Animal pens.
1,600 feet is a _big_ pen, and 30 feet thick seems overkill for a holding pen.
Imagine driving a panicking large herd of animals into these pens - where would you see a need for reinforcing the structure to withstand the force of a whole herd pressing against it?
30 feet is roughly 9 meters.
(I understand the "just google it" response, but just giving the numbers saves a few moments of work for many people.)
These remind my of walled off fields in Bolivia.
https://goo.gl/maps/xpnX311AH592
These seemed to be used both at fields for growing crops if the soil was still good. Or as pens/grazing areas for lama's if not. Bolivia's attempt at land reform was kinda disastrous with resulting over farming, over grazing and subsequent soil loss. Over just 50 years.
If someone claimed that there used to be thin volcanic soil that later blew away, would not be surprised.