Marshdale Gabbro
There are countless small, old quarries throughout Nova Scotia that helped build the province. This former quarry in Pictou County is an example.
Gabbro was extracted in Marshdale in the mid-1960s to provide rock aggregate for the building of the Harvey Veniot Causeway between Brown's Point and Abercrombie Point, connecting Pictou and New Glasgow. The causeway opened in 1968.
The quarry has been mostly inactive since that time except for the periodic removal of small amounts of gabbro rock left on the ground from the operation.
Gabbro is sometimes called “black granite,” which describes its appearance but is inaccurate since gabbro is a different type of rock than granite. Gabbros are typically dark grey, black or greenish because they contain dark-coloured minerals like amphibole, olivine and biotite. Granites tend to have light-coloured minerals like quartz, feldspar and muscovite.
The gabbro intrusion that the quarry worked was at least 90 metres long by 70 metres wide. (In geology, an intrusion is a body of rock that formed from magma cooling and solidifying before it reached the surface. The magma intrudes into rock that is already there by rising up through cracks in the rock.)
The site is interesting geologically because the gabbro is mostly surrounded by Carboniferous rocks - rocks that formed during the Carboniferous period, 360 to 299 million years ago. Carboniferous means "coal-bearing” and it was during this period that Nova Scotia’s coal deposits formed. The gabbro intruded into the Carboniferous rocks, which means the gabbro is younger than the rock that surrounds it. This is significant because it had been believed that Nova Scotia did not have intrusive rocks in Carboniferous rocks, and the Marshdale gabbro proved that belief to be incorrect. It is still not known when exactly the gabbro formed.
A prominent 10-metre-high spire of gabbro remains at the quarry’s north end (middle of pictures, partially hidden by trees). The spire shows the original height of the land prior to quarrying operations.
Another picture below shows an unusual piece of rock, which is not gabbro, taken from the spire. A number of geologists have examined it over the years but it is still not known what type of rock it is, another geological mystery of the site.
There area has a second gabbro intrusion, referred to as the Marshdale Schoolhouse gabbro because it outcrops as a small knoll behind the site of the old Marshdale schoolhouse. A Geological Survey of Canada report from 1892 reports the area behind the school had several prospect pits dug in search of specularite which occurred within the gabbro. (Specularite is a type of hematite, an ore iron, meaning iron is often extracted from it.)
Aggregate is the most-mined material in the world because it is used in virtually all infrastructure including homes, roads, schools and hospitals. Aggregate makes up about 80% of concrete and 94% of asphalt.
Building one kilometre of highway takes about 18,000 tons of aggregate - enough rock to fill four Olympic-sized swimming pools. Nova Scotia needs 10-15 million tons of new aggregate each year to build and maintain our infrastructure.
The causeway was named for Harvey Alfred Veniot who was born in Pictou in 1915 and served as MLA for Pictou West from 1956 to 1974, including seven years as Speaker of the House. He also served as Minister of Municipal Affairs and Minister of Agriculture and Marketing from 1968-1974. After his political defeat in 1974 he returned to practising law and in 1978 was appointed judge in the Provincial Court of Nova Scotia. He passed away in 2009.
