Clay deposits in Plymouth

I’ve been looking for clay deposits  in Plymouth for years

I’ve searched Budshead Woods. And I now have scientific evidence that Plymouth is covered in clay.  This evidence is an article published by the Ussher Society http://www.ussher.org.uk/journal/90s/1998/documents/Cattell_1998.pdf

In his article Mr Catell says deposits in Plymouth are 8m thick! EIGHT METRES!!! Where are they?!?!?! 
So when I did a craft fair over at Millbrook before Christmas , I found a  foundation trench around an old building and decided to investigate by digging out this material. It’s in a bucket in the boot of my car and I’m really excited. Sally Bazett was fairly excited about it.  But Millbrook is not Plymouth.  I think Plymouth has been covered over with rubble left from the WW2 blitz. There’s so much rubbish in the surface soils. I will need permission to dig holes in Plymouth.