Your probably correct what the water is concerned, as I mentioned i am not a proper expert. But think in prehistoric times it was more a matter of “that spot near the river has some cool sticky dirt you can make things with” The clay soil below my parents chicken coop seemed pretty much usable as it came from the ground.

I think clay rich soil is common enough to not need to bother with low clay soil and sand but that could be a skewed perspective from my local area. If there really is no other way its useful to know that technically any dirt will contain something to extract.