Encyclopedia of the Incas

Several kinds of terraces can be identified in the Andes, and they all share
common construction features. First, a retaining wall of rock was built to hold
the terrace soil. Generally, the wall has larger rocks at the bottom and smaller
ones near the top, which provides stability. Usually, the wall is slightly sloped in
an uphill direction. Soil was then brought in from elsewhere to fill in the terrace;
excavations have revealed that some terraces have pebbles and sand at the
bottom, aiding in drainage. The surface of the terrace may be designed to slope
from the back to the front, or it may be level.
Among the several terrace types known from the Andes, bench terraces and
valley-bottom, walled terraces are perhaps the most common. Bench terraces
follow the contours of a hillside. Sometimes called stair-step terraces, these often
have higher walls to make a wider surface for planting. Some of the elegant
terraces near Pisac in the Urubamba valley near Cuzco are six feet high but not
much wider. Such terraces sometimes have projecting stones that functioned as
steps laid into the retaining wall. Valley-bottom, walled terraces are constructed
where the natural slope is gentle. A retaining wall is only necessary at wide
intervals. Such terraces are usually found along valley bottoms and often contain
rich alluvial soils.