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Is it heavy soil, clay, loam etc. ? It's good to clean off the surface like mentioned by mowing, the disc you have may help there, but being a dearborn I assume it to be sized for a utility sized tractor, and may not be heavy enough. I've reclaimed some overgrown areas, and my small disc won't do much, reclaiming is a real hard task for light equipment until it's been turned with moldboard around here, especially if sod or root bound. If it's been left for brush and trees to take over, it needs to be ripped with a crawler or some other means to destroy the network of roots. The rocks you describe are like what we have here and my opinion is that 20 acres, is best handled with heavy tillage equipment, might be wrong with my limited knowledge, just know what works here. I use a small disc on my 850 ford, fairly heavy, but it does not cut in on firmed, or root bound soils that have not been worked, and the rocks can be real rough if you don't go slow. Once it's been turned, couple of passes, it does a nice job. I have a 7 acre field that has been in corn for years, and it has those kind of rocks, rounded mostly, were not that far from nice gravel veins, similar soils but a fair amount of clay. He uses a 6 bottom some years, if he does it 2 years straight, not sure why he bothers to disc, it looks like you could skip using a disc, last year, he just disc'd and planted, and I can see wanting to eliminate a step due to the costs of fuel, not sure if the yields were affected when it's not turned over and disc'd. 
For what you describe, you may consider hiring it done with a disc like this, it would probably be effective to chopping and breaking up the surface, maybe more depending on the soil, it was new when photographed spring '07, looks like extreme service, very heavy discs, never got any close ups of it, but around here with the rocks, I've often wondered what these guys have to repair after doing 1000 acres.
You can hear this thing clanking along in the fields for a long ways, all the rocks have rust scars on them from tillage equipment. View is from a hunting stand over looking a foodplot, then the field: 
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