Geology of Pennsylvania’s Sticky Boulders – By Jim Bowers

Eskimos have 50 names for snow but most climbers just have “rock.”

It’s all about friction

What makes Pennsylvania bouldering so unique and fun is the large amount of sticky rock in the state. By sticky I mean rock texture that has unusually good friction for hands and climbing rubber. Climbing on sticky rock is really fun because you feel like a Gecko – hands and feet sticking to holds smaller than you think possible at angles that defy gravity. Sticky rock glues you to sloper holds that shouldn’t work with feet sticking on impossibly steep angles. And that is really fun and satisfying and greatly opens up the possibilities for amazing sequences over rock.  

Now let me emphasize that super sticky climbing rock is actually rather rare and most people haven’t actually climbed on any. Yosemite granite, Gunks quartzite, western limestone, Colorado granite gneiss – none of these are particularly sticky. Let me explain why.

What makes rock sticky has to do with the size and shape of the tiny grains that make up the rock. Climbing friction is determined by the interaction between individual edges and points on rock grains and your skin or climbing rubber. The faces of rock crystals tend to be smooth as glass so friction is mostly a function of crystal points and edges “catching” on skin or rubber. The more crystal “points” and the farther they stick out the more “grab” the rock has on your skin. Finer rock grains mean more points available to grab – but coarser grains usually means the points stick out farther.

Now I’m going to introduce a brand new climbing geology term that you lucky folks are the first to be introduced to. Rock resolution. You won’t find this term in any previous description of climbing geology because for some reason all other climbing geology writings have focused on typical geology stuff such as tectonic history or depositional environments… in other words trying to trick you into learning some geology – which is a very big subject.  I’m going to focus more on how the geology of a given rock actually effects your climbing experience (related to that 50 names for snow thing).

Rock resolution is to brail as computer screen resolution is to vision. In other words you can’t get a climbing “edge” smaller than the size of the grains that make up that rock. If you climb on a granite made up of 1/2 inch wide crystals – you really can’t get a hold any smaller than 1/2 inch. Rock with poor resolution that gives just a few crystal points or edges for skin interaction ends up being painful. You can lay down on a bed of nails but just one or two alone will puncture you. Most climbing rock in North America is composed of individual mineral grains that are too large for optimal friction.

Now let’s talk about PA’s sticky rock types – let’s start with sandstone.

Layers of rock are called formations to enable you to give a name to individual continuous bands of rock. Seeing as sedimentary rocks were at one time a beach or the mud offshore you end up with a continuous layer cake arrangement as the beach migrates from sediment continuously deposited from land.

The sandstone band of the Old Port formation outcrops at numerous locations southwest of state college to west of Huntingdon – and includes the popular bouldering area Hunter Rocks. The interesting thing about Hunters sandstone is that it lies beneath (older than) the Tuscarora quartzite (which is the formation that makes up Seneca rocks) so it is really old rock (Devonian – around 400 million years ago). The odd thing about Hunter rocks is that it’s still a sandstone – when it should be a quartzite given its old age and depth of burial. Hunters consists of a pure white quartz sandstone so pure it’s mined for silica glass near Huntingdon. Trivia tidbit – the glass they used to make the 200-inch Mt. Palomar telescope came from one of these sandstone quarries. Hunters sandstone is older (buried deeper) than the similar composition Tuscarora quartzite located on the adjacent ridge top from Hunter rocks. Why is Hunters a sticky sandstone and not a smoother quartzite like the Tuscarora?

 
Quartzite is a metamorphosed sandstone where the individual quartz grains are fused into one mass by heat, moisture and pressure to form a solid quartz unit. When you break a quartzite it breaks through individual grains – and this makes for poor climbing friction as it forms smooth fracture surfaces instead of individual grains sticking up. One likely explanation of how the Hunters rock formation escaped the quartz grain fusing process is lack of water (needed to mobilize quartz). It’s likely this formation was filled with natural gas which kept the water out and enabled the rock to stay a sandstone. As a result the quartz sand grains only fuzed together where they were touching from the extreme pressure. This is fortunate for climbers as the rock is relatively strong for a sandstone but has enormous friction – the best of both worlds – quartzite strength and sandstone stickiness.

