Theses are carved rocks intended to become part of the water system of MachuPicchu which is an incredible system.  There are troughs brining water to all parts of the city from a spring in the mountainside and drainage ditches as well.  There was a constant, and still is, supply of water to the city – so much for the theory that drought drove the people away.  The more likely theories are: 1.  The caretakers of the royal retreat realized that the royals would never return and slowly drifted away to easier ways of life- or – 2.  The Inca ruler ordered the evacuation of the city to avoid a devastating attack by the Spaniards who, by the way, never did find MachuPicchu.