The Incas were accomplished engineers who built long-lasting structures high in the Andes Mountains, from Machu Pichu on a ridgetop to the Inca Road system that extended for thousands of miles throughout the empire. Moray is a sculpted landscape masterpiece that is less well known, located on a high plain about 20 miles northwest of Cusco. At this site, a set of large circles are lined with concentric rings of terraces; these circles began as deep natural sinkholes with unstable slopes. The Inca builders created features that are admired today, nearly five centuries later, for their precision, complexity, and beauty.

Terraces at Moray (Ana Tutui)

Moray was introduced to the larger outside world in aerial photographs taken in 1931. Speculation on how and why Moray was built has been ongoing in the decades since. The reasonable theories include Inca religious and ceremonial purposes, and/or some type of agricultural experiment station. There are also wacky theories, including landing pads for extra-terrestrials, as UFOs have allegedly been seen in the area (hmmm…).

The Incas didn’t have a writing system we recognize, so with no written records available, new theories will likely continue to emerge. What IS clear is that sophisticated engineering knowledge was essential to construct these enormous features, control water, and successfully address the geologic challenges of unstable slopes and landslides. I had the good fortune to visit Moray on trips to Peru, and this greatly increased my admiration of the monumental engineering achievements of the Incas.

Sinkholes, Soil and Lots of Stone Blocks

Constructed during the peak of the Inca Empire, before the Spanish conquistadores arrived in 1532, the bowl-shaped circles are built around natural sinkholes called muyus (the word for circles in Quechua, the language of the Incas).  The Moray region is underlain by extensive limestone deposits. The calcium carbonate and calcium sulfate in these rocks is subject to chemical dissolution that opens solution cavities and sinkholes in karst topography. Sinkholes are infamous for opening suddenly; frequently, this is due to human-induced disruptions to groundwater levels. Significant property damage, including dramatic examples of cars and buildings disappearing, happens regularly in Florida, Texas, Alabama, and other US states. (Check out a few exciting YouTube videos by searching on “sinkholes swallowing cars”!)

Volcanic rocks overlie the limestones, and within these rocks are springs that provided a water source incorporated into the muyus by the Incas. Andesite, a durable volcanic rock from a nearby quarry, provided the stone blocks that were shaped and used to build the terrace walls and structures at the site. (If the local limestone had been used in construction, it would have largely disintegrated over the past centuries.)

To build the muyus, hundreds of tons of rock and soil had to be hauled and then compacted. The largest construction is 720 feet long, 390 feet in diameter and 120 feet deep, with circular terraces of upward increasing sizes that flow out onto a large flat plaza and a series of oval-shaped terraces. Most of the retaining wall heights are around 5 feet high, and the terrace widths are typically 15 to 25 feet wide. Excavations on one of the plains connecting the muyus indicates that area was raised with more than 12 feet of imported fill.

Flying stairs, placed with geometric precision, are embedded in the walls of the terraces, allowing access between the different levels (visible in the photo below). The stair treads are quite high and far apart, requiring a bit of a leap between them (as I somewhat anxiously found out when I traversed the steps!). The smallest muyu is unfinished, and partially shaped stone blocks and tools scattered about this site indicate it was abandoned quickly, most likely when the workers learned of the Spanish conquistadores advancing into the Cusco region.

Flying stairs at Moray (FH Swan)

Controlling Water

Water management at Moray – collecting, transporting, and delivering water – indicates the exceptional hydraulic engineering skills of the Incas. The water that originally formed the muyus was also the medium that could have caused rapid destruction, so the Incas had to carefully control the natural springs and rainfall of about 20 inches/year. Reservoirs with stone linings were built to store water from the springs, and with a capacity of about 90,000 gallons, they were adequate to provide a consistent supply throughout the dry season. Stone-lined canals directed the water to the reservoirs and then downhill to the muyus. Vertical “drop structures” that created narrow waterfalls were used to connect each terrace level, ending at the deepest level in the depression. Carefully carved into large blocks of andesite, these drop structures were arranged in a single vertical alignment on the terrace walls in each muyu.

“Drop structures” for water and K. Wright (Source: Wright et al., 2011)

A significant engineering challenge was how to drain the water that flowed downward into the large depressions, as there are no surface outlets. Even after centuries of disuse, signs of subsidence at the bottom of these features are absent, indicating that precipitation and drainage from higher terraces can infiltrate in a controlled manner.

Although there have been no subsurface explorations at Moray, modern engineers believe that this drainage feat was accomplished by installing an “inverse filter” drain in each muyu. These were likely built by excavating to some significant depth (perhaps 10 to 15 feet?) and plugging the natural solution cavities with large andesite boulders, effectively stabilizing the structure while also maintaining the water flow paths between the rocks. Next, a series of layers of rock of decreasing sizes were placed, ending with a sand and gravel mixture. Since the muyus have not reverted to their rough sinkhole heritage after several centuries, it can be inferred that the placement of these below-surface drainage layers was impeccable.

Landslides are common in the region. An ancient landslide on the slopes of Wanumarka Mountain above Moray helped to sculpt the area around the muyus. The largest circle incorporates an ancient landslide, so the Incans constructed subsurface drains in this area to discharge water, a technique that modern engineers use on landslides today. Unusually heavy rains in 2009-2010 unfortunately reactivated this landslide and caused extensive damage. Repair work was undertaken, but the archaeological site is clearly at risk of further degradation. (Post-2010 Moray photos show the rocks from collapsed terrace walls collected into piles and temporary wooden shoring installed to reinforce damaged walls.)

Landslide activated in heavy rains of 2009-2010 (Andres Carreno)

Why did the Incas take on such a difficult construction project and how did they use Moray? While no one really knows what the Incas were thinking, there are clues about what might have happened at this site. In Part 2 of this post I’ll describe some of this information – and there are a few surprises.

Atwood, R., 2007. Mystery circles of the Andes. Archaeology60(5), pp.55-61.
Earls, J. and Cervantes, G., 2015. Inka Cosmology in Moray: Astronomy, Agriculture, and Pilgrimage. The Inka Empire: A Multidisciplinary Approach, pp.121-147.
Wright, K.R., Wright, R., Zegarra, A.V. and McEwan, G., 2011, March. Moray: Inca engineering mystery. American Society of Civil Engineers.
Feature image: Moray terraces with mountains in the background: Photo by Carlos Olaizola on Unsplash
Moray: Photo by Ana Tutui on Unsplash
Flying stairs at Moray; Photo by FH Swan
Landslide activated by heavy rains in 2009-2010: Photo by Andres Carreno on Unsplash

If you like my posts, please scroll down to the bottom of this page and leave your email address on my website. You’ll receive messages only when I publish a new post (about once a week) and my occasional newsletter. Join now to stay up-to-date about geology, geography, culture, and history.