Sense Sonnet - Chiller

Where's the Crust?

Earth is missing a bunch of crust
That should have been found 'twixt and 'tween
Certain time periods and it's just
A mystery what this must mean.
Where did all this rock layer go?
Geoscientists theorize
Ice then covered the planet, so
Roaming glaciers were on the rise,
And pushed the crust down deep into
The oceans where 'twas buried deep
Beneath  tectonic plates askew
From time and place it used to keep.

Called The Great Unconformity,
A big ice ball the Earth did be.