Welcome to this EarthCache! Unlike traditional geocaches, there is no physical container to find. Instead, your task is to carefully examine a unique geological feature, answer the questions below, and take a photograph to claim your find. This EarthCache focuses on "augen eyes" - a types of gneiss, a striking metamorphic rock that records both the mineral composition of its original protolith and the tectonic forces it has endured. All observations can be made within Marymount Cemetery during daylight hours. Parking is available nearby.
You'll be looking for the memorial for Terrence Stuart "Terry" Simpson, at the posted coordinates. This monument should stand out due to the usual "eye" formations within the stone. These "eyes" are called Augen.
Augen gneiss differs from regular gneiss in that it contains prominent, lens-shaped mineral grains known as "augen"—typically feldspar or quartz—that stand out from the finer-grained, foliated matrix. The German word “augen” translates to “eyes,” reflecting the distinctive eye-shaped appearance of these mineral grains. Regular gneiss exhibits uniform foliation or banding with no large isolated mineral grains, while augen gneiss is characterized by the contrast between the large, resistant augen and the more ductile surrounding matrix, which records differential deformation during high-grade metamorphism.
EarthCache Requirements
As with all EarthCaches, I do not expect technical, PhD-level answers. Your goal is to carefully observe, think about what you see, and enjoy learning something new. If you are completing the EarthCache with multiple caching names, please include all usernames in your responses. Individual submissions are not required.
To claim a find, send your answers to the questions below via the email or Message Center link at the top of this page, and include your required photograph in your online log. I will contact you only if clarification is needed.
Observational Task
At the posted coordinates, you will see a monument that features the classic shape of :augen eyes". Observe the size, shape, and colour of the augen in the rock, and note how they are oriented relative to the surrounding foliated matrix. Pay attention to whether the augen are elongated, rotated, or flattened, and look for any differences in mineral composition compared to the matrix. Take note of the overall foliation and any evidence of shear zones or deformation bands surrounding the augen.
Questions to Answer
-
Augen Characteristics: Describe the augen’s size, shape, colour, and mineral composition. Are they feldspar, quartz, or another mineral?
-
Orientation and Deformation: How are the augen oriented relative to the foliation? Do they appear stretched, rotated, or compressed?
-
Comparison with Regular Gneiss: How does this rock differ from ordinary gneiss in texture, structure, and appearance?
-
Photo: Include a photograph showing the augen clearly, along with yourself, your GPS, hand, or a signature item for scale. Each cacher must upload a unique photo.
Geology Lesson: Augen Gneiss
What Makes Augen Gneiss Unique
Augen gneiss is a high-grade metamorphic rock notable for its large, lenticular mineral grains embedded in a foliated matrix. The augen typically consist of feldspar or quartz, although mica and hornblende may also be present in some examples. These grains originated as coarse crystals in the protolith, often a granite or granitic gneiss, before undergoing metamorphism.
The surrounding matrix is generally fine-grained and foliated, composed of mica, biotite, hornblende, or other mafic minerals. The contrast between the large augen and the fine-grained foliation records the differential deformation that occurred during metamorphism. In other words, the augen were rigid enough to resist flow, while the matrix deformed plastically under stress.
Formation Process
Augen form under high-temperature (500–700°C) and high-pressure conditions (several kilobars), typically associated with tectonic shear zones or orogenic (mountain-building) events. During metamorphism, directed pressure and shear stress deform the rock, causing the surrounding matrix to flow around the more resistant mineral grains.
This deformation results in lens-shaped augen, oriented parallel to the foliation but often stretched or rotated depending on the stress direction. The shape arises because the surrounding matrix flows plastically while the augen resist deformation. Multiple phases of deformation can create nested or irregularly rotated augen, recording a complex metamorphic history.
Why They Are Rare
Augen gneiss is uncommon because it requires specific conditions: a coarse-grained protolith, high-grade metamorphism, and directed shear stress sufficient to deform the matrix but not the large mineral grains. Without these conditions, the protolith may form regular gneiss, amphibolite, or other metamorphic rocks.
Colours and Mineral Types
-
Feldspar augen: pink, salmon, or light cream
-
Quartz augen: grey or translucent
-
Mica/hornblende augen: dark green to black
-
The surrounding matrix is usually dark, fine-grained, and foliated, providing a clear contrast to the augen.
These colours make the augen visually striking and allow geologists to identify the type of mineral in the eyes.
Global Occurrences
Augen gneiss is found in ancient orogenic belts, high-grade metamorphic terranes, and shear zones worldwide. Notable examples include:
-
Canada: Canadian Shield, Ontario, Quebec
-
Europe: Alps, Scandinavian Caledonides
-
Asia: Himalaya, Aravalli mountains (India), Japan
-
Other locations: Portions of Australia and the United States (Appalachians)
The common factor is that augen gneiss forms in regions with significant tectonic compression, shear, and metamorphism.
Interpretation of Tectonic History
Augen gneiss is more than just visually striking; it provides insight into metamorphic conditions and tectonic stress directions. The orientation, rotation, and flattening of augen can be used to determine the direction of shear, relative stress, and degree of plastic deformation during metamorphism. Geologists can also infer the temperature, pressure, and duration of metamorphic events based on augen size, matrix foliation, and mineral composition.
Summary
Observing augen gneiss gives visitors a window into high-grade metamorphism, mineral mechanics, and tectonic history. The lenticular “eyes” record differential deformation, while the surrounding matrix records the plastic flow of the rock under extreme pressure. By studying this rock, we can visualize the forces that shaped mountain belts and deep crustal rocks, making it a fascinating and educational EarthCache.
Important Visitor Information
Please remember:
-
The cemetery is closed from dusk until 8:00am. Do not attempt this EarthCache outside of these hours.
-
Be respectful at all times. This is an active burial ground. If there is a funeral in progress, please come back at another time.
-
Stay on established paths, keep noise low, and do not disturb flowers, memorials, or visitors.
-
Follow all posted cemetery rules while completing the EarthCache.
Thank you for treating this space with care and reverence.
AI Content Disclosure
Some of the descriptive text and/or images on this page were created with the assistance of artificial intelligence tools. All information has been reviewed, verified, and edited by the cache owner for accuracy and clarity.
Placed by a Proud Platinum EarthCache Master.
