Thanks to cache crop for graciously sharing the space on this beach. It was their cache: GC143N9 - String of Pearls "E" that brought me here to see these rocks. Please ensure you visit & log both (and don’t forget to write down the clue to the final of their series!).
To the beach...
Access to this Earth Cache is via the public beach trail off Madrona Drive. Please use the parking coordinates on Cromar Road and be respectful of no parking areas. You will need to time your visit with low to mid tide to see the rocks that you need. They may be visible at high tide, but you might get your feet wet! Please see the link for best tidal times to plan your visit: Swartz Bay Tidal charts
Rocks at GZ...
To log this earth cache – answer the questions below. You may either email me or use the message centre. You may log your cache to keep your cache finds in order, sending in your answers within a few days of doing so and I will let you know if your answers are okay!
At GZ, look down at the stone involved.
1) Identify what type of rock you see the honeycombing on at GZ? Do you see these formations on any other types of rock here? Why is it important in the honeycomb formation here?
2) Look closer at the honeycombing. You should see an abundance of holes, but look at some of the largest and smallest. What are their averages sizes of the two sizes you see? Please measure or estimate the holes width and depth, in millimetres. Where are the larger ones in respect to the smaller ones? Describe why you think these variety of sizes are there?
3) You should see some bedding planes/fracture lines in the rock, creating some unique patterns on honeycombing at certain places. How large are these sections (on average)? What direction do you see them lying? (i.e. horizontal, vertical, are they angled/parallel with the ground and each other?).
4) What specific forces at this location have lead to the unique honeycombing here? Use your knowledge from the page write up as well as GZ observations for your answer.
5) Of course, optional for earth caches, but views at the shoreline are always beautiful and make the cache more special. Attach a picture at GZ of the view, the beach, the cool rocks, but not enough to give away all the answers…
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Honeycomb Weathering – North Saanich Rocks
As you would expect, honeycomb weathering takes its name from rock weathering that form into shapes akin to honeybee hive honeycombs. Honeycomb weathering can also be referred to as tafoni, fretting, cavernous weathering, alveolar weathering, stone lattice, stone lace weathering and sometimes boxwork, and fluting. It is a type of rock weathering that can be found both in sea and wind ravaged areas, in hot or cold deserts, and is seen on the stone faces of many historical buildings. Some examples of honeycomb weathering have even been found on Mars! It occurs in many different types of rocks including sedimentary (the most common), crystalline igneous, and even volcanic rocks, spread throughout the world.
World wide honeycomb weathering
There have been a few hypotheses developed to explain rock honeycombing, such as freezing-thawing effects, chemical weathering, thermal changing, varying moisture content, wind erosion, and salt weathering. Honeycombing occurs most often on inclined surfaces facing the sun with high tide immersion. The development of honeycomb in the intertidal zone in the Gulf Islands is helped by the especially favourable pattern of tides in the Strait of Georgia (Salish Sea). Low tide in the summer is often when the sun is strongest during the day giving the rock ample opportunity to dry out before being wet again. This salt water action can also be mimicked by the wicking up concentrated seawater by capillary action; by being wetted by waves; by absorbing spray; and by “groundwater” containing salt making its way to cliff faces in outflows. This is the most commonly accepted theory to explain honeycomb weathering: salt crystallization or haloclasty. The spray of salt water into the cracks and crevices of soft rock initiates honeycomb formation. When exposed to the sun, saline water within the pores of the rock is drawn to the surface. The water eventually evaporates, leaving behind salt crystals in the cracks and crevices. As they heat up, the salt crystals expand, and exert pressure on the softer elements in the rock. This leads to granular erosion (friction), new microfractures, and weathers the rock. Wind, rain, high tides, and more salt water spray clear out the eroded material and serve to continue the process of honeycomb weathering, creating larger and deeper formations. A process of chemical weathering may also come to play with the highly saline conditions.
Modified from Doe, N.A., November 2011.
In non-salt water locations where honeycomb weathering has been noted, haloclastic theory can’t be the causal force. Instead, scientists believe that there are a variety of factors at work. One thought is that fossilized algae can create small pockets of soft material amidst all the harder sandstone. The outermost layer will eventually erode due to wind, and rain, leaving behind a small depression. Once the outer layer is removed, the next layer weathers away, and then the next, eventually creating the formation. In honeycombed rocks, sometimes fracture lines and bedding planes can be visible. When present, these clearly influence honeycombing geometry. Variations in permeability and porosity from one stratum to the next result in different sizes of honeycomb holes. These may wear at different rates, causing intricate, repetitive patterns. Rows of holes often trace out bedding planes and fractures that are sources of moisture working its way to the surface. In some strata there aren’t any holes at all.
Congratulations to cache crop for the FTF! Thanks again for letting me share your beach!
Sources:
1. Doe, N.A., What makes holes in sandstone, SHALE 9, 2d ed., pp.12–40, August 2004.
2. Doe, N.A., Salt-weathering of upper Nanaimo Group sandstone, SHALE 23, pp.35–56, March 2010.
3. Doe, N.A., The geometry of honeycomb weathering of sandstone, SHALE 26, pp.31–60, November 2011.
4. Doe, N.A., Colonial corallites and honeycomb holes in sandstone, SILT 15, July 2015. Accessed 2018 Aug 15.
5. Rodriguez-Navarro, C., Doehne, E., Sebastian, E. "Origins of honeycomb weathering: The role of salts and wind", GSA Bulletin, vol. 111, no. 8, pp. 1250-1255, 1999.
6. Wikipedia: Honeycomb weathering. https://en.wikipedia.org/wiki/Honeycomb_weathering