45 minute read

The Inca civilization is often depicted as being shrouded in mystery, where how they achieved their incredible stonework is treated like an impenetrable enigma. But the Inca Empire, or Tahuantinsuyu, was still dominant throughout South American at the time of the European conquest. It stretched from Colombia down into Chile and Argentina, with a peak population of over ten million people before the arrival of European pathogens. Much of their culture was witnessed and documented by various early Spanish chroniclers in the 1500s.

Over the last decade, I’ve combed through many of these early colonial-period writings. Not only do they detail the Inca’s history and way of life, but to my surprise, they also included many references to their stonemasons’ ingenious building methods. This collection of early Spanish references presents a detailed picture of how their impressive stonework was accomplished.

These historical records were then cross-referenced with other lines of physical evidence, which does support what the Spaniards described. As part of this project, I returned to Peru to gather photographic evidence from over thirty Inca sites. The following collection of photographs illustrates various aspects of their construction techniques, including how the stones were quarried, transported, split, shaped, fitted, and dressed.

This paper also examines a number of what initially appears to be unexplained anomalies, such as a patchwork of masonry styles and seemingly ‘melted’ surfaces. These have fueled a number of “alternative-history” theories surrounding the origin of this stonework. These are, however, resolved with a better understanding of the region’s history and geology. In sum, what follows is one of the most comprehensive and evidence-based accounts of Inca construction techniques ever compiled.

Sections

• Introduction
• What Was Documented by the Spanish Chroniclers?
• Moving the Megaliths
• Quarrying, Cutting, and Shaping
• How Did They Precisely Fit Their Polygonal Stonework?
• The Purpose of the Nubs
• Clearing Up Stone-Melting Confusion
• The Many Problems of the Alternative-History Timeline
• Conclusion: The “Secret” Behind the Incas’ Monumental Architecture
• Sources

What Was Documented by the Spanish Chroniclers?

After the Spanish conquest, the Inca’s skilled stonemasons didn’t suddenly vanish. They continued their craft on post-colonial Spanish buildings and cathedrals, in what researchers call a “Neo-Inca” style (Nair, 2003; Kauffmann Doig, 1965). A few of these Spaniards recorded firsthand accounts. One of the most well-regarded of the chroniclers, Pedro Cieza de León, personally witnessed their masons at work while in Cusco:

“As for laying foundations, making strong buildings, they do this very well; it was they who built the houses and dwellings of the Spaniards, and they made the bricks and tiles, and laid large, heavy stones, putting them together so skillfully that it is hard to see the joinings. They also make statues and other larger things, and in many places it is clear that they have carved them with no other tools than stones and their great wit.”

• Pedro Cieza de León (1518-1554), The Chronicles of Peru (1553)

An example of this transitional-period stonework is seen in the walls of Casa de las Sierpes (House of the Snakes) in Cusco. This building was constructed by Don Pedro Bernardo de Quiroz, following his appointment as attorney general and judge in 1582. He reused pre-existing Inca stones from near the plaza of the convent of Santa Clara. For his lintel, he “commissioned two figures of mermaids with the heads of sea lions, female and male, to be carved for the portal of his house (Amado Gonzales, 2003).” Although not megalithic, this building does demonstrate that precisely-fitted stonework was still being produced here decades after the fall of the Inca.

Spanish chroniclers preserved much of the Inca’s oral history, which spanned back centuries. While the Inca didn’t have a complete written language, they did have wise elders in their society whose primary role was to recall and pass down their history. They converted their stories into songs as mnemonic devices and recounted the deeds of past rulers during important events (Cieza de León, 1554).

The Inca were meticulous record keepers, using their quipu system of knotted strings to record numerical data and some symbolic information. With these records, they tracked tribute payments from their conquered villages (akin to taxes). This included the number of workers that each dedicated to the empire’s monumental construction projects under their “mit’a” system of shared labor (Polo De Ondegardo, 1571; Cieza de León, 1554; Rowe, 1946; Acosta, 1590). Leading these construction efforts were the Inca’s elite class of architects and master stonemasons (Cobo, 1653):

“The Inca kings had a large number of architects and master stonemasons who became highly skilled in their occupation and made their living from it. All of the building that they did was for the king, who always kept them occupied with the many fortresses, temples, and palaces which he had built throughout all of his kingdom... there was no province in all of the Inca’s states that was not enhanced with these skillfully made stone structures.”

