We appear to be innately curious about the origins of many things. Take as an example the really big questions like the origin(s) of life and bigger still, the origin of the universe. However, we will leave those questions aside to focus on something more local, the Petitcodiac River.

Who am I? I am a retired biology professor from the Université de Moncton with a passion for rivers, streams, and lakes. In the following months, this blog will look at the physical features of the Petitcodiac River and the natural communities that depend on it for survival. Often these two facets will be combined or interrelated.

Geology is one of the main factors that determines a river’s characteristics. As a first step let’s look at the surrounding terrain and how it determines the direction of flow of the Petitcodiac River. The river follows a west to east drainage pattern as if it was on route for Shediac Bay. Instead, it takes a right-hand turn at Moncton to eventually exit into the Bay of Fundy and not the Atlantic Ocean. The expression “the bend” is often used to refer to this location and is believed to be derived from the “Mi’kmaq word meaning “bends like a bow” or from a Maliseet word meaning “sound of thunder”.1

Source: Wikipedia – Petitcodiac River (modified).

Why does the river bend at this location ? This is perhaps, the wrong question to ask, rather one should ask, why does it not immediately flow into the Bay of Fundy? The river finds itself confined by the Anagance Ridge to the north and the Caledonia Mountains to the south. On the north side of the Caledonia Mountains we find the Pollett River, the Little River and Turtle Creek that all flow into the Petitcodiac River. On the south side of the Caledonia Mountains streams and rivers flow into the Bay of Fundy. A break in the Caledonia Mountains allows the Petitcodiac River to descend through a large valley and into Shepody Bay, then Chignecto Bay and finally the Bay of Fundy. The drainage pattern at the bend is north to south with higher elevations to the north and east of the “bend”. It is tempting to ascribe this final straight section of the Petitcodiac River to an ancient glacial channel, but there is nothing that would support this conclusion.

The Caledonia Mountains are old, composed of precambrian rocks dating back from 740 to 550 million years. Note that dates are approximate and can vary from one reference to another. These mountains are mainly composed of igneous rocks (volcanic in origin) with much younger Carboniferous strata along the eastern boundary (Carboniferous period – 59.2 to 299 million years ago). Along the northeast border of the Caledonia Mountains one can find beds of Mississippian red conglomerate, sandstone, and siltstone ( 358.9 to 323.2 million years ago, see photo). At this time what was to become southern New Brunswick was located at the equator. This southern portion of New Brunswick is currently classified as part of the “Avalon Terrane” but back then was north of Protogondwana, the future supercontinent of Gondwana (200 million years). A “terrane” is a marginal part of a previous continent that has broken off. New Brunswick is composed of several slices of terranes, a bit like a birthday cake slice turned on its side with each layer a terrane.

Jumping forward in time to about 340 million years, the start of the Carboniferous period, there were deep lakes in southern New Brunswick, including one within what would become the upper Petitcodiac River.The Anagance Ridge to the north of the Petitcodiac River is also composed of carboniferous rocks3. This is the famous period of coal formation, about a span of 60 million years. We are still at the equator, and the supercontinent of Pangea is about to be formed (335 million years). These deep lakes lead to the formation of shale, and with the contribution of abundant microscopic organisms gave rise to the oil and gas in Hillsborough which were exploited for about eighty years3. During the Carboniferous period the  Panthalassica Sea invaded as far inland as far as central New Brunswick4. Repeatedly over 15 millions years, the Windsor Sea rose and fell, evaporating at each instant which gave rise to the gypsum deposits in Hillsborough, a village located in the lower part of the Petitcodiac watershed3.

There still remains a series of events to bring us to present day. There is the breakup of Pangaea, the formation and breakup of Gondwanaland, the birth of the Atlantic Ocean, and the migration to the north of what would become North America. However, we will pass over the details of these events only to say that the future Petitcodiac River lay within Pangaea and was originally located near the equator. 

     (Source: US Geological Survey Publication)

The final great event I wish to address, is the period of the last glaciation in New Brunswick, the Wisconsinian, which is the most recent stage of the Pleistocene Ice Age. In fact the ice has repeatedly advanced and retreated from New Brunswick are we are currently living within an interglacial period. As the glaciers eventually covered the Caledonia Mountains. New Brunswick has only been glacier-free for 10 000 years.

In the next issue I will look at dissolved oxygen, what determines its concentration and what levels are necessary to support life in the Petitcodiac?


1 Dr. Peter Paul interview with anthropologist Harald E.L. Prins and Bunny McBride, Hallowell, Maine, 12/02/1988. In Memoriam: Peter Lewis Paul, 1902-1989, edited by K. Teeter, 19-21. Hull:Canadian Museum of Civilization, Canadian Ethnology Service. Mercury Series Paper 26, 1993. https://en.wikipedia.org/wiki/Petitcodiac_River#cite_note-2 .

2 Zelazny, V. F., Martin, G. L., Toner, M., Gorman, M., Colpitts, M., Veen, H., … & Roberts, M. (2007). Our landscape heritage: the story of ecological land classification in New Brunswick. New Brunswick Department of Natural Resources, Hugh John Fleming Forestry Centre, Fredericton, NB E3C 2G6, Canada.

3 Fensome , A. & Williams , G. L. (eds). 2001. The last billion years: A geological history of the Maritime Provinces of Canada. Atlantic Geoscience Society/ Nimbus Publishing, Halifax, Nova Scotia.

4 New Brunswick. https://en.wikipedia.org/wiki/New_Brunswick

5 Glacial history of New Brunswick. https://www2.gnb.ca/content/gnb/en/departments/erd/energy/content/minerals/content/GlacialGeology.html