An EarthCache is a special type of geocache where there is no container to find. Instead, you explore a unique geological feature and answer questions to claim your find, along with posting a photo.
Log this cache as 'Found it' and send me your proposed answers either via my profile or via the geocaching.com messaging system (Message Center), and I will contact you if there are any problems.. Please do not forget to add a picture to your log, If it is a group photo you will EACH need to add a pic to your log.
At the posted coordinates you will see in front of you a talus slope, sometimes called a rock scree. The broken rock is thousands of years in the making from mountain building, weathering and gravity. The rock is likely to be unstable so you will not be able to get too close. Observations should be made from the posted coordinates.
Tasks:
1. Examine several rocks near the trail. Based on their visible minerals, describe at least three colours or mineral types you observe. Does the rock have the typical "salt-and-pepper" appearance of granodiorite? If not, what do you think it is?
2. Examine the scree. Are the fragments mostly very angular, somewhat angular or rounded? Explain what this indicates about how far the rocks have travelled.
3. Estimate the average size of the scree fragments you observed (for example: fist-sized, football-sized, or larger). Why do you think the slope contains both large and small fragments?
4. Compare the centre of the scree with its edges. Which area appears more stable, and what evidence supports your conclusion?
5. Looking uphill, identify one geological process that is still producing new scree today.
6. Upload a photograph of yourself or a personal item (such as your GPS or a piece of paper with your geocaching username) at the scree slope.
The Lesson:
Welcome to a classic scree slope. The loose rock covering this hillside records millions of years of geological history—from molten magma deep beneath ancient volcanoes to the relentless action of ice, water, and gravity.
The rocks surrounding you are likely granodiorite, an intrusive igneous rock that crystallized several kilometres beneath the Earth's surface approximately 40–90 million years ago during the formation of the Cascade mountain belt. Unlike volcanic rocks that cool rapidly at the surface, granodiorite cooled very slowly underground, allowing large mineral crystals to develop.
The steep hillsides of the Cascade Mountains are constantly evolving. Water gets into small fractures in solid bedrock. During cold weather, that water freezes and expands by approximately 9%, widening the cracks. After many freeze-thaw cycles, pieces of rock eventually break free from the cliff above.
Once they break off, gravity takes over and rocks tumble downhill until they come to rest, gradually forming the pile of angular boulders you can see here. These rocks remain sharp-edged instead of becoming rounded like river stones because the rocks have travelled only a relatively short distance,
The trees growing throughout the slope show that this scree has been stable long enough for soil to begin accumulating between the rocks. However, talus slopes are never completely stable. Heavy rain, snowmelt, earthquakes, or falling rocks can still trigger movement.
Granodiorite is common in the central Washington Cascades, where large bodies of magma slowly cooled deep beneath an ancient volcanic mountain range millions of years ago. The resulting rock is strong, but it is naturally fractured by joints that provide pathways for weathering.
The Rocks
The Cascade Mountains have three similar intrusive rocks: Quartz Diorite, Tonalite and Granodiorite. These look similar but have some important differences in mineral compostion.
Although these rocks formed from magma beneath the surface, they did not all crystallize under identical conditions.
Small differences in magma chemistry, cooling history, and mineral crystallization produced rocks with slightly different compositions.
Name
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Appearance
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Minerals
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| Tonalite |
Light grey
Speckled black-and-white texture
Glassy grey quartz is easily visible
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Abundant plagioclase feldspar
Quartz (typically 20–60%)
Very little potassium feldspar
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| Granodiorite |
Often lighter coloured
May contain pink or cream feldspar crystals
Quartz is abundant
|
Plagioclase feldspar
More potassium feldspar than tonalite
Quartz (20–60%)
Biotite and hornblende
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| Quartz Diorite |
Medium grey
Noticeable dark minerals
Quartz is present but not abundant
|
White to grey plagioclase feldspar
Dark hornblende and biotite
Moderate amounts of quartz (roughly 5–20%)
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Scree- What is it?
Broken Rock:
Although granodiorite is a very strong rock, it is not unbreakable. As the rock was uplifted toward the surface, pressure on it decreased. This unloading allowed the rock mass to fracture, forming a network of joints and cracks. These joints became the starting points for weathering.
During winter, water enters the fractures. When temperatures fall below freezing, the water expands by approximately 9% as it turns to ice. This expansion acts like a wedge, gradually widening the cracks.
Thousands of freeze-thaw cycles eventually pry blocks loose from the bedrock. Once detached, gravity carries the blocks downhill where they accumulate into the scree slope you see today.
Angular Rock:
One of the easiest clues that these rocks formed as scree is their shape. Most fragments have sharp edges and flat faces because they have broken directly from nearby bedrock and have not been transported very far. Compare this with river rocks, which become rounded as they repeatedly collide and grind against one another while being carried downstream. The angular blocks here indicate that gravity—not flowing water—was the primary transport mechanism.
Rocks of Different Sizes:
Look closely and you'll notice everything from fist-sized fragments to multi-tonne boulders. This reflects how granodiorite breaks apart. Small pieces may separate along closely spaced joints, while larger blocks remain intact until enough fractures connect to release them from the hillside. Rockfalls continue to add new material, while smaller fragments slowly work their way downhill between larger blocks.
How a Scree Slope changes over time
Fresh scree is difficult for plants to colonize because the rocks shift underfoot and contain very little soil. After a while, organic matter accumulates,fine sediment fills spaces between rocks, mosses and lichens begin growing, shrubs and trees eventually establish roots.Notice how vegetation is much denser around the margins of the scree than in the centre. These areas have generally been stable for longer periods.Each winter, freeze-thaw weathering creates new fractures. Heavy rainfall, snowmelt, earthquakes, or falling trees can trigger movement of loose blocks.Even today, the scree is slowly creeping downhill under the influence of gravity.
What you're observing is not the end result of erosion—it's a snapshot of an ongoing geological process.
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- This geocache has an approved Permit to be placed at this location on property managed by the Washington State Parks and Recreation Commission. Visitors are responsible for acquainting themselves with policies and rules pertaining to State Parks areas.
- The following items may not be placed in the geocache: food, illegal substances, medications, personal hygiene products, pornographic materials, hazardous materials, or weapons of any type.
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- Report any incident, problem, or violation to State Parks staff.
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