• Father Bernabé Cobo (1582-1657), History of the New World (1653)

These records trace the rise of the Inca, stretching back to Manco Capac’s founding of the Inca Kingdom in Cusco, through the line of Inca kings to the capture of Atahualpa by Francisco Pizarro. The most notable of these rulers was the great Pachacuti Inca Yupanqui, “Turner of the World”, who is said to have begun the rapid expansion of the Inca Empire. Their history tells of him reconstructing the city of Cusco from its foundations over two decades with 50,000 men:

“He had decided to rebuild the city of Cuzco… Inca Yupanqui outlined the city and had clay models made just as he planned to have it built. … the whole seat of the city was marshes and springs of water. He ordered that all of the springs be canalized in such a way as to be piped to the houses of the city and made into fountains to supply water to the city. At the same time, he ordered others to prepare the foundations of the houses and buildings of the city. … Then Inca Yupanqui ordered everyone from the city of Cuzco to leave their houses, take out everything they had in them, and go to the small towns nearby. As this was done, he ordered those houses to be torn down. With this done, cleaned up, and leveled, the Inca with his own hands, along with the rest of the lords of the city, had a cord brought; indicated and measured with the cord the lots and houses that were to be made and their foundations and structures. With all of this prepared, the foundations were dug. Having brought the necessary equipment, they started to build their city and its houses. While these buildings were being made, the work went on continuously with fifty thousand Indians on the job. From the time that Inca Yupanqui ordered the beginning of the improvements on land and rivers of the city and the construction of buildings until everything you have heard about was completed, twenty years elapsed.”

• Juan de Betanzos (1510-1576), Narrative of the Incas (1576)

Pachacuti Inca Yupanqui is also credited with rebuilding their most sacred Temple of the Sun, Qoricancha, in the same location of a prior temple constructed by Manco Capac (Cieza de León, 1554; Betanzos, 1576). And under his reign, construction began on Sacsayhuaman, the megalithic fortress overlooking the city. Construction on that site continued through reigns of Pachacuti’s son, Topa Inca, and grandson, Huayna Capac:

“From the time of the good King Inca Yupanqui, under whose reign the first material was assembled, until that of Huaina Capac, the Incas worked for over fifty years to erect this fortress; there are even those who say that they never finished it, and they give as proof this "weary stone" which, according to them, was to have been used for further construction, before civil war and the arrival of the Spaniards put an end to all these projects, and to the Inca Empire itself.”

• Garcilaso de la Vega (1539-1616), The Royal Commentaries of the Incas (1609)

“[Pachacuti] Inca Yupanqui was the Inca who started building the fortress of Cuzco, outlining and arranging it, [and] ordering that the foundation [stones] be quarried, the likes of which had never been seen before in Peru. ... “[Tupa Inca Yupanqui] also commanded that the construction of the fortress, which his father, [Pachacuti] Inca Yupanqui, had started be continued with great care, as these famous and renowned buildings would make his name last forever. He [also] ordered the streets and canchas of Cuzco to be straightened, and new buildings be constructed. He also ordered that the royal roads be organized from Cuzco to Chile and to the mountains above Charcas, and below to the coast [all the way to] Quito. So much effort was expended that it was as if he were personally involved in every aspect of the project.”

• Martín de Murúa (1525/1540-1618), The General History of Peru (1616)

How reliable is this historical timeline preserved by the Spanish? Numerous studies have attempted to answer this question by comparing these Spanish works to radiocarbon dating done at numerous Inca sites. The most commonly accepted historical timeline is John Howland Rowe’s interpretation of Cabello Balboa’s chronicle, which was based upon the Inca’s oral history. While stone itself can’t be directly dated, researchers are able to date other organic material used in these buildings’ construction and under foundations, in addition to dating other artifacts. These studies have found that, while mostly accurate, the Inca Empire’s expansion started a few decades earlier than the Rowe/Balboa chronology (Adamska & Michczyński, 2016; Marsh et al., 2017; Ogburn, 2012; Ziółkowski, 2021). This earlier expansion implies that they would have had more time to construct their numerous fortresses, temples, and outposts.

More broadly, these studies do confirm that these sites were indeed constructed by the Inca, not from some earlier civilization from thousands of years prior (a topic we’ll revisit in the final section, examining the “alternative-history” chronology). The construction of these sites wasn’t ancient history for the Inca either. Pachacuti is believed to have died in 1471, whereas Huayna Capac died around 1524, less than a decade before Pizarro captured Atahualpa in 1532. Those Inca descendants telling the Spaniards about this construction at Sacsayhuaman would have likely seen some of it firsthand.