Gritstone and Hunters sandstone should be pure white – the tiny spaces between the grains enables tiny green lichen to grow giving the overall rock a greenish grey color. This is actually a sign of sticky rock. The quartzite conglomerate in northeast PA is usually pure white, which means no space between the grains for tiny plants and much poorer friction.

Gritstone

The gritstone in the English Peak District is worshiped by climbers, among whom it has a cult status and is often referred to as “God’s own rock.”  There is a reason for that. Grit is amazing for climbing because it’s an incredibly durable rock with classic granite-like shapes and enormous friction – especially for feet. People are often confused as to exactly what Gritstone is as it’s not a proper geologic term. Essentially Gritstone is a hard sandstone with larger sized grains making up the rock. Think of sandstone as medium, 80-grit sandpaper while gritstone is a coarse 20-40 grit.  Provided the individual gritstone grains are angular it makes for crazy levels of friction. Climbing rubber can stick on a featureless gritstone slab at a steeper angle than any other type of rock! Grit makes for sticky hands but the smallest of finger holds tend to rip your tips after multiple attempts.  Fortunately, most grit holds tend to be whole hand slopes or pinches. 

Outside of England, Pennsylvania has probably the highest concentration of gritstone anywhere. There is some in West Virginia and New York but I haven’t encountered any elsewhere in America. Gritstone in the state occurs in Mississippian and Pennsylvanian age rock (around 300 million years old) mostly in western and northern PA. One exception is Squirrel Rock in Northeast PA.

One thing that makes Pennsylvania Grit unique compared to English Grit is the presence of strategically placed pockets which are often key to making climbs go. Brian McCall climbed with a bunch of Brits in Font and had a hard time convincing them PA had gritstone because they insisted their gritstone didn’t have pockets. The pockets in PA grit aren’t formed the same way as most sandstone pockets like at Red River Gorge or Hunters, but seem to be randomly placed – most likely the result of pieces of wood or other foreign matter embedded in the coarse beach sand that disappeared over time.

Diabase

Southeastern climbers are blessed with a large swath of Diabase rock in bands running from south of Gettysburg to Northern Bucks county into New Jersey. Unlike the sedimentary rock that makes up most PA rock, Diabase is an Igneous rock – intruded into dikes and sills as molten magma. PA diabase is Triassic age thought to be associated with continental rifting and the opening of the Atlantic ocean.  

Southeast PA is heavily populated, but the diabase tends to form hills and makes for difficult farming so it tends to be forested – escaping urbanization and farm use which occupies most of southeast PA land. Although diabase rock makes for difficult farming, plants actually love growing on this stuff. In the summer, diabase areas are a green humid jungle environment compared to PA’s sandstone and gritstone areas. But what the summer takes, the winter delivers as the relatively warmer Southeast PA climate and temporary snow enables many climbable days on this very sticky rock.
Friction-wise, Diabase is very similar to a finer gritstone or coarse sandstone as the size of the grains are similar and stick out nicely. Diabase is composed of pyroxene (the black grains) and Plagiocase (the white grains) giving the rock a dark grey color. Brown or reddish colors are from the iron in the pyroxene weathering to rust like minerals.

 
Fresh diabase doesn’t have particularly good friction, but you won’t find any unweathered diabase in PA aside from quarries, railroad ballasts and monuments (also known as black granite). When the diabase weathers it brings out the very angular nature of how the individual plagioclase and pyroxene grains tend to grow together when the rock forms – revealing lots of points and edges to stick to skin and climbing rubber.

An unfortunate aspect of diabase is that the minerals that make it up – pyroxene and plagioclase, are 5-6 on the Mohs hardness scale, while quartz is a 7. This means diabase polishes! I have already noticed this on the Porcelain slab at Gretna – 15-years ago, no polish. Now you will have an easier time using the smaller footholds as the larger ones are a bit polished and less sticky. The best way to mitigate this problem is to clean your shoes!  Every diabase climber should carry a clean rag to thoroughly clean the bottom of your soles before climbing anything diabase. The polishing comes from small bits of dirt embedded in shoe rubber. Your shoes need to SQUEEK or you are a rock polishing geek.