I too live in NWA, in Newton County. We have lots of clay and lots of rocks. But, what I have noticed is that there are in fact, veins of clay, like the other poster said. So, I think it depends on your land. It might be worth trying to find out some more detail on the geology of your land and area. There may be some clay beneath you that you must dig a little bit to get to. Furthermore, I drive past a construction site every day where they have scraped the ground of some great orange clay and piled it up so they could level the land and build a store. I am curious what they will do with this huge mound of clay soil. Would they
sell it to me? Or better yet, deliver it to my property to get rid of it? Just ideas.
I don't think the sifting is a problem because you don't have to get the clay entirely free of rocks. If fact, from asking this question and researching, I think that some rocks are acceptable as well as the river sand or river gravel we have around here. I made test cob bricks that held up great using 1/4 inch and less river gravel/sand. And, there are methods for getting out the large rocks. For instance, scooping a load of rocks and clay and moving them to a hillside and dumping on the ground. Then as you move the soil around, the rocks
should tumble downhill on their own due to gravity. I have seen this before. Again, just thoughts.