[Dave Flanagan]

Well? Granite has bigger grains so I think it should in theory.

[Nibile]

i think sandtone, cos being more flat give more contact surface for the whole hand.

[Dave Flanagan]

i think sandtone, cos being more flat give more contact surface for the whole hand.

I don't think this is correct. The rougher the rock the greater the contact area? Otherwise slate would have excellent friction.

[Fatboy]

I think the smaller grains in sandstone 'bite' more than granite maybe??  ???

[Nibile]

i think sandtone, cos being more flat give more contact surface for the whole hand.

I don't think this is correct. The rougher the rock the greater the contact area? Otherwise slate would have excellent friction.

yes, but theres the porosity also to be considered.
slate has no pores i think.

[AndyR]

Well? Granite has bigger grains so I think it should in theory.

Which theory is that then?

[Bonjoy]

I think the smaller grains in sandstone 'bite' more than granite maybe??  ???

It depends. River/delta deposited sandstones like grit have sharp high friction grains, but desert sandstones where the grains are rounded (from rolling about in the wind presumably) have less friction.

[Blunk]

There are certainly more factors involved than grain size. Grain sharpness, as Bonjoy noted. In granite, the proportions of quartz and feldspar seem to be important. Also what processes have weathered the rock, i.e. water, wind, sun, exfoliation, etc.

For my money sandstone generally has better friction in a wider range of temps. I have climbed on some rough granite such as at City of Rocks Idaho that was sticky as hell.

[bigphil]

My two-penneth.  The granite at Saddle Tor on Dartmoor is well rough and I found it too rough to give the same friction as say grit.  However, the granite at Carn Brae does not have cystals that big and had more friction.  I think the saddle tor granite is a good example of friction reducing as crystal size gets bigger.  A better example might be Hay Tor, which is riddled with massive quartz crystals that you can crimp.  The crystals are also generally too big to give good friction to the skin, but gave good friction to rock boots and even your clothes.

Fundamentally I agree that it is a function of crystal sharpness/roundedness and crystal size/contact area.  So there.

[AndyR]

I think the smaller grains in sandstone 'bite' more than granite maybe??  ???

It depends. River/delta deposited sandstones like grit have sharp high friction grains, but desert sandstones where the grains are rounded (from rolling about in the wind presumably) have less friction.

Both types (aeolian and fluvial) can have equally well-rounded grains - the main difference is in the sorting.
Desert sands (e.g. St Bees Head) are extremely well sorted - i.e. grain size is very uniform.
Grit is quite poorly sorted for a sandstone - i.e has grains ranging from mud size (<60 microns), right the way through to small gravel (i.e. pebbles) - the grains are also quite well cemented.

Regarding friction - I suspect this is dependent on a whole load of variables - I reckon porosity is pretty important given the amount of grease/humidity we exude on our fingertips.

As a generality - I agree with Blunk.

[AndyR]

A better example might be Hay Tor, which is riddled with massive quartz crystals that you can crimp.

K-Feldspar actually.
Sorry - I can't help myself 

[SA Chris]

I know very little about geology, but from an engineering point of view, if you took two uniform surfaces of cut granite and sandstone, and did a friction test on them, i would have thought sandstone would give a higher result, using rubber. Not sure how you would test it with skin.

[Paz]

Gun to your head which one would you rather rub the palm of your hand or your rock boots up and down twenty times?

If you had two big fuck off slopey breaks then once you'd cleaned the blood and gogarth grass lichen, or sea grease off the granite one it would be stickier, for the one go while your skin would last.  The sandstone one would constantly re lubricate itself in small grains of sand esepcially if you stand on the hold, and would constantly need re cleaning, in extreme examples basically causing the holds to change everytime. 

If you're talking about cut surfaces then I can't remember ever climbing on quarried granite, but the quarried sandstone I like has crimps and flatties

[SA Chris]

Paz, not all sandstone needs continual cleaning. Some sandstones are bullet hard and stay that way.

[Paz]

I've not been to 'thumberland or St Bees - is this what you're on about?  Even Font can get sandy.  I suppose I'm in for a treat at some point then. 

[SA Chris]

The County and St Bees are soft sandstone in a global sense. Stuff like Grampians, Table Mountain, and some of the US desert stuff are a lot harder. The treats will keep on coming, if you look hard enough.

[bigphil]

A better example might be Hay Tor, which is riddled with massive quartz crystals that you can crimp.

K-Feldspar actually.
Sorry - I can't help myself 

I stand corrected.  I knew it would be something like that but went with what I thought was the most likely.

[clm]

There must also be an effect of the granite grains being fused while the sandstones are just cemented.  I found etive slabs felt more insecure than a gritstone slab.  I had absent midedly hypothesised that it was for the above reason.