Indian Mortar and Pestle?

This was found in a newly tilled field in North East Missouri. I believe it is a mortar and pestle. The pictures do not do it justice. The rock is carved to fit a hand very well. If you hold the rock in your left hand it fits like a glove. It is almost as if you can feel the finger grooves on the bottom of the stone. There is a notch carved perfectly to fit your thumb to hold the stone. It is very heavy but the carving allows you to easily hold the stone against your stomach in your left hand and use the smaller stone in your right hand. The little stone also feels like it fits your hand very well. It seems like it especially fits a woman's hands. What are your thoughts?