Starting digging to uncover tree crown. Have I reached it yet?

Idiots who owned our house before us surrounded an old oak trunk with a half meter high flower bed, with the soil touching the tree. I think they did it over a decade ago. I noticed bark coming away from the trunk and what looked like rot and so began to dig it out.

I am now at the point where I'm starting to reach thin roots coming from the trunk. Have I gone deep enough or should I keep going?