Rob Countess wrote: " Well, in my mind, clay is a more specifically defined substance based on mineral content and particle size. Mud is larger particles, more organics, and less homogenous. Clay is what you make pots from and is less frequent in Canadian caves in my experience ..." Clay is something more of a technical term and means a sediment with particle sizes in the range 0.98–3.9 µm. This is finer than silt and much finer than sand. Mud, on the other hand is not a technical term being defined as a mixture of 'earth' or 'dirt' and water. It is difficult to think of a circumstance where a symbol for mud might be required that isn't covered by clay, silt or sand unless you are looking at fairly recently deposited material with a large organic component which might be better considered as 'flood debris' and possibly too ephemeral to mark on a map? Just my thoughts. Graham