Garden Bed Edging Inspiration: Boulders To Divide Bed From Neighbor

We have a bed that lives *right* on the property line in the IB2DWs area that has never had a real, proper edge other than the one that we dug with a spade.  The issue is that there's a little bit of elevation change - by design - between the two properties.  My plan for this season is to make that bed feel a little bit MORE finished by removing some sod, shaping the bed a little bit and edging it along the property line in some way.  This bed runs up against a tiny strip of grass next to my neighbor's driveway, so I'd like SOMETHING to keep the mulch from running/floating over onto their driveway.

I've thought about using the green metal edging that I used for our gravel paths.  But, Nat doesn't love it.  She wants something that works with the existing gravel, so I decided to start to keep my eyes open for edging that works to divide mulch beds from gravel and hard surfaces.  It wasn't long until I saw something like this below:

And, then...something similar like this other one below:

Boulders as edging.   

That's interesting, right?  And, easy to install.  I could dig a small, flat trench (or not) and line the boulders up using a string to set the line.  

The good news is that I already HAVE some boulders that I've picked up over the years.  To complete the project, I probably need 30-40 more.    

There's a little more to this project - like what to do with the cliff, how to kill the grass, what to plant, etc.  But, feels like this should help make the IB2DWs are a *little more* finished.