Rock / stone wall crumbling

I have a wall constructed out of a series of large rocks, 20-50lbs each I'd guess. Unfortunately, the rain has been taking its toll and washing out the dirt supporting the rocks. A couple have fallen and I could probably jury-rig them into place, but I'd like to know if there is a better way to do it.

For instance, I notice that there doesn't seem to be anything but dirt between the rocks and the garden it is retaining; shouldn't there be a barrier of sorts? And, if there is, what can be done without disassembling the entire wall?

Thanks.

Arphahat, dirt? Why don't you use concrete to hold the rocks together?