In Reality there is Several ways to Create a Swamp.
Both Natural and by Human Hands.
The Natural Way would usually be for an Area to be in slight water for long times and thus naturally washing out the Clay from the Soil and thus creating a watery clay put which results in a swamp after the water source dries up.
For example when a River or Lake dries out but remains with a low water level due to rain or ground water levels.
The Human way is usually to create pits and then lead a controlled amount of water from a river into it. Thus creating a constant water flow which is too small to sugmerge the earth but keeps it watery.
Now in the Game the World should actually be able to do this.
The Conditions would be Fairly Simple.
When a Pit of Fertile Soil is Formed it should become a Swamp.
The Swamp should then Produce Clay over time by converting the Fertile Soil below it into Clay and thus Form a Clay Pit.
The Conditions are Simply.
If an area of 6-30 Tiles of Fertile Soil Ground are Surrounded by Fertile Soil Tiles which are all at least 1 Tile Higher than themselves.
The World will Recognize it as an Pit.
If this Pit keeps untouched for at least 5 Raining Days it will become a Swamp.
For every Raining Day after it became a Swamp. 0.1 Tiles of Height will be Converted into Clay below the Swamp to a maximum of 3 Tiles below the Swamp.
This would allow the Map to Naturally Generate new Clay Deposits.
It would also Allow Players to Generate a New Clay Deposit Manually given they Invest the Time for it