How Were the Megaliths Moved?

One of the greatest engineering challenges faced by the Inca construction was moving the enormous megaliths. The largest stones at Sacsayhuaman are estimated to be 100-200 tons (Šaravanja et al., 2023; Scarre, 1999). How could the Inca, without modern machinery or strong draft animals like horses and oxen, have moved such massive stones from their quarries?

A few of the chroniclers addressed this very question. The solution was the collective strength, ingenuity, and coordinated effort of multitudes of workers. During their construction of Sacsayhuaman, thousands of laborers hauled the megaliths using thick ropes (Cieza de León, 1553):

"He ordered twenty thousand men sent in from the provinces, and that the villages supply them with the necessary food, and if one of them took sick, that another should be sent in his place and he could return to his home. These Indians were not permanently engaged in this work, but only for a limited time, and then others came and they left, so the work did not become onerous. Four thousand of them quarried and cut the stones; six thousand hauled them with great cables of leather and cabuya [fibrous rope]; the others dug the ditch and laid the foundations, while still others cut poles and beams for the timbers... Overseers went around watching what they did, and masters who were highly skilled in their work. Thus, on a hill to the north of the city, at its highest point, slightly more than a bow-shot distance, this fortress was built which the natives called the House of the Sun, and we the Fortress."

• Pedro Cieza de León (1518-1554), The Chronicles of Peru (1553)

Other Spanish chroniclers also mentioned them pulling these stones with ropes:

"They found that there was an abundance of stone and large quarries in the vicinity of Salu. … Immediately after arriving there, they gave the order about how they were to bring and transport the building stones. For this purpose, they had a large number of ropes, thick cables of sinew, and sheep [llama] hide."

• Juan de Betanzos (1510-1576), Narrative of the Incas (1571)

“To this labour was added the conveyance of stones from great distances by force of men’s arms. Any one who has seen their edifices, will not doubt their statements that thirty thousand men were employed.”

• Polo De Ondegardo (1520-1575), Narrative of the Rites and Laws of the Incas (1571)

Some Andean cultures still make exceptionally strong traditional rope using various materials. While visiting a Kichwa community in Ecuador (who speak a dialect of the Inca’s Quechua), two women demonstrated how they made rope from a succulent called Furcraea cabuya (mentioned in one Cieza de León translation). Just a few thin strands wound together were already too strong for someone to break.

Another enduring example of traditional rope making can still be seen at Q’eswachaka. Every year a Quechua community joins together to reconstruct a traditional Inca bridge. The collective effort likely resembles how Inca era workers once joined together to haul stones. The festival takes place over four days, where first the women weave the ropes out of ichu grass, which is widespread throughout the highlands. Dozens of men then haul the heavy thick ropes across the canyon to re-erect the bridge. Rope made from llama leather was then used for cross-support bands.

A 1996 engineering study tested the strength of the ichu grass ropes (Ochsendorf). Each could support at least five thousand pounds when two inches thick. When braided into thicker cables, they were estimated to hold 50,000 pounds. That strength was crucial, as Inca rope bridges once carried long caravans of llamas ladened with goods.

When calculating the force required to move megaliths, one point that’s often missed is that their entire weight would not have been lifted. Rather, to move them horizontally, workers only needed to overcome friction and generate enough lateral momentum. On an incline, only a fraction of the stones’ total weight added to the pulling force required. This leads to the Inca’s extensive use of earthen ramps and other friction-reducing methods.

Father Bernabé Cobo, a Jesuit missionary, witnessed their use of ramps in Cusco:

“The stones were taken to the work site by dragging them, and since they had no cranes, wheels, or apparatus for lifting them, they made a ramp of earth next to the construction site, and they rolled the stones up the ramp. As the structure went up higher, they kept building up the ramp to the same height. I saw this method used for the Cathedral of Cuzco which is under construction. Since the laborers who work on this job are Indians, the Spanish masons and architects let them use their own methods of doing the work, and in order to raise up the stones, they made the ramps mentioned above, piling earth next to the wall until the ramp was high as the wall.”

