Everything from Shells
Downs go up and downs go down,
As wave on wave of frozen ocean
Built each ridge and vale and crown
With ev’ry ancient tide in motion.
Tiny creatures swarmed the sea
And dropped their tiny plates all over,
From Stonehenge to Normandy
As deeply as the Cliffs of Dover.
Singular alga sounds all wrong, as if the term has become strictly a mass noun.