Hi Paul
Nice to hear from you;
Yes, sorry; didn't mean to sound facetious with my comment about charts but detailed soundings do indeed include information about the nature of the bottom.
Soundings are traditionally taken using tallow coated leads with a big wad of tallow (a soft waxlike semi-sticky substance) in the bottom concavity. The tallow would bring up part of the bottom sediment (sand, pebbles, clay, shells, etc.).
No doubt there there is some modern piece of kit which performs the same function. But I guess the nature of regularly drying areas only requires visual observation at low water and that info would appear on the chart.
Happy to have been able to help you to shore, food and Stella. Hope you enjoyed your sojourn in Ilfracombe. Yes, I will get around to writing up my adventures with "Temeraire" in the Bristol Channel.
Kind Regards ☺