Scratching the surface

Striae and slickensides, marks made by glaciers and marks made by great movements of the Earth [1200 words 5 mins

High above the Buttermere valley, there's a tiny hut once used by the quarrymen, and now available for overnight use by anyone who can manage to find it among the rock outcrops and piles of slate-quarry spoil. The sleeping platform, made of unmortared slate stone, is uncomfortable enough to make sure you wake up nice and early, for the dawn light along the perfectly formed glaciated valley with its two ribbon lakes.

Truncated spurlines and two ribbon lakes: looking down the former Buttermere Glacier from Warnscale Bothy

One that's still there: Boval Glacier, Switzerland

Five minutes above the hut, I paused on a nicely rounded rock to take the photo. And then I looked down at my feet; and saw the absent glacier right there underneath my bootsoles.

Glacier scratches are 'striae', which is just saying 'scratches' in Latin. Because whatever they tell you about U-shaped valleys – and Buttermere is a classic U-shaped valley. All that stuff in Standard Grade geography about ribbon lakes, and chopped off spurlines, and smoothed-off rounded rocks – and yes, the spurs of High Stile and Grasmoor are classically cut away and steepened above the valley floor, Warnscale's rocks are as sheep-shaped and moutonnée as anyone could ask for. But when you actually start to believe in that absent glacier, to actually see it yourself in your mind's eye, is when you see the scratch marks on the rocks. The marks left not by the moving ice itself but by the rocks that dropped into the crevasses to be dragged along inside it, like a sheet of very slow sandpaper 200 metres thick.

The moment when you actually start to believe in that absent glacier, to actually see it yourself in your mind's eye, is when you see the scratch marks on the rocks.

Glacier scratches are easily worn away by rainfall and lichen, even over the brief 20,000 years of the current break in the Ice Age. So a good place to see them can be where path erosion has just exposed fresh rock. The first geologists, carefully checking out alternative hypotheses, wondered whether those scratches in the path might just be the metal wheel-rims of ox carts and horse-drawn carriages. But not here where I'm now standing, high on the side of Haystacks…

Rocks above the Warnscale Bothy scratched left to right by the glacier

Striae, glacier scratches, aren't easy to see under the midsummer sun, or on typical grey Lakeland days: they’re best spotted in early morning, or under the low sunlight of winter. And the marks themselves, obviously enough, indicate the direction the glacier was moving. It's left to the observer to work out whether this was downhill here, towards Crummock Water and the Solway, or on the contrary line up the way towards Burnmoor Tarn. Okay, that’s an easy one – but as the next photo shows, glaciers don’t necessarily have to flow downhill.

Glacier scratches emerging from Loch Lomond beside the car park at the foot of Ben Lomond. The view is towards the head of the loch, meaning that the glacier underfoot was flowing uphill at this point.

Across the top of Haystacks, there's plenty to look at: the odd erratic boulder dropped off by that same glacier, the jumble of a volcanic landslide that now forms the scrambly rock-knoll called Little Round How – not to mention the charming little tarn and the grand spectacle of Great Gable rising overhead.

Volcanic rubble forms Little Round How.

A couple of years later, I was coming up Hastacks the other way, on the scrambly path above Scarth Gap. Coming up it quite slowly, because I was listening in on someone’s Uncle John. At a youngish 89, Uncle John was enjoying the scrambly bits above Scarth Gap just as much as anyone 80 years his junior, with enough spare breath to tell us youngsters about a thrilling winter rock-climb on Pillar Rock, just across the valley, back in the 1960s. A little late reaching the summit of Pillar Rock: in fact it was already dark. A cold night of it, without any spare clothing, and a quick descent of Slab & Notch at sunrise to avoid being rescued off the top… The Mountain Rescue called out by anxious family said good on you for self-rescue, and brewed them up a hot cup of tea to help with the hypothermia.

All of which gave me plenty of time to study the path underfoot, as it runs up a terrace of narrow rock-slabs. And there on the smoothed rock-surface underfoot, some more scratchy markings.

Scratched rock surface on the path above Scarth Gap, Haystacks

The scratches here are running slantwise across the rock-slab. But can a glacier, in its inexorable slide downhill, creep into that tiny crack on the left at the base of the rockwall? Then again, friction heat at the base of the glacier can generate melt-water: that's what turns it into a slidey-downhill glacier rather than a frozen-on icecap. But enough heat to melt the rock itself, or at least the quartz mineral within it? Because that's what's happened here: quartz smeared across the rock surface like margarine across a slice of toast.

Scratched rocks on Haystacks path: closer view. Did a moving glacier melt this quartz, before creeping like a mouse into the tiny crack in the rocks on the left of the photo?

So no, this isn't ice moving across rock. This is rock itself moving across another rock. It's a fault plane, where one rock mass has been shoved bodily along the surface of another. A smoothed and scratched surface like this is called a slickensides. Running your fingers along the scratches, you might be able to work out the direction of the movement. But even if you can't, what you're looking at is the crushing and crunching of the Lake District, caused by the coming together of England with Scotland in the Caledonian mountain building of 450 million years ago.

Mostly, when rock masses move past, they shatter each other to bits. The Lake District's had some pretty severe mangling, and fault planes emerge as the line of broken-up rock that erodes into a gully. But just sometimes, when the angle's just so, the two rock masses move past each other smoothly, leaving a scratched, flattened surface that's sometimes smeared with melted mineral. And if you climb up out of Scarth Gap again on the path towards Seat and High Stile, you'll see the slickensides again, almost as clear as it was back on Haystacks.

Geological jargon is great for geologists: if nobody can understand what you're talking about, they're bound to take you for a very clever person. Well, that's the theory. But at their best, geologists show a wide-minded love for every language in the world: aa lava from Hawaii; boudinisation from the delicious greasy sausages of Belgium; scraps of foreign rock in a magma chamber called xenoliths, strangely deposited out of ancient Greece. And from the Latin 'striae' of educated Renaissance Europe we move to the German miners of the Middle Ages for the slickensides, the smeared, smoothed and scratched surface left by the movement of one mightly rock mass across another.

November frost creates a ghost memory of the former St John's Glacier below Helvellyn