@Scott_Draper yes, we would change our recommendations knowing that you're carrying a bag of drainage rock! John was thinking you were carrying a bunch of bulky individual rocks that would add up in weight to ~50lbs, which would require much more space!
Probably the most important feature for carrying that much weight is a substantial hipbelt, so you can shift much of the weight from your shoulders to your hips/leg muscles (which are bigger and stronger than your shoulders alone). The hipbelt also gives you some flexibility to shift the weight between your shoulders and your hips as needed, giving each set of muscles a break. So, for a 50-lb bag of drainage rock, here's a curated search to give you some options and within that list, you might look at the following packs:
The numbers you see refer to the volume capacity of the pack, so the bigger the number, the more room you'll have in the pack. Tighten down those cinch straps if you find the bag of rocks doesn't take up all of the space in the pack; and if there's still a little room, you could consider @REI-JohnJ's advice around adding a light blanket around the rock bag to ensure the rocks stay in place while you work out!
Hope this helps!