What are mound builders for?

The various cultures collectively termed "Mound Builders" were prehistoric, indigenous inhabitants of North America who, during a 5,000-year period, constructed various styles of earthen mounds for religious, ceremonial, burial, and elite residential purposes.

Sorce:

Mound Builders - Wikipediaen.wikipedia.org

why is a nation of the tood recurre.

the causes of the dust bowls were severe droughts,   lands became dry and hard, temperature was higher than usual, hot and dry conditions killed crops, high winds. because of those factors, the winds kicked up dry soil and created clouds of dust.