The Book of ROCK

"The harder it is, the better it is for your hole."


Bolts are only as strong as the rock you put them in. Some rock is soft and some rock has more layers than onions. The rock, not the bolt, is holding your life. The bolt is just hanging onto the rock. So let’s learn the basics to have a “rock solid” foundation!

Chapter 1 - General Rock Biology

HowNOTtoHighline Bolting Bible Book of Rock Chapter 1 Rock Type Joke
The Book of Rock Chapter 1 Rock Joke

It is important to understand the KIND of rock you are working with so you know what bolt will be the safest to use. To keep it simple, we will divide rocks into 3 categories. Hard rock, soft rock and layered rock.

Rocks are made up of minerals. Minerals are what make, or break, the rocks’ strength. What you need to know about minerals are their hardness and their resistance to weathering. Quartz, a common sand grain in sandstone, and a common crystal in granite, is incredibly hard and resistant to weathering. Feldspar, like quartz, is incredibly hard, but weathers easily and turns into clay. Clay minerals are soft, and also weather easily into other soft clay minerals.

Chapter 2 - Hard Rock (not the cafe)

Hard Rock would be any rock that is, wait for it, incredibly hard, typically unweathered granite and quartzite sandstone.

HowNOTtoHighline Bolting Bible Book of Rock Sandstone Granite Limestone
The Book of Rock Chapter 2 Types of Rock

Granite is the result of magma having cooled very slowly underground (plutonic rock) allowing the quartz and feldspar crystals (among other minerals not nearly as important) to grow big and tightly against each other, leaving no spaces. Yosemite granite is smooth from glaciers polishing them and Joshua Tree granite is rough because it sees very little water to erode it. But both formed and cooled under ground and therefore are very strong. And both haven’t seen the excessive chemical weathering (think acid rain and/or seawater) to have turned the hard feldspar to soft clay.

HowNOTtoHighline Bolting Bible Book of Rock Joke 2
The Book of Rock Chapter 2 Rock Joke

Quartzite Sandstone is quartz-rich sand, cemented together with quartz cement. So the whole rock is nearly 100% quartz. Quartzites are the typical ridge-formers of sedimentary basins, like the Sandstones at New River Gorge, WV.

Bolts in hard rocks can be installed closer to the edge (within a foot or two). Strength of hard rocks is above 14,500 psi and goes up to 30k and even 50k psi for some.

  • MECHANICAL BOLTS - Granite or other hard rock is perfect for all kinds of mechanical bolts, just be sure they are stainless steel.

  • GLUE IN BOLTS - These work just fine in hard rock but there are 2 schools of thought. Glue ins can be frowned upon in granite as it can be considered overkill and makes replacing them very difficult later unlike a mechanical bolt. The reason some people choose to use glue ins with fixed eyes is because there are no threads to get screwed up, no one can steal hangers, and the fixed eye allows highliners to thread static rope through rather than needing quick links. Glue ins typically have less visual impact than a standard climbing hanger.

Chapter 3 - Soft Rock

Soft Rock can be sedimentary or magma based. These are typically non-quartzite sandstones (where the sand-grains, or the cement, or both, are not quartz-rich, and thus prone to weathering and weakening), limestone (calcium carbonate), volcanic rocks, and excessively weathered granites. Just like tall tales, porous rocks with a lot of holes don’t usually hold up!

HowNOTtoHighline Bolting Bible Book of Rock Soft Rocks Sandstone
The book of Rock Chapter 3 Sandstone

Sandstone is literally compressed sand, glued together with chemically precipitated cement. The cement and sand grains are made of various minerals, each with different hardness and resistance to weathering. When either the sand or the cement is not quartz, the sandstone tends to be much softer, because the grains/cement may have weathered to clay (or weathered out altogether, leaving pores). To “scientifically” test a rock’s hardness out in the field, hit it with a hammer! If the rock sparks, its hard rock. If the rock breaks, it’s soft!

HowNOTtoHighline Bolting Bible Book of Rock Soft Rocks Limestone
The Book of Rock Chapter 3 Limestone

Limestone is calcium carbonate (mineral name: calcite), often from dead marine life, squeezed together to make rock. Calcite is moderately strong, but it weathers into CO2 and water (i.e. nothing) when it reacts with acid rain. Think sinkholes and caves. So its structure tends to have a lot of microscopic holes in it which makes it weaker.

