At some depth you will reach the "bottom", where the soil is the original undisturbed layer from before there were people. To get an understanding of what the original soil surface was, you may have to explore what the first 12" (or more) below the surface is like in a bunch of test pits around your original dig. Two feet of digging should have given you some clues whether you are over a deep hole that has been backfilled (or not). So try to locate the perimeter of the depression and follow one edge down. You might try finding somebody with a metal detector to scan your whole yard, especially the borders of your property and around foundations, stone walls. Outhouses were usually situated away from other buildings & about 100 feet behind the house where possible. Besides outhouses, you should be looking for trash dumps and small lost valuables like coins & keys. Check out your local public library for books on bottle collecting that can provide more clues about how to conduct a dig; while there, inquire about whether your area has been illustrated in a "street atlas", as many American cities and towns were mapped between 1875 and 1920's in commercially sold atlases that show much detail of that period, often including out-buildings, property boundaries including stone walls, passable trails & roadways, owners' names. [How about some experienced diggers jumping in & givng squibby some more solid ideas?!]