Carving a message through time
This earth cache will help you learn what it takes to use geology to leave a lasting message. That knowledge will help you identify the type of rock in front of you. That is your mission.
Pictographs are ancient rock paintings and there are some even around idaho, displaying messages from centuries ago. Most of them have survived thanks to well protected locations, such as caves, where rain and wind can’t erode away at whatever paint was used and fade the image.
Petroglyphs, on the other hand, are rock carvings typically made by pecking directly on the rock face using a stone chisel and hammerstone. The chiseled contours can survive a lot more weathering and erosion than just about any paint. This means they can be in open and exposed locations.
Petroglyphs from hundreds or even thousands of years ago are supremely more awe inspiring than those made within the last few decades, even though modern methods provide a more professional, and crisp image. Modern tools go beyond chiselling and introduce sand blasting, laser etching, or power tools like a Dremel. You see these modern petroglyphs or ‘rock carvings’ everywhere. There are business signs, gravestones, sidewalk poems, and more.
At these coordinates you can find a sidewalk poem engraved in rock and set into the sidewalk so seamlessly that many walk past it daily without noticing. It is part of a set, scattered throughout Old Town. They are all made of the same rock, harvested from the same parent rock, and at the same time. Your mission is to identify which type of rock is in front of you. To do that, consider what you would want from a rock to do engraving yourself.
Ductile materials can be stretched out or reshaped without snapping or rupturing. Imagine how warm putty or chewing gum can be shaped and moulded. That means it is extremely ductile and can perform what is known as ‘plastic deformation’. When it is cold, a sharp sudden pull can snap putty clean in half, meaning cold putty is less ductile. It is the same with rocks and minerals. Some can perform plastic deformation and be carved, or in extreme cases, bent. A lot of rock types can be completely bent out of shape without being snapped or shearing off when they are extremely hot, typically due to magma. Check out this rock that was bent under extreme heat and pressure:
But there is a down side to more malleable materials. Using a material with massive amounts of plastic deformation, like putty or unfired clay could easily be shaped but it wouldn’t last. After spending hours carving and shaping the finished result, it would almost as easily lose that shape. Someone stepping on it would change the shape. Luckily rocks tend to only be this ductile under immense heat and pressure deep underground, but you can see the downside to very malleable rocks on the extreme end of the scale. It ends up looking up looking like the picture above. If you can’t picture this happening, look at busy traffic intersections. The tracks where vehicles are sitting, applying their weight for long periods of the day can clearly be seen. That weight pressing on the somewhat ductile tarmac gradually leaves grooves.
On the other end of the spectrum, if a material isn’t ductile at all, a slight change in shape causes it to snap, also known as brittle failure. Picture a thin glass window pane. A slight bend and it fractures and it is not going to bend back into shape. This is called being brittle. This would be bad for rock carving too. An effort to chisel away at an extremely brittle rock could just cause the entire slab to crack in half, instead of just a small detailed dent. A rock type that isn’t too brittle or too ductile is perfect.
Tiny differences in the crystalline structure on a microscopic level make big differences in how brittle something is. Sure most glass can’t take a hit, but then there are some types of glass that are bulletproof and have been used as a snowboard. That is largely down to the way layers are composed, as well as grain and crystalline structure and It is the same with rocks. If the crystal structure lines up predictable it can sheer off easily in lines of ‘cleavage’ where the rock cleaves apart. Biotite is a great example of a rock with perfect basal cleavage, it flakes away in layers just like a croissant because the crystaline structure is lined up so neatly in sheets that very little connects the layers. Graphite is pretty similar and is largely how pencil leds can leave gray lines as you write. This becomes an issue in carving rock when a fault suddenly cleaves off a chunk that was supposed to stay put. Marble carvers struggle with this when a pocket or fault line is hidden and breaks off a piece.
Some crystalline structures are regular but still strong because they don’t have predictable regular lines of cleavage, like diamonds. It is a regular pattern but connected in four directions, stopping the risk of being split down a recurring weak fault line. Regular structure doesn’t mean weakness, but it can do if it also lines up with lines of cleavage.
Take a glance at the other end of the crystalline structure spectrum and you have chaos. Mismatched random grain size and position all over the place. Picture granite, how it looks a bit like white noise on a television screen. There are no predetermined fault lines or lines of cleavage. This makes it stronger and less likely to have a chunk sheer off unexpectedly.
Sedimentary rocks, like sandstone, will crumble easily but that is just being too soft due to clast and cement composition. This is beyond crystalline structure and is how the ‘cement’ is formed with granules forming together like sawdust and glue forming MDF wood. Sandstone is easy to carve and easily eroded, meaning your finished shape won’t stand up to the elements very well if exposed.
So now with a basic knowledge of how malleability, crystalline structure, and lines of cleavage impact how good a material is for carving a lasting message, look at the rock in front of you and try to identify it. Pick from the rocks in the gallery using the logging requirements / questions beneath.
Before you log this earth cache as found, first email me answers for these four questions. Make sure not to include any answers or spoiler photos when you log this as found.
1 When you feel the rock, push your feet against it or try to reshape it, how does it feel? How malleable do you think it is from that?
2 Looking at the carved surfaces, do you think this was done by hand like with a chisel, or by a mechanical method? Why?
3) Are there any clear repeating crystal structure or fault lines that could be lines of cleavage or is it just random? How many directions of repeating fault lines are there in the structure and what does that tell you about how this rock will shear apart when being sculpted?
4) Using your answers above, look at the rock options in the photo gallery and pick which rock you think this message is carved in.