Tom, you're the mathematician of the party.

How much ice is there here ?" "If I could see better I could tell you better," sniffed Reade.
"Hundreds of tons of it, anyway." "How did the stuff get here ?" asked Dan wonderingly.
Dick was now at the edge of the ice pile, and flashed the light at the roof of the cavern.
"See the rifts in the rock up there ?" he asked.

"Water must have leaked in here during the heavy winter rains.

It was cold water, too.

Then, in extra cold spells, such as this country experiences, the water must have frozen.