How to get there
You need to find the Ambidextrous trail on the lower slopes of Ledgeview / McKee Peak. The Geocaching Trails map shows the maze of the trails in reasonable detail to get you to the posted coordinates. OpenStreetMaps also shows the trails in the area in detail.
Background Information
Tafoni
Tafoni are small cave-like features found in granular rock such as sandstone, granite, and sandy-limestone with rounded entrances and smooth concave walls, often connected, adjacent, and/or networked. They often occur in groups that can riddle a hillside, cliff, or other rock formation. They can be found in all climate types, but are most abundant in intertidal areas and semi-arid and arid deserts .
Conglomerate
Conglomerate is a coarse-grained sedimentary rock that is composed of a substantial fraction of gravel-size clasts, (e.g., granules, pebbles, cobbles, and boulders) larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification of gravel. Conglomerates typically contain finer grained sediment, (e.g., either sand, silt, clay or combination of them) called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay. In terms of origin and depositional mechanisms, conglomerates are closely related to sandstones and exhibit many of the same types of sedimentary structures.
How Does Tafoni Form?
Sandstone is permeable rock; the minerals that hold the sand grains together in the sandstone are water-soluble. A chemical weathering process known as ‘solution’ dissolves the microscopic mineral cements that hold the sand grains together in the sandstone. Falling rain picks up carbon dioxide to make carbonic acid that dissolves the cements: H2O + CO2 —->> H2CO3. This is natural acid rain which has persisted for many, many years; long before air pollution, however at reduced levels.
In the moist temperate environment of the Lower Mainland, this weathering process involves algae that colonize rock surfaces. When algae form a micro-thin layer on rock surfaces, the algae inhibit absorption of water and so hamper solution. Algae may or may not be readily apparent on rock surfaces; however, these cliffs are pervasively colonized with algae in various types and colours.
Acidic solution attacks an entire rock face, which may weather away evenly. When something occurs to damage the surface and remove, or kill the algae, solution attacks the cement immediately, and the sand grains fall off the steep surfaces. Each falling sand grain leaves a tiny pit in the rock which can be exploited by solution. The holes, only millimeters across, grow inward as solution migrates into the rock.
Surface damage could be caused by freeze-thaw expansion and fracture of the rock, the impact of a rockfall, a hard freeze that locally kills the algae, or a particularly permeable contact between rock layers.
Algae may re-establish on the sunny surfaces at the opening of the developing tafoni hole. Once algae recolonize the rock surface, solution is again reduced. However, algae may not grow well on shaded rock surfaces inside the entrance to a solution hole, so acidic water can dissolve cement more readily there and the holes migrate inward.
Since sandstone is permeable rock, tafoni may grow laterally behind the algae-protected surface, and the developing tafone (singular of tafoni) is larger on the inside. The process is variable and complicated.
Logging Requirements
Send me your answers to the following questions. Please, Do not post your answers in your log.
- What discourages the formation of tafoni?
- What is the name of the process that causes tafoni to form?
- Which of the types of surface damage listed above are the most likely to have taken place at this site?
- Select a tafone, and describe the interior surfaces:
- Describe the inner surfaces of the tafone you selected (ie. smooth, rough, bumpy, or what?)
- Does the tafone you selected seem to be growing in sandstone or conglomerate?
- Measure the size of some of the conglomerate clasts (e.g., granules, pebbles, cobbles, and boulders) in the tafone you selected. If there are none in the tafone you selected, look for conglomerate clasts elsewhere along the tafoni cliff and measure those instead.
- Optional: Post a photo of you (and your group) anywhere along the tafoni cliff.
References
This Earthcache was created for the Fraser Valley Cache Bash 2018.