Welcome

This EarthCache was placed at the entrance to a trail.  While this is a publicly accessible location, it is important that you follow all rules to demonstrate how responsible geocachers are with the environment. Once you complete the EarthCache at the trialhead, you should definiltey experience the falls if the park is open. It is closed for much of the year so you do not have to enter to complete this cache. If you do decide to visit the falls, please stay on the trail and follow all rules as posted. At no point, should you collect anything, or disturb the environment except where specifically directed. Please enjoy this EarthCache responsibly. 

Experience this quick stop right off of Boulder Canyon Drive. Boulder Falls is a short and easy walk to a small but mighty waterfall. Bring a picnic and enjoy sitting on the cool rocks while watching water crash down the falls. There is no typical trailhead for Boulder Falls, but there is a parking area across Highway 119/Boulder Canyon Drive from the falls.

Boulder Falls is located 11 miles west of Boulder, on the north side of Boulder Canyon Drive (SR 119) between Boulder and Nederland. It consists of five acres of mining claims that were given to the City of Boulder by Charles G. Buckingham, president and co-founder of Buckingham Brothers Bank (now Norwest Bank). Buckingham had held a U.S. Patent since 1881 on the American Mill site that included the Falls. He donated it to the City of Boulder for recreational purposes in 1914, hence "saving this beautiful spot from the encroachment of the great tungsten boom."

For many years, Boulder Falls – sometimes referred to as the "Yosemite of Boulder Canyon" – was the popular destination for picnic groups. Carryalls brought visitors up for a visit to the Falls, especially after the narrow gauge railroad washed out in the great flood of 1894. When the railroad was rebuilt four years later, stage and tourist travel was diverted, and the popularity of Boulder Falls declined.

Earth Science Lesson

They often look like this… but don’t take that for granite! (I just had to do the pun, sorry)

Granites are some of the most distinguishable rocks out there, and even amateur geologists usually have no problem in identifying them. But if you’re new to the rocky world of EarthCaching, identifying any rock can be a daunting task. Fret not, we’ve got you covered. Here’s what you need to know.

How granites form

Granites are igneous rocks, sometimes also called plutonic. This means that they’re not formed as a result of a volcanic eruption, but rather through the slow, gradual cooling of magma. They tend to have pretty big, coarse crystals (visible with the naked eye) — as a rule of thumb, the bigger the crystals, the slower the cooling. For instance, in other eruptive rocks which cooled extremely fast, you can’t see crystals with the naked eye, or there aren’t any crystals at all — because they’ve had no time to form. But with granites, named for their granular structure, you don’t have this problem — you just see the crystals with relative ease. No equipment is needed, though a loupe can help sometimes.

This granite is pink-reddish due to the alkali feldspar. Don’t let color be the only factor you look at. Here, the feldspar is pink-reddish, the mafic minerals are black, and the quartz is grey. Image credits: Khruner / Wiki Commons.

Since granites are a pretty diverse group, their chemistry can also be varied. They contain, above all, silicon dioxide, which can make up to 70% of their composition. They also contain alumina, (around the 14% mark), and to a lesser extent, sodium oxide, potassium oxide and iron oxide. Other chemical compounds typically account for less than 1%.

Minerals in a granite

Granites usually contain both “felsic” and “mafic” minerals. Those names might not mean much if you’re just interested in IDing simple rocks, but it helps to remember them for your geological forays, especially if you want to identify more rocks.

As pretty a granite as you’ll ever find. Try to spot the different minerals. First, mentally divide the mafic from the felsic. Don’t bother too much with the mafic, just know that it’s there. Then, try to separate the quartz from the feldspar. Can you do it? Image credits: James St. John.

Basically, felsic minerals are white(ish) white mafic minerals are dark. One tool that may be helpful is that “mafic” sounds a lot like “mafia,” (of the Memfis Mafia) which is bad, so it’s black. Felsic sounds a bit like “fair” (I know it doesn’t but whatever — it helps), so they’re a bit whiter.

Overall, granite is a felsic rock, with mafic minerals being secondary or accessory in nature. The two main minerals in a granite are quartz and feldspar — in an extreme case, you could have a granite comprising of just those two minerals, though that almost never happens.

So, the first two (quartz and feldspar) are felsic, they’re the white ones, while the other ones (generally amphiboles, pyroxenes, biotite) are black or very dark. You can also have accessory minerals such as tourmaline, beryl, topaz, zircons and apatite, but that really doesn’t help you with the identification. Let’s look at the minerals that do help you:

Quartz

Quartz is a very common mineral in the Earth’s crust and is almost always well represented in granites. According to the generally accepted definition, a granite must have at least 20% quartz.

