LIGHTHOUSE PARK GEOLOGY� (SELF-GUIDING TOUR)� DAVID COOK P. ENG (RETIRED)� OCTOBER 2025�
The dominant rock-type on the North Shore is a suite of rocks collectively known as granitic rocks which are at the southern end of an 1800 km long complex, the Coast Plutonic Complex, shown in red on the map of the Province to the right. In Lighthouse Park they are primarily granodiorite. Its composition, in relation to the other granitic rocks, is shown in the QAP diagram to the lower right.
Granitic rocks are light-colored igneous rocks composed of varying percentages of mineral grains large enough to be visible to the unaided eye. These minerals are quartz (Q), alkali feldspar (A) & plagioclase feldspar (P) with lesser amounts of dark minerals such as mica & amphibole. They form by slow crystallization of liquid magma below the Earth's surface. Only the first three minerals are used in the classification.
Quartz
Feldspar
Mineral texture of a typical granitic rock
QAP diagram
1 cm
I will now take you on a journey through Lighthouse Park to see some interesting features of these rocks. Many are features formed during the Fraser Glaciation that waxed and waned between 25,000 & 10,000 years ago during the Pleistocene epoch which lasted from 2.6 million years to 11,700 years ago. The ice reached a thickness of more than 1,800 metres thick.
Other features are related to plate tectonics, weathering, water action & the emplacement of the rocks themselves.
Stop 1: Pothole in granodiorite near the beginning of Beacon Lane Trail
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Stop 1: Remains of a pothole in granodiorite: about 6 metres up a cliff-face, east side of Beacon Lane Trail near the beginning of the trail near the yellow gate
Grinders
Pothole
This pothole was formed by a river probably during an interglacial period. The surrounding topography has since been shaped by subsequent ice action (glaciation).
Water flow
Cook
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Stop 2: Jointing, east side Beacon Lane Trail
Stop 2: Jointing, east side, Beacon Lane Trail
…Vertical joints are due to regional stresses from the west & from the east as the Juan de Fuca plate dives beneath the NA Plate causing stress from the west & the Mid-Atlantic rift causing stress from the east
Horizontal joints due to melting of weight of ice & thus release of the crust & it rises (isostatic release)
Vertical (2 sets) & horizontal joints (1 set) in granodiorite were formed by two different forces…
Cook
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Stop 3: Roche Moutonnee (Sheepback) southern end of Beacon Lane Trail east of the picnic tables
Stop 3: Roche Moutonnee (Sheepback), southern end of Beacon Lane Trail, east of picnic tables
Ice direction
Stoss end (smoothing by ice)
Lee end (plucking by ice)
of jointed bedrock
of bedrock
Iice direction
Roche Moutonnee
Cook
Stop 4 (East Beach): Ultra-basic dyke
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Stop 4 (East Beach): Ultra-basic dyke intruding granodiorite
Granodiorite
Ultra-basic dyke
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Legend
Dykes
Note: Large ultra-basic dykes (in red) mapped by a previous worker all trend in the same direction
Ultra-basic dykes are rocks low in light-colored minerals such as quartz & feldspar and high in dark minerals such as pyroxene, that are intruded as sheets into preexisting rocks (in this case granodiorite) filling joint fractures pulled apart by tectonic forces (see stop 2).
Cook
Stop 5
(Lighthouse Viewpoint): Chatter marks, joints, striae & glacial polish
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Stop 5 (Lighthouse Viewpoint): Features expressed by ice on granodiorite. Chatter marks formed by cobbles imbedded in base of the ice. Striae. Glacial polish. The joint fractures are not a glacial feature. They are described in Slide 7 (Stop 2).
Ice direction
Striae & glacial polish
Joint fractures
Concentric chatter marks
Ice direction (N to S)
8 cm scale
Stop 6 Viewpoint: Roche moutonnee or sheepback & glacial striae
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Stop 6 Viewpoint: Details of a Roche Moutonnee or Sheepback
Stoss or smooth side
of the sheepback
Main part of the sheepback
Lee-end of the sheepback where ice-plucking takes place
Glacial striae
Plucking took place here
8 cm scale
Ice direction in all images is left to right
Cook
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Stop 7 (Shore Pine Point or Jack Pine Point): Synplutonic ultrabasic dyke intruding granodiorite
Stop 7: Synplutonic ultrabasic dyke: Shore Pine Point
A Synplutonic dyke is a dyke intruded when the granodiorite rock is still plastic
8 cm Scale
Synplutonic dyke
Cook
Stop 8 (West Beach Point): Ultrabasic dyke.
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Stop 8 (West Beach Point) : Wide ultrabasic dyke (or dike) intruding the younger granodiorite. See Slide 11 (Stop 4) for explanation of ultra-basic dykes.
Large ultra-basic dyke
Cook
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Stop 9 (Arbutus Knoll): Quartz veins injected into granodiorite & a minor fault
Stop 9 (Arbutus Knoll): Quartz veins injected into granodiorite. One vein is offset by a minor fault
Veins of quartz intruding granodiorite
Minor fault off-setting the quartz vein
8 cm scale
Cook
Stop 10 (trail leading to Arbutus Knoll): Exfoliation of granodiorite
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Stop 10: Exfoliation (separation of successive thin sheets, or spalls) of granodiorite on Arbutus Knoll Trail
Exfoliation is caused by expansion of rock from heating during the day followed by contraction from rapid cooling at night causing the separation of thin slabs from large blocks of rock at the surface. It is a weathering feature & definitely not glacial.
Sheet of granodiorite separated from bedrock (exfoliated) which has remained in place.
Sheets of granodiorite separated (exfoliated) from bedrock & removed.
8 cm scale
Cook