In message , The Natural Philosopher
writes
I've always wondered - how thick is a the clay layer likely to be (feet,
meters, 10's metres?)

meters. But it depends.

Clay is finely ground rock, thats all. Where I live its boulder clay,
formed by the ice sheet scraping the top ff the UK and dumping it in
Suffolk over pre-existent chalk, as the glaciers retreated. Its a huge
terminal moraine, and in some places its tens of meters thick, and in
other places where the water ran different its pure sand and gravel,
those being a coarse version of clay.

Up the top of a hill were I lived, it was about 4-6 feet into chalk: 10
miles west the chalk is at the surface, and they have a horse racing
town. a few miles east is a gravel pit, and further in in a valley its
clay deeper than who knows what.

There is no 'normal'

Fair bit of Herts. came from Norway hence the lumps of pudding stone I'm
told.

--
Tim Lamb