• Father Bernabé Cobo (1582-1657), History of the New World (1653)

Several ramps remain visible today at Inca quarries and archeological sites, such as these Chullpas (funerary towers) near Puno. A sign at Sillustani mentions that one of the ramps still had a couple large stones left upon it leading to a Chullpa that was still under construction. At Cutimbo, a ramp extends toward the edge of cliff, presumably to haul the loose stones up from the lower quarry. And at other quarries, ramps can be seen leading to individual megaliths that were in the process of being hauled out (Protzen, 1985; Protzen & Nair, 2013).

In some areas, quarries were located near construction sites, so stones didn’t need to be moved particularly far. At Machu Picchu, some alternative-history tour guides give the false impression that stones were moved from the valley thousands of feet below, while ignoring an upper quarry next to the Temple of the Three Windows. Similarly, two quarries exist nearby and uphill from Sacsayhuaman, and large stones are scattered across the landscape around there.

Other stones were indeed transported from distant quarries. Much of the stone from Cusco was sourced from the Rumicolca quarry, 18 miles (30 km) away. Although distant, this was through a relatively flat river valley, along their sprawling road network (the Collasuysu branch of the Qhapaq Ñan). It would have generally been smaller stones that were transported this distance. The stone used in Cusco’s pre-colonial stonework averaged roughly 100lbs, with some of the larger stones at Qoricancha only being around 300lbs (Sacred Geometry Decoded, 2022).

While there is also an upper quarry at Ollantaytambo, a primary source of its stone was a hillside across the river valley at the Kachi Qhata / Cachiccata quarry. Hauling megaliths through this valley is undeniably an impressive feat. To accomplish this, the native workers built a well-engineered road, leading down from the quarry and then directly up to the megalithic zone of Ollantaytambo. Those trying to exaggerate the sense of mystery surrounding this site tend to ignore the existence of this gigantic ramp, as it undermines their “lost advanced technology” narrative. This ramp leads up the side of the mountain at a gradual eight-degree incline, and is 4-8m wide, enough for a few columns of men pulling with ropes (NOVA, 1996; Protzen, 1993; Miano, 2021).

Along this route from the quarry there are numerous “piedras cansadas” (tired stones) that have been abandoned while in transit, with one resting at the base of the ramp. These provide further insights into their transportation methods. An excavation under one of these megaliths revealed how they constructed roadbeds to help move these massive stones (Protzen & Nair, 2013). This carefully prepared surface contained multiple layers of soil, gravel, and clay. This surface would have lowered friction and reduced the likelihood of stones catching on other rocks. Soil was pushed up along the leading front-edge of this stone, as evidence of it being dragged. Drag marks can also be seen on some large stones.

Decades earlier, another megalith near Ollantaytambo revealed a different friction reducing technique: “Sr Luis Llanos informs me that a treasure hunter at Ollantaytambo dug under a large Inca block abandoned between the quarry and the site and found remains of wooden rollers (Rowe, 1946).” Softball-sized spherical stones found at some sites are also thought to have been used as rollers. Additionally, numerous bronze pry bars have been found at Inca sites, which could apply leverage in helping to shift and re-position stones (a technique still used today by Peruvian stone masons).

In 1996, a NOVA documentary successfully replicated this manual method of hauling megaliths, confirming that the tired stones could be hauled by a coordinated team pulling with ropes. They gathered 250 men, women and children from around the village of Ollantaytambo to move a 15-ton block up an incline approximating the large ramp leading to the Sun Temple. Although the stone was initially a challenge to raise from the hole where it sat, once it became dislodged, they were able to maintain a relatively fast momentum.

This same method can be scaled up to more men (and excluding women and children), with more ropes, pulling even larger stones. An example of what this may have looked like can be seen in the Nias people from Indonesia, who similarly moved megaliths with ropes until about the 1950s. Although the size of the Inca megaliths is remarkable, numerous other examples across the world and throughout history have shown that larger stones can be moved by manpower alone.

How Were the Stones Quarried, Cut, and Shaped?

The Inca-era masons crafted some of the finest stone monuments in human history. In their worldview, the natural world was permeated with consciousness. Rock to them held a sacred and spiritual significance. Their mythology even spoke of humanity as being born from stone. This reverence is reflected in the painstaking care and precision of their interlocking masonry, shaped through countless hours of work. Their creations sought a harmonious balance with the landscape, with passageways and court yards opening to dramatically framed vistas. Other structures appear to emerge out of the bedrock, with bricks closely hugging the curves of natural outcroppings.

