I found some ammo, which I think is from World War II.
The cartridges are stamped WRA 42 and DEN 42.
I do not know what the big piece is. It has a stamp: N2117 III
Does anyone know what the big piece is?
From the cartridge stamps, I'd say most likely you have cartridges from Winchester Repeating Arms and Denver Ordinance, both likely manufactured in 1942.
Looks like a fragment of an exploded mine, maybe an S-type? At the end of the war sometimes they would pile up ammo and mines into a big heap then detonate the mines all at once in an effort to destroy everything.
If you can find the center of the dump explosion, you may be able to recover a lot more stuff. Look for a crater.