Click here to skip to the content

Sugarloaf Mountain


Reg Wilson, Geologist, Geological Surveys Branch, Energy and Mines, New Brunswick

My name is Reg Wilson and I’m a geologist with New Brunswick’s Geological Surveys Branch and my job is to carry out regional bedrock mapping surveys and produce geological maps like the ones you see behind me.

Among many other areas, my work has taken me to Campbellton, where the most prominent local landmark is Sugarloaf Mountain. The Campbellton area is underlain by a great thickness of volcanic rocks dating from the Early Devonian Period, more than 400 million years ago. Many people who are aware of this volcanic heritage, as well as some who are not, believe that Sugarloaf, with its distinctive cone shape, is the actual vent where lava and ash, etc. once erupted.

In fact, this is only partly correct. First, consider that there may have been an additional thickness of 3 or 4 kilometres, by some estimates, of volcanic and sedimentary rock that was deposited on top of the rocks that form the present land surface; and that hundreds of millions of years of erosion followed by glacial scowering during the recent Ice Ages, has created the landscape we see now.

Sugarloaf is composed of a dacite porphyry that represents magma that cooled and crystallized in a volcanic neck, or conduit, part of the “plumbing system” through which magma moved from a deep subsurface chamber, up to the ancient (paleo)surface. These dacites are harder than the surrounding rocks and more resistant to erosion, which explains why Sugarloaf stands out as such a prominent feature.

So whenever you look at one of our landscapes, bear in mind that what you see is the product of slow erosion operating over vast spans of time. Similar erosional remnants are present elsewhere in New Brunswick; for example, Bald Peak in the Tobique region is also a dacite porphyry that crystallized in a volcanic pipe.

The thick black line on this image represents the present erosional land surface, after erosion of a thick layer of volcanic and sedimentary rocks that once lie above it, and Sugarloaf Mountain would be here at this volcanic conduit or neck which is composed of more resistant or harder rock and stands out as a peak compared to the surrounding softer rock.

This image will demonstrate what I was trying to explain. This is not specific to Sugarloaf in particular but it gives a general picture of the subvolcanic magma system that supplies lava to a volcano. And this part up here would be the rocks that once overlie the present surface. The present surface now being the erosional surface outlined by this dark black line and Sugarloaf Mountain being the volcanic neck of hard resistant rock that is more resistant to erosion.