How was their remarkable masonry accomplished? Modern masons have become accustomed to working with strong steel chisels, diamond-tipped saws, and high-speed grinders. Few today are aware of how this work was done in the centuries before the advent of these tools. For those looking for some lost technology, the answers presented here may not be particularly satisfying - it had more to do with their process and techniques than it did with any advanced tools.

The ingenuity of Inca-era masons is evident in the tool marks that show how they split rocks apart. They carved a line of grooves and hammered in wedges to split the stone cleanly in two. This same technique is still used by some modern quarry workers. They may have also used an ancient Roman-style variation, pounding in wooden wedges later soaked with water to expand and split the rock. Rocks with rows of chiseled out holes can be found at numerous Inca sites, including Machu Picchu’s quarry, a tired stone at Ollantaytambo, Tipón, and on stones in Cusco. While this stone splitting technique appears to be relatively common, other times they would have just selected from loose rocks that scattered the landscape, or by prying them out of bedrock along pre-existing fractures (Protzen, 1985).

Another stone with a long thin cut at Ollantaytambo has led some to suspect that these masons must have had some kind of drag saw or circular saw. However, the groove is wobbly, not straight, so couldn’t have been made with a saw blade. Upon closer inspection, you can see crystals inside of the crack, revealing that this was simply a calcite or quartz vein. Those minerals have since mostly dissolved, leaving behind the misleading appearance of a thin saw cut.

Some mistakenly assume that this stone at Ollantaytambo shows saw cut marks, but closer examination reveals that these are natural cracks filled with mineral deposits, not tool marks.

After splitting rocks to their rough size, their masons used hammerstones for further shaping. The use of these hammerstones is described in various Spanish accounts (Cieza de León, 1554; Polo de Ondegardo, 1571; Father Bernabé Cobo, 1653; Garcilaso de la Vega, 1609; Murúa, 1616).

“The implements that they had to cut the stones and work them were hard, black cobblestones from the rivers, with which they worked more by pounding than cutting.”

• Father Bernabé Cobo (1582-1657), History of the New World (1653)

“This work was very toilsome, for all their buildings were of masonry, and they had no tools of iron or steel, either to hew the stones out of the quarries or to shape them afterwards. All this was done with other stones, which was a labour of extreme difficulty. They did not use lime and sand, but adjusted one stone to another with such precision that the point of junction is scarcely visible.”

• Polo De Ondegardo (1520-1575), Narrative of the Rites and Laws of the Incas (1571)

Beyond these Spanish accounts, two types of physical evidence confirm that hammerstones were the primarily tool used. First is the prevalence of hammerstones found at both their quarries and construction sites. Hiram Bingham’s team found hundreds of these hammerstones during his excavations of Machu Picchu (1952). Some had depressions for the thumb and finger to help with grip. These stones had been selected for their toughness (resistance to cracking), and included cobblestones, dolerite, quartzite, olivine basalt, or hematite, which naturally contains iron (Bingham, 1952; Protzen, 1985; Rowe, 1946). The hematite or basalt hammers are probably the “black stones” that Garcilaso de la Vega called hihuana (1609). At the Rumicolca quarry, Jean Pierre Protzen found 68 river cobblestone hammerstones amongst the fractured andesite, of a variety of stone that he notes were easily determined to be foreign to the site (1985).

The second type of physical evidence that confirms the use of hammerstones are the impact marks seen on these stone faces. These are especially visible on the limestone blocks used at Sacsayhuaman and neighboring sites. This calcium-carbonate rich material metamorphizes into white dots when struck (Rowe, 1946; Protzen, 1985). Notable too are the larger hammerstone impact marks on their front-facing surfaces versus smaller dots on the beveled edges where they shifted to a smaller tool for the detailed work. When you look closely at this limestone, you can even see some jagged angular lines where flakes chipped off. During the Chincana Project’s excavations at the base of those walls, there was a layer of small angular gravel chips in the soil, which would be consistent with them flaking off material (Adams, 2025). While less noticeable, similar hammerstone tool marks can even be seen on granite and andesite blocks. And yes, these marks are also seen on even the precise-fitting, polygonal megalithic stonework.

Some readers may question the feasibility of using these simple hammerstones. However, humans have a long history in skillfully working stone in this manner. Over twelve-thousand years ago, Paleolithic hunters in the Americas were crafting finely-made, bi-faced spear points out of hard varieties such as flint, quartzite, obsidian, and jasper. By just using simple hammerstones, struck at the correct angle, they were able to quickly chip away flakes of material into sharp, symmetrical blades.

