Campbell River’s Big Rock has long been the focus of legend
and has an air of mystery surrounding it. The 30 ft high rock,
sitting perched between the Island Highway in Campbell River and
the ocean, appears to have no physical relationship to anything
surrounding it. There are no other big rocks nearby, no deep holes,
no cliffs or mounds… the area is quite flat. Geologists have
determined the Big Rock is a Glacial Erratic; the area’s
First Nations have differing explanations for the rock’s
presence.
The Rock is used by locals to celebrate all kinds of events –
from Hallowe’en to the Olympics! In October 1986 the Great
Pumpkin emerged from the fog. It was made by sewing 238 orange
garbage bags together using a mile of thread.'
An Inukshuk showed up on the rock at the time of the recent
Olympics on the Mainland.'
FIRST NATIONS' EXPLANATIONS:
One legend has it that a boastful grizzly bear turned to stone
after not heeding the Great Spirit’s warning that if he tried
to jump over the water from the mainland to Vancouver Island, and
missed, he would be turned to stone. He just failed to complete his
attempt to jump from the mainland to Vancouver Island. The tide was
high and his back foot touched the water. As the Great Spirit
warned, the bear turned to stone. To this day, Grizzly bears are
plentiful along the mainland coastal inlets but they are very
rarely found on Vancouver Island (those few that are seen are
recent arrivals, swimming -successfully! - over from the mainland
).
There is more than one legend, each group that settled the area
created their own legend. At the Campbell River Museum, the
animated video story of Big Rock is based on a K'ómoks First
Nations legend, and involves an octopus K'ómoks First Nation
Legend of the Big Rock
GEOLOGISTS’ EXPLANATIONS:
I. What is An Erratic?
An erratic is a piece of rock that differs from the size and type
of rock native to the area in which it rests. "Erratics" take their
name from the Latin word errare, are most often carried by glacial
ice, often over distances of hundreds of kilometres. Erratics can
range in size from pebbles to large boulders. Geologists identify
erratics by studying the rocks surrounding the position of the
erratic and the composition of the erratic itself. Scientists have
determined that many erratics pointed to an ice age in Earth's
past. Geologists have suggested that:
- landslides or rockfalls initially dropped the rocks on top of
glacial ice or
- the glaciers picked up rocks by scouring the ground
underneath.
The glaciers continued to move, carrying the rocks with it
(ice-rafting – sounds fun!). When the ice melted, the
erratics were left in their present locations. Glacial erratics
give us information about the direction of ice movement and
distances of transport.
II. How Was It Formed? Erratics are formed by glacial ice
erosion resulting from the movement of ice.
1) Glaciers erode by multiple processes, and each of these
processes can result in Erratics with differing
characteristics.
a. Abrasion/scouring: In an abrasion process, debris in the
basal ice scrapes along the bed, polishing and gouging the
underlying rocks, similar to sandpaper on wood, producing smaller
glacial till.
b. Plucking (pull something off or out). Glaciers crack pieces
of bedrock off in the process of plucking, producing the larger
erratics.
c. In Ice Thrusting, the glacier freezes to its bed, then as it
surges forward, it moves large sheets of frozen sediment at the
base along with the glacier.
d. Ice Lens formations with the rocks below the glacier spall
off (spall = break up into small chips, flakes, or splinters)
layers of rock, providing smaller debris which is ground into the
glacial basal material to become till. An ‘ice lens’ is
formed when moisture, flowing through soil or rock, accumulates in
a specific spot. The ice initially accumulates within this spot,
and, as long as the conditions remain favorable, continues to
collect in the ice layer or ice lens, wedging the soil or rock
apart.
2) Another formation option is that rock avalanches fall onto
the upper surface of the glacier (supraglacial). Rock
avalanche–supraglacial transport occurs when the glacier
undercuts a rock face, which fails by avalanche onto the upper
surface of the glacier. The characteristics of rock
avalanche–supraglacial transport include:
a. Monolithologic composition - a cluster of boulders of similar
composition are frequently found in close proximity. Mixing of
multiple types of rocks throughout the glaciated basin, has not
occurred.
b. Angularity - the supraglacially transported rocks tend to be
rough and irregular, with no sign of subglacial abrasion. The sides
of boulders are roughly planar, suggesting that some surfaces may
be original fracture planes.
c. Great size - the size distribution of the boulders tends to
be skewed toward larger larger boulders than those produced
subglacially.
d. Surficial positioning of the boulders - the boulders are
positioned on the surface of glacial deposits, as opposed to
partially or totally buried.
e. Restricted areal extents (which means ‘magnitude of the
area) - the boulder fields tend to have limited areal extent; the
boulders cluster together, consistent with the boulders landing on
the surface of the glacier and subsequently deposited on top of the
glacial drift.
f. Orientations - the boulders may be close enough that original
fracture planes can be matched.
g. Locations of the boulder trains - the boulders appear in
rows, trains or clusters along the lateral moraines as opposed to
being located on the terminal moraine or in the general glacial
field.
