Sambro Lighthouse
The Sambro lighthouse was built in 1758 and is the oldest working lighthouse in North America. It is so old that it is no longer known where its granite blocks came from, so geologists tried to determine the source.
Nova Scotia's first elected government passed an act to build the lighthouse because of the need to make shipping safer at the mouth of Halifax harbour. Construction was financed partly by a tax on spirits and partly by the proceeds of a lottery. (One thousand tickets were sold at £3 apiece with prizes ranging as high as £500.)
The lighthouse is made of granite blocks with wood shingles around them to protect the mortar that binds the blocks.
The lighthouse was restored in 1998 by the Canadian Coast Guard. As part of that initiative, a study was done to try to determine where the granite had been quarried.
Several theories had been floated over the years. One suggested the blocks had been ballast on a ship from Massachusetts. Various quarries were suggested as the source, including five quarries shown on early geological maps of the Halifax area and the possibility of a quarry in the Duncan’s Cove-Chebucto Head area. It was also suggested the stone could have been extracted on-site since Sambro Island is granite.
Another theory was that the granite was from Fortress Louisbourg. After capturing Louisbourg a second time in 1758, the British wanted to make sure it could not be used by the French again, so miners tunneled under its defenses and set off explosives to destroy them. Some of its cut stone was reused in buildings in Halifax and Sydney, including in St. George’s Church, Sydney’s first building. However, Louisbourg was mainly built of sandstone, not granite, so the fortress could not have been the source of granite for the Sambro Island lighthouse.
To determine the source of the granite, a forensic petrological study was done by the Nova Scotia government’s mines branch in partnership with the Nova Scotia Lighthouse Preservation Society. (Petrology is a branch of geology that focusses on the origin, small-scale structure, and composition of rocks.)
Three chips of granite were taken from the lighthouse, and granite samples were extracted on Sambro Island and at various points along the shore between Sambro and Chebucto Head.
The samples were compared with a database of geochemical data and mineral chemistry studies of granitic rock in the Halifax area. The analysis showed that the characteristics of the lighthouse’s granite were consistent with some parts of what we call the Halifax Pluton, or granite bedrock in the Halifax area. (A pluton is a body of rock made of cooled magma that rose toward earth’s surface through pre-existing rocks because magma is less dense than solidified rock. Plutons solidify several kilometers underground but are often eventually exposed at surface by overlying rock being eroded.)
Since the granite blocks were consistent with granites in the Halifax Pluton, the blocks did not come from Massachusetts as ballast. They are also not from Sambro Island because the island is not part of the Halifax Pluton and its granite has different characteristics. The blocks also did not originate at the Brookfield quarry in nearby Terence Bay since the lighthouse’s granite also did not match Terence Bay’s.
The analysis found that the lighthouse’s granite blocks likely came from somewhere on Chebucto Head because the blocks and the Chebucto Head granite are a close match. The precise location on Chebucto Head was not determined.
The Sambro Island lighthouse study was done for historical purposes, to increase our knowledge of the lighthouse.
However, forensic petrology, and geology in general, are often used to understand where rock came from for quite different reasons. For example, when restoring stonework in historical buildings, it is often necessary to find the source of the original stone, or at least a replacement stone that matches the original, so the renovations appear as seamless as possible. However, this is often harder than it sounds.
All stone is unique, its characteristics determined by the geological circumstances in which it formed. For example, granite from one site can look quite different from granite from another site. Stone can even have significant variations in colour and composition within the same quarry or area.
Also, the original rock is no longer in a quarry because it was extracted, so it is impossible to do a direct comparison between extracted stone and stone that is still in the ground. That is why characteristics like mineralogy, texture and chemical composition are analyzed.
See how replacement stone for the recent Halifax Armoury renovation was found at https://notyourgrandfathersmining.ca/halifax-armoury-renovation
Geology is even used to help investigate crimes, as was the case when a murder victim was found in an historical Nova Scotia copper mine: https://notyourgrandfathersmining.ca/lansdowne