The techniques used by the Incas’ workers weren’t too dissimilar to these Paleolithic knappers. Stones like granite, andesite, and rhyolite, commonly used in their buildings, can similarly be chipped away using the right technique. In his experiments, Jean Pierre Protzen observed that striking the rock at an angle of 15-25 degrees, and rotating the hammerstone at the moment of impact, increased the amount of flaking. This relates to a common misperception about the Mohs hardness scale, which some use to argue for the seeming impossibility of using simple hand tools on harder varieties of stone. But this scale only ranks how easily each will scratch, not how easily it will crack or flake. In the words of John Howland Rowe, “the process of working stones with stone hammers is not as slow and laborious as many people who have never tried it are inclined to believe (1946).”

While hammerstones were the primary tool used in the final shaping and dressing, chisels were required for carving out sharp inner corners. Most commonly, they likely used shards of sharp rocks as chisels. Any loose shards of the same material or harder would serve this purpose. During their study of Tiwanaku, Stella Nair used obsidian shards to replicate carving out sharp 90-degree inner corners within Rhyolite, a high-silica volcanic rock with similar hardness as granite or andesite (2013).

Although Inca sites don’t appear to have inner corners as tight as at Puma Punku, they did frequently carve out flat religious offering platforms on their “huacas” (sacred places of worship). These intentionally carved altars are often mistaken for quarries by those who aren’t familiar with Inca religious practices. The Spanish chroniclers note that roughly four hundred of these huaca sites were established at key points along their road network, which they used to venerate their various ancestors, oracles, and deities (Cobo, 1653; Polo De Ondegardo, 1571; Ayala, 1613).

The Inca were also skilled in metallurgy and created bronze chisels for this work. While raw copper is a relatively soft metal, the local variety also naturally contained arsenic, which was mixed with tin and silver. This created a durable bronze that was two to four times harder. They then cold-hammered this metal, increasing the hardness by another four times. The end result was that these “Arsenical copper and bronze tools were of sufficient hardness and substituted for iron (Peterson & Brooks, 2010).”

How Did They Precisely Fit Their Polygonal Stonework?

While the prior questions of how these rocks were quarried, moved, and shaped is no longer a mystery, how exactly they fitted such precise masonry still needs to be addressed. Rather than invoking some lost technology, this final step is better framed as a matter of lost methods or techniques. Once again, Spanish accounts and remaining physical evidence do provide revealing clues as to how this was done.

As a quick tangent, it’s worth noting that the Incas’ architects used a range of styles depending upon the varieties of stone available and the type of structure. Not all Inca structures used the tightly-fitted masonry for which they’re renowned. In the coastal deserts, where earthquakes were less of a concern and quality rock wasn’t as readily available, they commonly built with adobe, such as at Acllawasi, Tambo Colorado, and Huaca la Palma. Even around Cusco, adobe and rough rubble masonry were common on less significant buildings.

The finest stonework was reserved for the higher status buildings, such as temples, fortresses, and palaces. Here we commonly see two main masonry styles: ashlar, which uses four-sided rectangular blocks, and polygonal, which can have various irregular angles. Their ashlar masonry was typically coursed, meaning that each row of stones is flattened on top, so that subsequent rows are laid upon a relatively flat surface. The joints of this style are simpler than in polygonal masonry. Because the stones join along straight edges, only the joint angle needs to be correct for a precise fit. However, to shape irregular rocks into the rectangles of ashlar masonry requires that more material be removed.

Polygonal masonry, in contrast, allowed builders to take advantage of the wide variety of shapes and sizes of rock that are available to them. With this style, they could instead carefully select stones best suited to filling each gap, as if they were playing a game of Tetris. They then only needed to incrementally shape the edges to make a tight fit. So, while polygonal masonry may initially look more complicated, this style would generally have been less time consuming for them than ashlar, since the final stone shapes were closer to that of the original rocks. Their polygonal masonry also offered superior earthquake resistance because of its tightly interlocking design.

When looking at the tightly-mated joins between these stones, many assume that the precise fit continues beyond their outer faces to the internal joins, but typically only the faces of rising joints have this tight fit. Internally, they are often slightly wedge shaped, angling inwards and leaving gaps inside between adjoining blocks. These gaps were packed with a sticky red clay (llàncac allpa) and rubble (Cobo, 1653; Protzen, 1985).

