drdawg said:... Perhaps you would like to research "Yazoo Clay".

It is impressive that there are mechanical engineering studies of your clay!

"The Yazoo Clay is a calcareous fossiliferous mudrock"

" Caliche (calcium carbonate deposits) and gypsum (calcium sulfate) are common."

Only a few spots in my yard are "pure" blue-gray clay so the rest might range from 'clay" to "silty clay". I don't think I have any Caliche gluing it together.

If I wet down the surface and leave some water standing overnight, my clay will soften 6-18" deep by morning, so SOME water does perk through it. A hole with standing water appears to evaporate more than it perks, but I can't be sure. I can treat it as "almost waterproof" for most purposes.

When soft, a pick does dig it up easily, so I have to defer: your clay IS worse than my clay!

Certainly true, Ken, that temperature/zones don't follow lines of latitude. Alaska covers zones 1 through 7b according to the US National Arboretum:
http://www.usna.usda.gov/Hardz...

I grew up in the UK and gardened there in the equivalent of USDA Zone 8-9, then moved south to Ontario, Canada (roughly same latitude as southern France) and dropped to zone 4!

Apparently, Rick, all clay is not created equal.

That's quite a change of zones, Sue.