III. How Did It Travel?
1) Glacier-borne erratic: Erratics provide an important tool in
characterizing the directions of glacier flows, which are routinely
reconstructed used on a combination of moraines (a mass of earth
and rock debris carried by an advancing glacier and left at its
front and side edges as it retreats), eskers (a long narrow winding
ridge caused by a glacial stream), drumlins (a long narrow ridge
left by a glacier – blunt at one end and tapering at another)
, meltwater channels, and similar data. Erratic distributions and
glacial till properties allow for identification of the source rock
from which they derive, which confirms the flow direction. Erratic
materials may be transported by multiple glacier flows prior to
their deposition, which can complicate the reconstruction of the
glacial flow.
2) Ice-rafted erratic: Glacial ice carries debris of varying
sizes from small particles to extremely large masses of rock along
with their flow. This debris is transported to the coast by glacier
ice and released during the production, drift and melting of
icebergs. The location and altitude of ice-rafted boulders relative
to the modern landscape has been used to identify the highest level
of water in proglacial lakes (e.g., Lake Musselshell in central
Montana) and temporary lakes (e.g., Lake Lewis in Washington state.
Ice-rafted debris are deposited when the iceberg strands on the
shore and subsequently melts, or drops out of the ice floe as it
melts. Hence all erratic deposits are deposited below the actual
high water level of the lake; however the measured altitude of
ice-rafted debris can be used to estimate the lake surface
elevation.
3) Large erratics: Large erratics consisting of slabs of bedrock
that have been lifted and transported by glacier ice to
subsequently be stranded above thin glacial or fluvioglacial
deposits are referred to as glacial floes, rafts (schollen) or
erratic megablocks. Erratic megablocks have typical length to
thickness ratios on the order of 100 to 1!These megablocks may be
found partially exposed or completely buried by till and are
clearly allochthonus (found in a place other than where they were
formed), since they overlay glacial till (unsorted glacial
sediment). Megablocks can be so large that they are mistaken for
bedrock until underlying glacial or fluvial sediments are
identified by drilling or excavation. Such erratic megablocks
greater than 1 km2 in area and 30 meters in thickness can be found
on the Canadian prairies! An erratic megablock located in
Saskatchewan is 30 km by 38 km and up to 100 m thick! Their sources
can be identified by locating the bedrock from which they were
separated; several rafts from Poland and Alberta were determined to
have been transported over 300 km from their source!
4) Nonglacial erratics: An erratic is any material which is not
native to the immediate locale, but has been transported from
elsewhere. The most common examples of erratics are associated with
glacial transport, either by direct glacier-borne transport or by
ice rafting. However other erratics have been identified as the
result of kelp (seaweed) holdfasts, which have been documented to
transport rocks up to 40 cm in diameter, rocks entangled in the
roots of drifting logs, and even in transport of stones accumulated
in the stomachs of pinnipeds (eg walrus, sea lions, seals) during
foraging!
To log this cache, you must e-mail us some information regarding
the site that you learned while there. This could be done by
answering the following questions:
1. a) Given the information provided above, which formation
process do you believe resulted in the depositing of the Big Rock
in Campbell River: Glacial Erosion? Rock Avalanche-Supraglacial
transport?
b). What 2 of the Big Rock's characteristics lead you to this
conclusion?
2. a) Which transportation method do you believe the LEAST
likely method which resulted in the Big Rock being located here:
Glacier-borne erratic, Ice-rafted erratic, Large erratic or
Non-glacial erratic?
b) What is your reasoning?
3. What navigational aids can you see while standing at the
Rock?
4. Find the Benchmark (a circular metal disc attached to the
Rock with some writing on it). What direction does the line run in
the middle of the disc: vertical, horizontal, diagonal?