot of glaciers? those that i saw near columbia icefield were all in more or less soggy soil, i think, not sure if they ever get dried out in mid summer--i was there in july, i rather doubt at that altitude there is much time between melt and new snow, and as you say, what 'soil' there is among the stones is clay, so not much drainage other than on slopes or pockets that are mainly stone (such as the moraines a bit farther back, which i think were drier than those nearer the glacier which probably still have seepage)..

On your last point, yes, I was going to mention something about that but it seemed too complicated and too contrary to accepted rock gardening assumptions to wade into, LOL!  Yes, some of those plants are growing in what is essentially till - it's melting out directly from the glacier, and where it is not being winnowed and sorted by running water, it's just till (as opposed to any more sorted and permeable substrate).  Yet alpines grow there.  I would venture to say that much of what has been written about and accepted as the only truth about growing alpines comes from observations of what may be necessary in very wet and warm(er) climates (i.e. the UK) where perfect drainage and shelter from humidity/rainfall may be necessary.  It is not necessarily always so in nature.

The new Wrightman's catalogue describes Halda's method for planting tufa crevice gardens by splitting tufa slabs along bedding surfaces, then buttering one side with a clay paste, inserting bare root alpines then sandwiching on the adjoining slab.  I expect this sounds pretty unorthodox from the "perfect drainage" viewpoint.  (In this case, however, drainage is still being provided by these slabs being elevated around the level of the surrounding soil in the trough or bed, and by the tufa itself.)

[cohan]

 So from a limited dataset and with much broad assumption applied, I'd venture that pre-existing soils were largely planed off in this area.


On your last point, yes, I was going to mention something about that but it seemed too complicated and too contrary to accepted rock gardening assumptions to wade into, LOL!  Yes, some of those plants are growing in what is essentially till - it's melting out directly from the glacier, and where it is not being winnowed and sorted by running water, it's just till (as opposed to any more sorted and permeable substrate).  Yet alpines grow there.  I would venture to say that much of what has been written about and accepted as the only truth about growing alpines comes from observations of what may be necessary in very wet and warm(er) climates (i.e. the UK) where perfect drainage and shelter from humidity/rainfall may be necessary.  It is not necessarily always so in nature.

The new Wrightman's catalogue describes Halda's method for planting tufa crevice gardens by splitting tufa slabs along bedding surfaces, then buttering one side with a clay paste, inserting bare root alpines then sandwiching on the adjoining slab.  I expect this sounds pretty unorthodox from the "perfect drainage" viewpoint.  (In this case, however, drainage is still being provided by these slabs being elevated around the level of the surrounding soil in the trough or bed, and by the tufa itself.)

i was also, without ever having really thought about it, assuming that everything here was left by glaciers, but once stopping to think about it, i wasnt sure--bedrock seems awfully far away, here, and i assume some of the soil is from plant deposits post glaciation, but hard to imagine that is very deep in this climate, apart from wetlands, where peat/grass/sedge deposits seem much thicker;  i only have to dig a foot or so down before the soil seems largely inorganic..

the halda article  sounds interesting, i will have to take a look to see if its on the website also..

no doubt those plants growing in that heavy wet soil are a particular group-such as those mentioned in catalogues as growing in 'cold screes' with snowmelt, etc; i was already planning some spots in my rock garden with some sort of plastic lining or containers-i was thinking about the thick black rubber basins used for feeding/watering livestock-beneath the soil a few inches or more, to hold moisture in some areas for plants that seem to grow in those kind of spaces, yet not wet right up to the crowns; i thought some areas like this could also hold extra water after precip and trickle it out to other parts of the garden(i havent fully decided on the details, will partly depend on how it seems to work when i actually try it!)...
i have a plant of Saxifraga aizoides that i collected last year (not by the glacier in the national park, where it was also growing, but lower down outside the park) it was growing with primula and others on the edges of  a wet gravelly clay slough/pond; i potted it in local clayey loam with gravel and set the pot in a saucer of water and it did very well, ditto for Primula mistassinica (sp?) from the same location, so my aim is to sort of duplicate that pot in a saucer of water effect for a section of garden..

no doubt many of those plants that need perfect drainage are from much different environments-an exposed rocky outcrop or crevice would be a quite different thing than these dense areas behind glaciers; and as you suggest, a focus on drainage is likely to be much more crucial for places that have large seasonal precipitation at a time certain plants may not want it...
my old rock garden, built when i was a teen, and didnt think much about soil or drainage, did have a lot of rocks, with soil filled in around them, so there would have been good drainage, but the soil itself was just our local clayey 'grey wooded' loam..not that i was growing anything too fancy-- a few natives from the foothills and drylands, and a number of semps..

[cohan]

Cohan, thought you might be interested in these pics of the rock garden at Tilliepronie House in Aberdeenshire.  The rocks are massive and would have needed heavy machinery to move.

thanks for those, roma--it looks like a really wonderful garden! i particularly like the look of age, where each species seems to have spread a bit rather than being one little dot of each thing..
now if only i had a pile of stones like that! (and the requisite moving equipment or swedish moving skill

[David Sellars]

Lori:

I agree with your comment about growing conditions in the garden not necessarily replicating nature.  I find it fascinating that alpines in the mountains often grow in fine material.  In his book 'Alpine Plant Life'  Christian Korner notes that " alpine soils often contain - counter to expectations -  large fractions of very fine grain sizes".  But he also observes that grain size increases with altitude because more recently weathered material is coarser. High alpines stay dry under the snow in winter and then can get soaking wet during snowmelt and spring rains.  But I guess the steep slopes compensate for the poor soil drainage and the period of wetness is relatively short compared with our rainy winters on the coast.  In the rock garden we use a coarse soil mix, not because it replicates natural soil conditions in alpine areas but to compensate for the different climate conditions to which we are subjecting our plants in the garden. On top of the glacial till, most of our rock garden 'soil' is coarse sand at least half a metre deep and much deeper in some locations.

[Stone Rider]

Cohan, thought you might be interested in these pics of the rock garden at Tilliepronie House in Aberdeenshire.  The rocks are massive and would have needed heavy machinery to move.

this is very elegant execution with irregular igneous rocks. To work with them remains me one saying of Joyce Carruthers: there is many ways how to skin a cat.