Nice pond demonstration on a small scale.
Years ago I used to do some soil testing and found that many times the contractor thought that the soil at the job site contained clay. When we did lost by wash and sieve analysis most of the time a high percentage of silt was present which also seals a pond well. A simple field test that we did was to take a small ball of moist soil in your hand and squeeze. If you get an inch or longer ribbon of soil between your pointer finger and thumb, chances are good that it contains some clay.
Hope this helps someone better identify their soil and isn't to off topic.