Welcome Deception Falls, an earthcache garden!
This cache is the first in what I hope will be a series of earthcaches showcasing the many varied types of rocks that are found all throughout Washington State. Getting a deeper understanding of the many types of bedrock that make up this state helps you to appreciate its geologic history.
Mt. Stuart Batholith
The map page you see below is an excerpt from the 1:100,000 scale maps that you can download from the Washington State Department of Natural Resources for their surface geology maps. This excerpt is from the Skykomish River map and it shows the area near Stevens Pass, including the area here at Deception Falls.
The colors on these maps show the boundaries of the different types of surface bedrock and the labels that you can see for them are annotations for what kind of rock they are, which is shown on the side of the map in a glossary of sorts. As you can see from the map, the rock in this area is labeled as Ksw. OK, so what is type Ksw then?
The important thing to note here is that these rocks are part of the Mt. Stuart Batholith. Perhaps the first question you might ask is, what in the world is a batholith? Wikipedia defines a batholith as A batholith (from Ancient Greek bathos 'depth', and lithos 'rock') is a large mass of intrusive igneous rock (also called plutonic rock), larger than 100 km2 (40 sq mi) in area,[1] that forms from cooled magma deep in Earth's crust. The Mt. Stuart Batholith is approximately 13x16 miles in size - plenty large enough to be a batholith.
The Mt. Stuart batholith formed 85 million years ago as magma that was ten times hotter than boiling water oozed up from underground, but didn't reach the earth's surface before cooling. Over the eons since then, the ground has eroded and the batholith itself has been uplifted, leaving the batholith exposed today.
The particular type of rock that is exposed here is granodiorite. Granodiorite is composed of two principle types of minerals, quartz and plagioclase feldspars. It looks very similar to the more well known rock of granite, but granite has orthocloase feldspars rather than plagioclase feldspars. All feldspars are aluminum silcon oxides, but orthoclase feldspar contains potassiam, whereas plagioclase feldspars contain a mixture of sodium and calcium. Thus granite and granodiorite are chemically different even if they look quite similar.
In terms of the two main minerals in granodiorite, pure quartz crystals are clear. In real rock samples impurities that often make them look translucent. Plagioclase is usually white to greyish-white in color, with a slight tendency for more calcium-rich samples to be darker.
A Magnetic Mystery
As these rocks contain iron in some of their crystals, the orientation that the crystals align to when the magma solidifies tells you a lot about what the magnetic field was like where they were at when the rock soiidified. The Mt. Stuart Batholith dates to the late Cretaceous period and, somewhat surprisingly, the magnetic orientations of the crystals in the rock grains don't line up to the known magnetic field of the earth at that time. That means that the rocks must have moved since then.
Given the amount of geologic time since then, there are two plausible options, either the Mt. Stuart batholith tilted, or it got pushed around from another place via continental drift of sorts. In their paper Paleomagnetism and tectonics of the Cretaceous Mt. Stuart Batholith of Washington: translation or tilt?, authors Beck, Burmester and Schoonover from Western Washington University argue that it has to be translation from some other place. Why? Mostly because rocks from all of the other areas nearby that don't have the same origin as the batholith also show the same angle mismatch, meaning they too would have to be tilted the same amount (even though the rocks aren't connected).
If you work out the mathematics of where these rocks would have had to come from based on the angles of their paleomagnetic crystals, the answer turns out to be somewhere off the coast of what is now northwestern Mexico, a term that is now popularized as the Baja - BC hypothesis which involves both translation and rotation of the terranes of rock that were once located in that area and now make up large swaths of the aggregated land from the central Cascade range all the way to southern Alaska.
It should be noted that this theory is controversial. The paleomagnetism indicates that this is where the rocks came from, but there isn't a lot of understanding of how this could have happened from a continental drift point of view - so there is an ongoing dispute amongst geologists about it.
Logging Questions
Some of these questions can be answered by reading the description, some by reading the sign that is present at GZ, and others will be answered by field observations. Send your answers to me via the geocaching messenger service within a few days (at most) of logging your find. If you are sending the answers for a group of cachers, please make it clear in your message who all is in the group.
- From the description, what is a batholith?
- Find a piece of granodiorite from the Mt. Stuart batholith and examine it. Describe the color(s) and grains that you see in the rock, along with the sizes of those grains. Which of these do you identify as quartz and which do you identify as plagioclase feldspar?
- Estimate the ratio of quartz grains to plagioclase feldspar grains that you see in your piece of granodiorite.
- From the description, what is the Baja - BC hypothesis?
- Take a picture of the granodiorite from the Mt. Stuart batholith along with something related to geocaching (a pen, a travel bug, a GPS, a piece of paper with your geocaching nickname, whatever) and attach it to your log.