QuÃ©bec City sits between the Laurentian highlands of the southeastern Grenville Province of the Canadian Shield and the Appalachian Mountains that were formed during the Taconic and Acadian orogenies. Bedrock here is the Upper Ordovician Utica shale that “overlies the predominantly shallow marine carbonate facies of the Cambrian-Ordovician St. Lawrence Platform” (or St. Lawrence lowlands).The adjacent St. Lawrence River, which I gather formed post-Pleistocene glaciation by cutting into the relatively less-resistant sedimentary rocks sandwiched between the Laurentians and the Appalachians, is part of the Great Lakes – St. Lawrence Seaway system.
As a sign by one of the many higher-up river outlooks explains, the land beneath Quebec City was not chosen by the French because of the overwhelming tectonics over an equally stupefying period of time that created it but purely for defense strategic reasons. To each their own time scale.
In a time-traveling nutshell: Canadian Shield forms the core of the North American continent –> happy passive margin forms with the buildup of a carbonate platform and the transgression of the sea –> BAM BAM Taconic and Acadian continental collision events creating the Appalachian mountains –> some quiet time as the Atlantic Ocean forms to the east –> glaciation from the north –> glacial retreat –> uplifted QuÃ©bec City and associated river –> some French dude named Samuel de Champlain surveys the Great Lakes – St. Lawrence area, claims the high cape of QuÃ©bec City and territory all the way from north of Minnesota down to and including Louisiana for New France in 1608 and his people put up a bunch of ramparts against, well, everyone –> the Brits take over in 1763 –> Canada forms in 1868 and tells everyone to sod off in exchange for putting limey monarchs on its currency –> Canadian geologists find economic natural gas in the Utica shale. (Someone call They Might Be Giants and set this to music.)