Search Challenge (2/10/16): Why an arc of lakes?

You know that odd sensation you get... 

... when you first notice something that you should have seen before now?  

I mean, sometimes you'll look at something you've seen for years, and then a new feature just sort of pops out at you.  

Ever have that happen to you? 

It happened to me recently when I was looking at a map of North America.  We've all seen this about a zillion times, and yet, this time, I saw something different.  Here's the map as we usually see it: 

From Google Maps. The blue dot in California shows you where I am. 

And THIS time, it struck me that there's a big line of big lakes stretching across Canada and into the US.  Here's another way to look at it that makes the arc of lakes very clear: 

Here's another version of the same map.  (I've labeled some of the lakes to save you a couple of minutes of searching.)  

Image from Google Earth, with my annotations of the major lakes on this arc.

See that?  Although I've looked at maps of North America just about my entire life, I'd never noticed that the lakes line up so neatly. This striking observation leads to today's Search Challenge. 

1.  Is this arc of lakes across the top of North America just an accident of geography?  Or is this actually the consequence of some gigantic geological process?  

I suspect I know the answer to this, but I'm going to enjoy the search process along with you. 

The Search Challenge here is "how do you look for something like this?"  If you're not already a geologist, how can you find out enough about the topic to ask a reasonable question of Google?  

Let us know your answer... and your process (including the most useless time sink diversions that take you off the path to the answer)!  

I'll be reading the comment stream this week, and will chime in from time to time.  

This will be fun.  

Search on!