Making a Sloped Bed
I am looking to create a 8 foot deep 20 foot wide bed with my kids school letters at their elementary school created from bushes. The roadway in front of the school is significantly elevated and I would ideally like to slope the bed upward gradually front to back to enable even better readability (the ground currently is completely level). In other words so the back bushes are elevated above the front - so the letters are easier to read.
How best to accomplish this? I would like to not use edge material if possible. If I build the topsoil up significantly with the slope and cover well with mulch will it hold its shape or will erosion be a problem?
Thanks in advance for any ideas!