Geological Notes
The
geological map below was adapted from White [Ref. 1]. It shows the approximate
location of the 3 JaneÕs Walks. The bedrock in the area is mainly granite (technically granodiorite to
monzogranite of the South Mountain Batholith, in pink) and metamorphosed sedimentary rocks (sandstones and slates: in gray tones,
Halifax Group; in yellow, Goldenville Group [Ref. 1] well-known for its gold
deposits).
Granite
is exposed south of Colpitt Lake (Walk 2), and at Piggy Mountain (Walk 1). Granite is an igneous rock, relatively
resistant to erosion in comparison to metamorphic sandstones and slates; thus it
typically makes coastal headlands and highlands or mountains. At Piggy Mountain,
the light colored coarse-grained granite has a pink or orange tinge coming from
glistening flat crystals of potassium feldspar
(in rectangular laths of up to 5 cm) and smaller crystals of glassy quartz and shiny mica (<2 cm) of two varieties: one black (biotite) and one
yellow (muscovite). There are a
few white veins or dikes (up to 10
cm wide) of fine-grained granite, with localized clusters of larger crystals of
quartz, feldspar and mica (called pegmatite). These dikes represent remnants of
molten material injected into fractures when the granite magma was solidifying
at about 600oC. ÒBubblesÓ of hot fluid that formed (ÒmiaroliticÓ) cavities
in the molten mass were filled with quartz, the last mineral to crystallize; one
such rounded accumulation (~60 cm) can be seen near the summit of Piggy
Mountain. The granite magma formed at great depth and ca. 380 million years ago
and invaded pre-existing metamorphic sandstones and siltstones such as those
that can be observed at Point Pleasant Park or along the coast at York Redoubt,
or at Williams Lake (Walk 3). The rocks are layered (stratified) and have a
dark colour mostly due to carbon accumulated in a deep ocean in the Cambrian
and Ordovician Periods (ca. 500 million years ago). When the molten magma rose
as a fluid mass (with a consistency of porridge), it invaded, engulfed and
incorporated some fragments of these dark rocks, which are called xenoliths (foreign rocks). Many small xenoliths
can be observed in the granite outcrop at Piggy Mountain and south of Colpitt
Lake.
The
surface we walk on at Colpitt lake or Piggy Mountain represents a level that was
about 6 to 8 km deep when the granite cooled [Ref. 3], at the root of a
mountain chain probably as high as the Himalaya. Over millions of years,
erosion removed all that rock and exposed the rocks we can now observe. The
last erosional episode was during the last Glaciation, which ended about 10
thousand years ago. At that time thick ice (several kilometres thick) moved
predominantly from the North, plucked blocks of rocks that scraped the bedrock
like coarse sandpaper, locally creating smooth NW-SE elongated ridges that
resemble the back of a whale, hence ÒwhalebacksÓ.
When the ice finally melted away, it left large blocks that have travelled
large distances, which we call ÒerraticsÓ;
several examples can be seen in the area.
Large
accumulations of glacial debris (glacial
till) formed elongated oval hills that we call drumlins. One such drumlin is Citadel Hill. A drumlin occurs west
of Williams Lake, and a smaller one across the Royal Nova Scotia Yacht Squadron
in PurcellÕs Cove. Soft drumlins are often exploited for sand and gravel (or
for underground defenses in Citadel Hill and Georges Island) and because they allow the development of
deep roots, are characterized by tall, healthy trees; drumlins are a precious
heritage and should be protected.
Hard
rock such as granite was scoured less than the dark metamorphic sandstones and
siltstones, which tend to be the sites of lowlands, lakes, or coves. Faults
(large fractures) are accompanied by broken rocks and therefore are easier to
erode, forming linear valleys such as the Northwest Arm and the valley occupied
by McIntosh Run (Herring Cove Fault).
Because granite and metamorphic rocks are very impermeable, most of the
groundwater in the backlands circulates through faults and fractures. Granite
and ÒbluestoneÓ metamorphic rocks were quarried in PurcellÕs Cove, for
buildings and fortifications.
Walk
3 - at the eastern end of Williams Lake - traverses an area where the bedrock
is dark, metamorphosed sedimentary rock (in gray in the above geological map).
These rocks are similar to those exposed at Point Pleasant Park, and the Bluestone
Quarry in PurcellÕs Cove. They were originally deposited on a relatively deep
ocean floor during the Cambrian and Ordovician Periods, about 500 million years
ago.
The
predominant mechanism of deposition was one of underwater landslides (turbidity
currents; similar to those that broke underwater cables after the Magnitude 7.2
Grand Banks Earthquake and tsunami in 1929 [Ref. 4]. A jumbled mass of sand and clay stumbles down a slope, and
when it stops, the larger and heavier grains of sand settle first, and the fine
sediment last; the top layer of that sediment often develops small ripples and
dune-like mounds of a few cm, reworked by bottom currents. Each landslide
episode is recognizable as a Ògraded bedÓ, from 5 cm to more than 1 m in
thickness.
Plate
tectonic forces closed the ocean basin [Ref.
4], and lateral
compression caused the once-horizontal beds to fold accordion
like into wavy folds called synclines (concave upwards) and anticlines (concave
downwards). One such syncline can be observed in Point Pleasant Park, and its
extension in Williams Lake (in the map, indicated by a line with head-to-head
arrows). This deformation occurred about 400 million years ago, during the
Acadian Orogeny (Mountain Building Event).
Although
the Williams lake walk does not attain the granite outcrop, the rocks show the
effect of its heat (650oC) when it was intruded. The granite ÒbakedÓ
the folded metamorphic sedimentary rocks in contact with it. The layers were
welded together and the original sedimentary features were obscured; grain size
and mineralogy changed up to 3 km away in the Halifax peninsula [Ref. 3]. However,
the sedimentary features can still be discerned in some large blocks near
Williams Lake.
References:
[1] C. E. White, et al., 2008. Geology of the
Halifax Regional Municipality, Central Nova Scotia in Mineral Resources
Branch, Report of Activities 2007; Nova Scotia Department of Natural Resources,
Report ME 2008-1, p. 125-139.
http://www.gov.ns.ca/natr/meb/data/pubs/08re01/19White.pdf
[2] Potter, D.P., and Goodwin, T.,
2013. A Teachers Guide to the Geology of York Redoubt National Historic Site of
Canada , Geological Survey of Canada, Open File 6964, 31 p.
doi:10.4095/292865
[3] Jamieson,
R.A. et al., 2012. The contact aureole of the South
Mountain Batholith in Halifax, Nova Scotia: geology, mineral assemblages, and
isograds, Can. J. Earth Sci. 49: 1280–1296.
[4] Atlantic Geoscience Society, 2001
The Last Billion Years, Nimbus Publishing, Halifax, Nova Scotia. http://ags.earthsciences.dal.ca/AGS_Pubs.php
Marcos
Zentilli, April 28, 2014