Clast lithological analysis (“pebble counting”) offers evidence that the middle and lower reaches of the Thames once flowed through East Anglia before being blocked by the Anglian ice-sheet and diverted to their present course. It’s amazing what you can learn from counting pebbles. . .
Father Thames, as we’ve often been told,
Flows through London, en route to the sea.
We may think that his route’s very Old,
But the Counters of Clasts disagree.
These folk, while they’re out on their travels,
Dig up terraces (flood plains of old),
Returning with sacks full of gravels,
As happy as if it were gold!
Each gravelly lump is derived
From a chip off some parental block,
So its rock-DNA has survived
For the Counters of Clasts to unlock.
They allocate each little stone
To its rock type, and stack them in piles;
Then each pile is counted. It’s shown
That some pebbles have travelled for miles:
From the Midlands come Permo-Triassics;
While volcanics arrive from North Wales;
And the Rhaxella cherts are real classics
From the North Yorkshire moorlands and dales.
So gravels have character, too!
This mixture of rock types, you see,
Is the clast-counting folk’s major clue
To where river beds once used to be.
So that’s how they know OFT
In his earlier days used to flow
Further north to the proto-North Sea,
An ice-age or longer ago.