On the map I am currently playing nearly all sand resources are exhausted. I uncovered a large part of the map and found ~15 clay deposits while having only 3 sand deposits. In comparison there seem to be 40 iron deposits on the same wetlands map. In reality I would expect as much sand as clay, and in my opinion both should be more common than iron.
This being said - what is the idea how to get exhausted resources? Trade them all with the trader? Start a new map? Also long supply lines seem to be problem in the game (e.g. getting the sand from one corner of the map to other). I wish I had some way of managing supply lines e.g. like in Timberborn. Usually I catch the employees walking around collecting food rather than working in the sand pits. I think that a local food storage that is supplied from the town should help keep them working.