JACK POINT HONEYCOMBS
The odd erosion patterns you'll find at this earth cache are
contained within the sandstone of the upper Nanaimo Group
formation, formed approximately 65 million years ago. Just before
the Nanaimo Group was formed, the micro-continent of Wrangellia
(which included most of Vancouver Island and some other areas)
joined onto the edge of North America. When these two continents
collided, there was great geological upheaval - rocks uplifted and
mountains were created. These new mountains were subjected to rapid
erosion, and the mud, sand, and silt that swept rapidly down these
young peaks was deposited into a warm, stagnant, marine basin.
Eventually the sediment deposits became the very same sandstone
you're looking at today.
What's causing the honeycombing and cavernous weathering at this
site? Wind and waves play a role, but largely the cause is thought
to be salt-weathering-related (Doe 2010). As waves crash up on the
rocks and as fresh rain water hits salt-encrusted rocks, the salty
water seeps into the sandstone through tiny cracks and holes. Just
how the salt drawn into the rock causes erosion is still debated by
geologist. The most popular theory is that crystallization pressure
is responsible. When water that is supersaturated with salt changes
from a solution to a mix of crystals and water via evaporation, the
salt grows in volume. This increased volume ups the pressure on the
micro cracks within the sandstone, causing erosion. Another theory
of salt weathering is that highly concentrated salt solutions
weaken the mineral "cement" that holds the sandstone together.
Salt-weathering is a big problem in Europe, where it causes
deterioration of heritage buildings and statues.
Sources: Doe, N.A., Salt-weathering of upper Nanaimo Group
sandstone, SHALE 23, pp.35-56, March 2010. Mustard, P.,
(1994) The Upper Cretaceous Nanaimo Group, Georgia Basin, British
Columbia, in Geology and geological hazards of the Vancouver
Region, southwestern British Columbia, J. Monger (ed),
Geological Survey of Canada Bulletin 481, p. 27-96.
THE LOCATION
This earthcache is located in Biggs Park at Jack Point. This is
one of my favourite parks in Nanaimo because of the natural
scenery, history of the area, interesting artwork, and bounty of
seabirds to observe. Parking is at the Jack Point parking lot: 49
08.973, 123 53.243. It takes about 1.5-2 hours roundtrip to walk
from the parking lot to the very tip of the point and stop at all
the geocaches along the way. If you're walking to the point (and
you should because it's beautiful!), keep an eye out for a series
of caves on the land side of the trail. There's a couple of them
between this cache and the point.
If you decide to go down to the water's edge from the bench,
watch where you step. Some of the rocks that look solid are
actually just the top of sandstone that's been carved out by
erosion. If you lightly tap the rocks and hear a hollow sound,
you've found one of these spots. I suggest you place your feet
elsewhere.
Don't miss the modern petroglyphs at N 49° 08.964 W 123° 53.284
(home of GC2EEND) and N 49° 09.282 W 123° 53.471.
HOW TO LOG
To log this cache send the answers to the following questions to
the cache owner. Logs posted without answers will be deleted:
1. Describe the "honeycombing" you see (colour, texture, size,
etc).
2. Depending on the tides, you might spot several shallow,
circular sandstone areas that have been subjected to erosion,
ringed in white. What is the white ring around these areas? How
does it contribute to erosion? If you don't see the white-ringed
areas because the tide is too high, look at the photo below.
2. There is a bench at GZ. What are the names of the people to
whom the bench is dedicated?
3. Go to 49 09.388, 123 53.534 and you'll discover one of my
favourite sandstone structures. Describe the structure. What does
the sandstone look like at the top versus the bottom? Why do you
think there's such a difference between the top and bottom? Bonus
point: what vegetable does this structure remind you of?
4. Take a photo and post it (optional).