UKC

Scrittly gritstone question

New Topic
This topic has been archived, and won't accept reply postings.
 tehmarks 13 Aug 2020

I've just been bouldering on the Cracoe Memorial Boulders at Rylstone the past few days (great fun, complete solitude - grades are a bit stiff though!? ). The surface of the boulders is very rough and crystalline, but also very loose. Even carefully placing a foot on a hold is guaranteed to convert some of it to sand. I've not properly fondled any of the boulders there, but probably quite similar to the Woolpacks?

When this loose surface layer is eroded (by hands, feet, brushing, however), will it eventually leave a more solid surface in its place, more akin to what you might find at say Stanage, or will it continue to expose more loose, scrittly rock and erode away quickly with the passage of hands and feet?

 JoshShaw 13 Aug 2020
In reply to tehmarks:

In my experience, there isn't a simple answer to this one. Sometimes the surface will stabilise leaving something a bit more solid. But, sometimes it's just a sign that the rock is a bit softer a more prone to wear.

Given your description (crumbling underfoot) I'd be leaning towards the latter, it's probably just softer than other areas.

OP tehmarks 14 Aug 2020
In reply to JoshShaw:

Makes sense. I was assuming the latter myself, just based on the quality under the sandy grit, but thought it was worth asking.

Good job it's not a popular area

 wbo2 14 Aug 2020
In reply to tehmarks: There's been another thread on Millstone grit geology recently..however here's my suggestion , that may be wrong.  The rock is a mixture of grains, in a cement.  The rock is also heavily eroded/weathered, which will tend to remove cement and leave solid crystalline grains , and a surface texture you describe.  The upshot is that the uncleaned surface is a poor guide to what's underneath.  

What's the other rock around there like?

 ste_d 14 Aug 2020
In reply to tehmarks:

The latter ☹️

 Will Hunt 14 Aug 2020
In reply to tehmarks:

Good question. The answer is probably "both" in most cases. Sometimes there is some surface scrittle which, once gone, leaves hard rock underneath. Sometimes you'll start to find soft rock almost straight away when brushing with even a relatively soft brush. In any case, there's only so much brushing that any gritstone will take, so I'd advise using a brush but being careful about it. Rock quality can vary dramatically even on the same crag or buttress.

For persistent offenders (there's a foothold at Yeadon that started to deepen after only a little traffic which I did this on), a little bit of glue from a tube of runny Loctite can be dripped onto the hold and allowed to soak in. It's very important to always test the glue on an unimportant bit of rock to check that you're not using something that will leave a glassy surface or have any other undesired effect. This is a very easy and useful way of stabilising a hold.

 AJM79 14 Aug 2020
In reply to tehmarks:

Not all grit is equal quality. It's made up from, mostly quartz grains which are cemented together. This cement can be silicate or calcite based (silica is stronger but generally doesn't fill all available spaces like calcite). On top of the different types of cement there are other factors such as amount of cement, amount of pressure applied during formation (this will mainly affect silica cements). There are many also factors affecting the size of grain deposited (i.e where in the basin it was deposited). All of this means that the quality of grit can be very variable of a short distance (think of all the deep sandy breaks surrounded by quality grit at millstone). There's a few places near me where the grit is solid at one crag and a crag a few hundred meters away is scrittly crap, it's probably the cement and it'll always be like that so don't brush too hard.

Also the surface, even on shit grit, is usually stronger as it undergoes chemical changes on contact with the atmosphere. So once your through this, erosion will take place faster. Just go to bridestones (West Yorks) to see.


New Topic
This topic has been archived, and won't accept reply postings.
Loading Notifications...