Depth varies sharply accross the estuary, but it's all mud ??

[simonfraser]

I sail out of Brightlingsea, accross the estuary.
At quite a few places the depth varies from over 10m to basically nothing in a few hundred meters.

It's all mud, or not ? how does it stay in such a steep profile ?
Or is there rock in the estuary holding the tall 'mud', flats up ?

 

[sailorman]

 

[apollo]

Happy Days, my parents live where it says "Furze Mill"....