“The Indians did not use mortar in these buildings, that all of them were made of dry stone; the first reason for this is that they did not use lime and sand for construction (never having discovered this type of mortar), and the second reason is because they set the stones together with nothing between them on the exterior face of the structure. But this doesn’t mean that the stones were not joined together on the inside with some type of mortar; in fact it was used to fill up space and make the stones fit. What they put in the empty space was a certain type of sticky red clay that they called llanca, which is quite abundant in the whole Cusco region.”

• Father Bernabé Cobo (1582-1657), History of the New World (1653)

By mating only the faces of rising joints, the masons significantly reduced the time and effort required. In certain high-status buildings, they took the additional effort to align adjoining stones within the walls as well. At Qoricancha they did this with an interlocking, jigsaw-like pattern for better earthquake resistance. Bedding joints also required closer fits internally for better stability. Concaved imprints were often carved into lower rows to securely seat the above stones.

The question remains, how did they fit these stones together so perfectly? Additional clues can be found at Machu Picchu, where it appears as though the stonework around the Temple of the Three Windows was never fully completed. This work may have been abandoned while in progress due to the Inca civil war or the Spanish conquest. A number of rocks here are still in the process of being fitted and dressed. Two large stones are still tilted slightly back with smaller rocks wedged underneath. Their lower edges are in the process of being shaped to fit with the lower rocks, as explained by Fernando Astete (chief supervisor of Machu Picchu), in the documentary “Ancient Superstructures: The Secrets of Machu Picchu” (Kissous, 2020).

According to some of the Spanish chroniclers, an essential part of this process was simply perseverance, where they did repeatedly need to re-check their joints until a tight fit was made:

"If we consider the number of times they must have fitted and taken off one stone before this accuracy was attained, an idea may be formed of the toil and of the number of workmen that was required."

• Polo De Ondegardo (1520-1575), Narrative of the Rites and Laws of the Incas (1571)

“All this was done through the power of many people, and with great suffering in the work: because in order to fit one stone with another, as they are so well adjusted, it was necessary to try the fit many times, since most of them were neither even nor full.”

• José de Acosta (1540-1600), The Natural and Moral History of the Indies (1590)

“This work certainly is very hard and tedious. In order to fit the stones together, it was necessary to put them in place and remove them many times to check them, and since the stones are very big, as we see, it is easy to understand what a lot of people and suffering were required.”

• Father Bernabé Cobo (1582-1657), History of the New World (1653)

There are a few techniques that they may have used to make this process simpler, requiring fewer movements of the stones. They could have also used their sticky red clay to create imprints when testing the joints, to show where additional material needed to be removed (Sacred Geometry Decoded, 2017). A similar method uses stone dust or sand to leave impressions while creating bedding joints. During his research at Ollantaytambo, Jean Pierre Protzen used this technique with only hammerstones to tightly fit a block to a bedding joint (NOVA, 1996). The entire process took him a few hours.

Although this may initially seem time consuming by modern standards, it was feasible given the tens of thousands of laborers who worked over decades on these monuments. For efficiency, the largest megaliths would be placed first and then remain seated while the smaller stones would then move in to be fitted with them. Despite the fixation on the largest stones, the vast majority of their stones were small to medium sized and could have been easily fitted by small teams of workers (see Sillustani workers photo above).

Another hypothesis, proposed by Vince Lee, is that they used a scribe technique to first match edges of closely positioned neighboring stones (Lee, 1986; Miano, 2021). He noticed that carpenters doing log home construction similarly used scribes, allowing them to only move logs once for the final placement. Those skilled in this method would commonly get a tight fit on their first attempt.

With the stones placed next to each other, the scribe would travel along the edges of one stone to check for a consistent spacing with its neighbor, removing material as necessary. In doing so, the two separated stones could be carved to fit, and then ideally moved together in one final movement. Using a technique like this would have saved a lot of effort in fitting some of the larger rocks.

A key element of this scribing method was the plumb-bob, which are small weights hanging from a string, used to find “true vertical” during construction. John Howland Rowe noted that “the Inca know and used the plumb-bob, for which there is a Quechua name (WIPAYCI) in Gonzalez’s dictionary of 1608. Two specimens are illustrated by Bingham (1930), and I picked up a small stone one in the ruins at Ollantaytambo (1946).” The hanging plumb-bob would ensure that the scribe maintained a consistent angle and distance throughout the process.

