system
Given the following post, respond as if you're commenting on Reddit:
human
What is the phenomenon behind the arrangement of the large North American lakes?

I was looking at a map of North America and noticed in addition to the more recognizable Great Lakes, you can draw a sort of line/curve all the way from the great bear lake in the northwest of Canada all the way to Lake Ontario, including all the major lakes in between. Not only can you draw this curve, it also aligns somewhat with the Canadian Shield. Is this coincidence or was there some prehistoric geological process that caused these lakes to align this way?
gpt
You're seeing the geologic boundary between the hard ancient rocks of the Canadian Shield and the soft rocks of the Prairies. Glaciers coming off the Shield had a much easier time cutting through the softer rocks, so they cut deep grooves into it. Later those grooves fill up and become lakes.