This is something to be aware of if you are creating a new town just you begin into a new age - for me it was Stone Age into Classical.
I When I created my new town I had just researched Polytheism. This meant that I could ONLY build crop fields. However, I had not yet researched mining, which meant I had no Iron so I could not build a Level 2 crop field. That meant that I could not build a storage shed for food/crops.
So I was stuck with a new town in which I could not create storage for crops until I had finished researching Mining.
If I had chosen to research Strategy after Polytheism I would have been stuck with this situation for at least a week.
Just something to keep in mind.