We have the same thing with our Queenston Shale Clay deposits. I've an old government document that talks about all the different types of clay in Ontario and even gives the chemical breakdown of each type. This is a paragraph from the document:
Thin green bands parallel to the bedding, and
more rarely at right angles to it, have been
formed by the reducing action of acidic groundwaters.
Green Shale constitutes 5-25 percent of
most Queenston sections. Queenston Shale is
readily broken down by weathering, forming
ultimately a red clay soil with superior ceramic
The Queenston Shale is remarkably uni
form in chemical and mineral composition from
top to bottom of the formation; minor varia
tions have been pointed out in a previous
report (Guillet 1967, p.59). Lime content
is most variable, ranging 3-18 percent in brick
quarries of the Toronto-Hamilton area, re
flecting the proportion of green shale beds in
the sampled section. Green shale is harder, more
limy and less easily broken down by weathering.
Colour and porosity of the fired ware is there
fore dependent to a large extent on the propor
tion of green shale in the section.
You can also see this in the Caledon Badlands: Take a peek at the image.
Is it similar to this?