Québec

That area was "A Natural Fortress"

First of All: That Area Was
"A Natural Fortress"

(1,000,000,000 to 9,001 years ago)

The unique geography of the Québec City area made it, even before there was any Fortified Wall, "a natural fortress", with high cliffs dominating the St. Lawrence River on both sides of that river (the Québec City north shore and the city of Lévis south shore).


The Québec City area is located at the junction of three "geological regions": the Canadian Shield (including the Laurentian Mountains), the St. Lawrence Lowlands and the Appalachians (including the Appalachian Mountains) … which indicates that it must have had a very tumultuous geological past.
© 2008 Her Majesty the Queen in Right of Canada / Brochure entitled "Québec, a Fortified City: Geological and Historical Heritage", page 3 / Graphic reproduced with the permission of Natural Resources Canada.

In order to better understand where this "natural fortress" came from, we have to look at the geological history of the Québec City area, which dates back from 1,000,000,000 years ago (yes, one "billion" years ago), when our planet had attained approximately its present size and there was only one supercontinent in the middle of only one superocean.

In fact, the previous existence of "a natural fortress" in the Québec City area can better be explained with reference to plate tectonics.


The oldest known supercontinent was called "Rodinia" and formed some 1.1 billion years ago, when there also was only one superocean, which was called the "Panthalassic Ocean" (or "Panthalassa") and eventually became the present-day Pacific Ocean. At about that time, the Laurentian Mountains also formed in the continent "Laurentia" (the future North America), and their high peaks were very similar to the present-day Himalayas. Today, the Laurentian Mountains are part of the Québec City area to the north of the St. Lawrence River and belong to the Canadian Shield.
Image Credit: NASA / By H. L. Levin / http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html / The supercontinent Rodinia; from "The Earth through Time" / Public Domain.
Image Source: en.wikipedia.org/?wiki/File:Rodinia.jpg / Uploaded on July 27, 2009 / As "The supercontinent Rodinia 1,100 million years ago".
According to the theory of plate tectonics, our planet's crust was broken up into moving plates that rubbed against one another and became modified over time. In the area where Québec City lies today, mountains rose up when the continents collided, and oceans or seas formed when they split apart. These phenomena occurred at an imperceptible rate of 1.6 to 2.4 inches (4 to 6 centimeters) per year, which is about how fast our fingernails grow. Over tens of millions of years and then hundreds of millions of years, however, the position of the continents changed substantially. During such a long time frame, the position of the continents relative to the poles and the equator changed to the point of affecting the climate. We have to remember that even if the continents appear to be stationary when we look at them today because they move so slowly, they have moved over time in the past, are still moving in the present and will move over time in the future. The same logic applies to the different climatic conditions those continents have experienced over time in the past, are still experiencing in the present and will experience over time in the future. In the particular case of the Québec City area, these considerations explain why that area became, gradually and over a very long period of time, "a natural fortress".


The supercontinent Rodinia started to split apart about 750 million years ago. As for the Laurentian Mountains that are part of the Québec City area, erosion wore them down progressively, over a very long period of time, to the size they are today. Geologically speaking, as time is measured in millions of years, the roots of what the Laurentian Mountains were 1,000,000,000 years ago (similar to the Himalayas) are all that remain today. About 650 million years ago, the supercontinent Rodinia was clearly broken up into different pieces, with the continent "Laurentia" (the future North America, including the Laurentian Mountains) being one of those pieces.
© 2002 / Plate Tectonic Map / By Christopher R. Scotese / Reproduced with the permission of Christopher R. Scotese.
Image Credit: Plate Tectonic Maps by C. R. Scotese / PALEOMAP Project (www.scotese.com).
Image Source: scotese.com/precambr.htm / PALEOMAP Web site.


Over a subsequent period of 136 million years, the Iapetus Ocean (the precursor of the present-day Atlantic Ocean) was born and kept spreading. Also during that period, the sedimentary rocks of the St. Lawrence Lowlands were formed. To this day, they have undergone little or no deformation and are still wedged between the Laurentian Mountains and the Appalachian Mountains, in the Québec City area. These same sedimentary rocks of the St. Lawrence Lowlands were used much later as the excellent building stones of Old Québec's Fortified Wall. About 514 million years ago, the Iapetus Ocean closed up and the Appalachian Mountains began to form (also as part of the continent "Laurentia", the future North America), placing the Québec City area at the heart of mountains on both sides of what became the St. Lawrence River.
© 2002 / Plate Tectonic Map / By Christopher R. Scotese / Reproduced with the permission of Christopher R. Scotese.
Image Credit: Plate Tectonic Maps by C. R. Scotese / PALEOMAP Project (www.scotese.com).
Image Source: scotese.com/newpage12.htm / PALEOMAP Web site.


About 255 million years ago, as the formation of the Appalachian Mountains in the Québec City area was well advanced and would take almost 55 million more years, all the continents once again came together and formed the youngest known supercontinent, which was called "Pangaea" (or "Pangea", as in this illustration). At that time, there also was only one superocean, which was also called the "Panthalassic Ocean" (or "Panthalassa") and eventually became the present-day Pacific Ocean.
© 2002 / Plate Tectonic Map / By Christopher R. Scotese / Reproduced with the permission of Christopher R. Scotese.
Image Credit: Plate Tectonic Maps by C. R. Scotese / PALEOMAP Project (www.scotese.com).
Image Source: scotese.com/newpage5.htm / PALEOMAP Web site.


Subsequently, over a very long period of time, the supercontinent "Pangaea" broke up into the continents we know today, and the landscapes of those new continents changed drastically. About 18,000 years ago, the Québec City area slept under tons of ice, when the last expansion of the polar ice sheets was taking place and the glaciation was at its maximum extent, during what became known as "the last glaciation period" on our planet, which had begun about 110,000 years ago.
© 2002 / Plate Tectonic Map / By Christopher R. Scotese / Reproduced with the permission of Christopher R. Scotese.
Image Credit: Plate Tectonic Maps by C. R. Scotese / PALEOMAP Project (www.scotese.com).
Image Source: scotese.com/lastice.htm / PALEOMAP Web site.


About 12,000 years ago, the Québec City area was immersed under a glacial sea, during the last glaciation period on our planet, which ended about 10,000 years ago.
© 2008 Her Majesty the Queen in Right of Canada / Brochure entitled "Québec, a Fortified City: Geological and Historical Heritage", page 11 / Graphic reproduced with the permission of Natural Resources Canada.


About 9,500 years ago, as the last glaciation had retreated and the last glaciation period had come to an end, the Québec City area saw the appearance of the St. Lawrence River (pretty much as we know it today). So, the present-day course of the St. Lawrence River and the fact that it "narrows" in the Québec City area are the geological result of a very long tectonic, glacial and marine history.
© 2008 Her Majesty the Queen in Right of Canada / Brochure entitled "Québec, a Fortified City: Geological and Historical Heritage", page 11 / Graphic reproduced with the permission of Natural Resources Canada.