Worth sharing. Click to enlarge. I found this great photo of clay-with-flints resting on an undulating surface of broken chalk -- this pic is from the Chilterns.
The irregular bedrock surface is the [product of many different processes. I suspect that here solutional rills are a part of the scenario, but maybe we are also looking at enlarged cracks associated with permafrost in the past, and there is also a possibility of cracks and pits associated with deep root penetration at a time of woodland cover.
The clay-with-flints is a highly variable and somewhat mysterious material -- containing flint nodules weathered out of the chalk but also erratic pebbles assumed to have come from stripped away Cretaceous layers which once rested on the chalk. I have always felt that glacially derived material may also be contained -- .but much more systematic studies will be needed before the full story is known
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