If they run into hard pan you get stubby deformed carrots.
In an area that has been gardened for a while the hard pan is about six to eight inches below the surface. If you rotate where you plant the carrots every year and deep dig that area you will end up having deep dug your entire garden in a few years resulting in a better garden with not a bunch of effort.
Right. Heavy soil makes bad carrots. My grandfather would dig a trench in the rich Iowa soil, put down seeds, and fill the trench with sand.