Geological Notes

Map White adapted.pngThe geological map below was adapted from White [Ref. 1]. It shows the approximate location of the 3 Jane’s 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. “Bubbles” of hot fluid that formed (“miarolitic”) 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 Pictures 2.pngxenoliths 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 “whalebacks”. When the ice finally melted away, it left large blocks that have travelled large distances, which we call “erratics”; 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 Purcell’s 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.

Picture 3.pngHard 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 “bluestone” metamorphic rocks were quarried in Purcell’s 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 Purcell’s 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 “graded bed”, from 5 cm to more than 1 m in thickness. 

Plate tectonic forces Williams Lake.pngclosed 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 “baked” 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

http://geoscan.nrcan.gc.ca/starweb/geoscan/servlet.starweb?path=geoscan/downloade.web&search1=R=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