Quartz is typically light-grey and kind of looks like an opaque window. If you look at the crystals closely, they should stand out from the surrounding rocks. Quartz is pretty sturdy and you can’t scratch it with your fingernail (unlike other minerals).

Feldspar

Feldspar is often the most abundant rock in a granite — this is why the rock looks white with dark spots, and not dark with white spots. There are two main types of feldspar: alkaline and plagioclase, with most feldspars being a mixture of the two. It doesn’t really help you to tell the two apart (unless you want to go much deeper into the mineralogy of granites), and in most cases, you can’t. However, identifying feldspar is key for identifying a granite.

Feldspar pops up pretty much everywhere in geology, in igneous to sedimentary rocks. It makes out some 40% of the Earth’s crust, and knowing how to recognize it is key for any geologist — amateur or professional. Feldspars are hard minerals, almost always white (sometimes with a pink hue), typically sporting a glassy luster. A big difference between feldspar and quartz, for example, is how it breaks. While quartz breaks irregularly, often in curvy shapes, breaks readily along flat faces, a property called cleavage. Feldspar tends to look quite blocky as a result, as opposed to other minerals whose individual crystals might stand out.

Amphiboles and pyroxenes

We’re taking these two as a group because of their many similarities. They’re both black, and if you could look at them closely, they look like prisms rising up. The main visual difference is that amphiboles are generally taller than pyroxenes, though this distinction is extremely hard to make for a non-specialist.

Chemically, amphiboles contain water while pyroxenes don’t but again, this doesn’t really concern basic identification. Look for black prisms which are a bit tall, not flat like…

Biotite

Biotite is black mica.

Amateur geologists often have a rough time distinguishing between biotite and other black minerals, but there are a few things which can help you. First of all, biotite is basically a series of planar sheets, unlike amphiboles and pyroxenes which are prisms. So biotite is flat, the others are tall. Because of its sheet-like structure, it often starts to split up at the ends, much like an old book (can be hard to see with a naked eye). Biotite is also easily scratched with a fingernail, that can be a good tell. Granites can also feature white mica (called muscovite), though that is much rarer than biotite.

What makes a granite a granite

Overall, it’s not any one of these minerals that make a granite a granite, but rather the way they all fit together. The main takeaway is to look at:

– the structure and texture. After all, granite is named for its “granular” or phaneritic texture — you can see
– types of minerals: look for quartz (at least 20%) and feldspar, often with mafic minerals

A granite with large crystals. Can you see the difference between the quartz and the feldspar? This will help when answering the questions below. Image credits: James St. John.

Granites are some of the easiest rocks to identify, and it’s all a matter of training your eyes and mind, it shouldn’t take too long. Also, if you’re just starting, you might read in some places that many rocks look like granites, but aren’t really granites — they’re called granitoids. For now, you shouldn’t really bother with that and go with the ‘if it looks like a granite, it’s a granite’ approach.

Logging Tasks

1. Can you identify mafic within the sample at GZ? Describe what you see and how you are able to identify it.
2. Can you identify mafic at Stage 2? Describe what you see and how you are able to identify it.
3. Can you identify Felsic at GZ? Describe what you see and how you are able to identify it.
4. Based on the description do you believe the Felsic is Quartz or Feldspar?
5. Can you identify Felsic at Stage 2? Describe what you see and how you are able to identify it.
6. Based on the description do you believe the Felsic is Quartz or Feldspar?
7. On to the mafic, can you identify any amphiboles, pyroxenes, or biotite at Stage 2? Please describe what you see.
8. Post a photo of yourself (or a proxy) at Stage 2. (Required)

References

1. Geology ABC -- how to identify granites. (2021, January 29). Retrieved March 7, 2022, from https://www.zmescience.com/science/geology/how-to-identify-granites/

2. King, H. M., PhD. (n.d.). Granite. Retrieved March 7, 2022, from https://geology.com/rocks/granite.shtml

3. Britannica, T. Editors of Encyclopaedia (2021, March 25). granite. Encyclopedia Britannica. https://www.britannica.com/science/granite

4. Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
5. Softschools.com. (2019). Granite Facts. [online] Available at: http://www.softschools.com/facts/rocks/granite_facts/2976/ [Accessed 13 Mar. 2019].
6. Helsinki (2015) Introductıon: The Rock, Granıte: About The Rock, Granıte-Research-Book-Reduced
7. Atlas-hornin.sk. (2019). Atlas of magmatic rocks. [online] Available at: http://www.atlas-hornin.sk/en/home [Accessed 13 Mar. 2019].