HowNOTtoHighline Bolting Bible Book of Rock Soft Rocks Volcanic
The Book of Rock Chapter 3 Volcanic

Volcanic rock is from lava, so it cooled rapidly on the surface, not allowing the crystals to grow big and interlock. Also, lava tends to have lots of gasses dissolved in it, so as it cools it tends to be porous from the gas bubbles getting trapped. Small crystals and pores make volcanic rocks weaker than a slow cooling (unweathered) granite. Rhyolites and basalts, and volcanic ash “Tuffs” are typical volcanic rock names.

Bolts installed in soft rock need to be placed further back from edges in the most solid part you can find. It’s also quite unsafe to put pressure on bolts installed in soft rock when they get wet, like after a rainstorm. Cyclic loading mechanical bolts in sandstone compromises placement integrity. It’s highly recommended to let the rock dry out thoroughly before highlining on those bolts. Soft rock strength (dry) can be as low as 500 PSI and up to 7,000 PSI

  • MECHANICAL BOLTS - The softer the rock, the less force these will hold. Wedge bolts are not recommended because the engagement contact point is quite small which can compromise the placement in soft rock. Some sleeve bolts are ok because there is more contact area (like 5 piece Power bolts - STAINLESS).

  • GLUE IN BOLTS - This is where glue shines. Glue gets into all the rock pores therefore can pull on the entire rock surrounding the bolt stud rather than all the pressure being placed on a few millimeters of the sides of the hole with mechanical bolts.

HowNOTtoHighline Bolting Bible Book of Rock Classification of Rocks
The Book of Rock Classification of Rocks

NOTE: Rocks fall (get it?) all over the hardness chart - and this is because of their varying mineralogy. It’s good to know the area you plan on drilling so you can install the safest bolt possible. Some sandstones can be very very hard like New River Gorge, WV (quartzite) and some granites can crumble in your hand like the kind at Cabo San Lucas baja tip (feldspar, aka “Arkose”, now weathered to clay-rich granite).

Chapter 4 - Layered Rock

NowNOTtoHighline Bolting Bible Book of Rock Layered Rock Example
The Book of Rock Chapter 4 Layered Rock

Layered Rock is rock that has well-developed horizontal partings. In sedimentary rocks, these partings are bedding planes, where sedimentation stopped, and then restarted again. Sandstones, shales, and limestones can all be layered. In igneous rocks, especially granite, horizontal partings can occur that are called “exfoliation fractures”. These occur when igneous rocks are exposed at the surface and the pressure (of having been buried for millions of years) is released. The rock expands, and cracks like an onion.

Metamorphic rocks (slates in particular) tend to be layered as well, due to the shearing forces typical with metamorphic processes. Shearing causes the rock to foliate (or makes all the minerals in a rock align with one another in a preferred plane), causing onion like flaking, or parting along the shearing plane.

Layered rocks can be tricky to bolt. The rock may be hard but comes with crack, and anybody on enough crack isn’t going to be reliable! An example of this is the Dinorwic Slate Quarry in Llanberis, Wales. (David Thexton ) Another example of this is the limestone in Ontario as the limestone has been compromised by vegetation from the roots cracking it.

  • MECHANICAL BOLTS - can promote more cracking and are NOT recommended

  • GLUE IN BOLTS - Glue can grab the whole sandwich of rock layers giving better holding power.

Chapter 5 - Rock Inspection

HowNOTtoHighline Bolting Bible Book of Rock Cracked Rock inspection Carabiner Hanger
The Book of Rock Chapter 5 Carabiner Hanger

SOLID ROCK - Regardless of what rock you are drilling into, make sure it is a solid piece! Hit it with a rock density detector… aka… a hammer. You can hear the difference of solid rock with a high pitched ping rather than a dull thud on hollow rock. You can also put your hand close to the spot and if you can feel the vibrations, the rock is NOT solid.

HowNOTtoHighline Bolting Bible Book of Rock Inspect Your Anchors Bolted Rock Break
The Book of Rock Inspect Anchor Rock Chunk

BIG ROCK - Be sure you are not just on some large flake or suitcase sized rock. Check to see if the rock is thoroughly attached to all the other rock around it and not just a boulder sitting in the dirt. Sometimes even car-sized rocks are just barely attached to a rock face. Your bolt might hold but the rock might not! Think about the big picture of how that rock is attached to the earth.


If you have feedback, additional information, more correct information, or just good ol' typos that need fixing, please email me at or leave a comment below.


It costs about $10k a year to operate HNTH and no we don't make money on youtube. Please spot us $20 on paypal/venmo or support us per episode on Patreon.


Check The Bolting Bible Home Page!

Subscribe to our channel on youtube and hit the bell so you get notified every time we put out a new episode. We talk about bolts, highlining, climbing, gear, break tests and more.

850 views0 comments

Recent Posts

See All