The Purpose of the Nubs

There’s much speculation surrounding the protrusions on these stones, often called “nubs.” These are commonly suggested to be lifting bosses (Rowe, 1946; Lee, 1986; Protzen, 1985). Others question if they were used for astrological purposes, whether they’re remnants of where liquified stone was poured into molds, or if they hold hidden messages from an undeciphered stone-nub language. While there are a few variations, a close analysis of these protrusions does reveal their true purpose.

Initial clues can be gathered by examining their placement. They tend to be located towards the lower edges of stones, often with only a single, centered nub for narrow stones, and with a pair of nubs on wider stones, one in each lower corner. While common on medium or larger stones, they are rarely found on the very bottom layer of stones that didn’t need to be placed upon other stones.

Many stones show traces of where the nubs were previously located. This can be seen on the south side of Hatunrumiyoc, in Cusco. Here a wall has been crudely dressed in what appears to be rushed or unfinished work, and some of these nubs haven’t yet been completely removed. Another stone at Machu Picchu is in the final process of being dressed. On its bottom right, where a nub would conventionally be located, the nub is now missing where the face is being flattened to match neighboring stones, whereas the bottom left appears to be in the process of being chipped away. The removal of these protrusions after fitting indicates that they had an initial function relating to construction, rather than a purely aesthetic or astrological role.

Another common idea is that these may have been useful in the transportation of stones. However, they are almost always on only one side of the stone, whereas their backs are rough and unworked when used as retaining walls. Because of this, and since ropes would slip off the relatively rounded nubs, this suggests that they were not used to help secure ropes to aid in hauling stones. They are also not found on the tired stones left in transit leading from quarries.

All of these observations are consistent with these nubs being utilized as lifting bosses, used to push the front-edge of stones upwards during the fitting process. Picture what this work would have entailed for masons trying to get a tight fit. If they placed a stone onto a lower row of blocks, but the joint wasn’t quite right, they would have needed a way to tilt the heavy stone back up again. Trying to jam one of their pry bars into the crack would have damaged stones’ carefully shaped edges.

Creating lifting bosses to push up upon is the obvious solution to this problem. For balance, the logical placement of lifting bosses would be exactly where they are found, either with two positioned towards the corners, or one in the center. This also explains why these lifting bosses aren’t as common on smaller stones, since those would have been easier to lift back up again without needing these extra protrusions. It’s little wonder why stone masons in other parts of the world arrived at the same conclusion, adding nubs to their own stones to serve a similar purpose.

"Protuberances were often left on the blocks of stone to provide a purchase for the pry bars, and, in many walls, the protuberances were never removed. They were probably considered decorative, as similar ones were in Greece in the 4th century B. C. The protuberances are usually near the bases of the blocks. … Many stones at Sacsahuaman have indentations instead of protuberances at the base, no doubt for the same purpose."

• John Howland Rowe, Inca Culture at the Time of the Spanish Conquest (1946)

Further confirmation of their function comes from their shape and wear patterns. These nubs aren’t perfectly symmetrical above and below. Rather, their tops are commonly more rounded whereas their undersides have more abrupt edges, sometimes slightly cupped. This would have both reinforced the protrusion from above while providing a more supportive overhang. It also would have allowed workers to push upwards with logs, temporarily propping it back. For those questioning the strength of wood, a 12” diameter log will support in excess of 50 tons (Lee, 1986).

When examining the undersides of these lifting bosses, damage that appears to originate from below is often seen. These chipped or broken nubs are apparent on about half of those remaining nubs at Hatunrumiyoc. Marks such as these would be expected if they were indeed levering upon these with pry bars.

If they could get a tight fit on the first attempt, then there wouldn’t have been as much need for these nubs to re-lift the stones for further shaping. The degree of damage to some of them also suggests that they had been used repeatedly to lift the stones. This is consistent with the multiple Spanish references saying that they needed to re-work and re-fit each stone until a tight fit was made.

While these lifting bosses are the most common form of stone protrusion, there are other varieties that served different functions. The upper edges of walls sometimes have a series of stone pegs extending about a foot. These were used to tie ropes securing their thatch roofs (Bingham, 1952). A few modern replicas roofs have been re-created at Machu Picchu. Another style of protrusion are the diagonal stone steps, which they used for ascending agricultural terraces and walls.

A more enigmatic style of nub can be seen at what appears to be a huaca at Ollantaytambo. These are slightly inclined, and are carved directly into the bedrock, so clearly wouldn’t have been used as lifting bosses or tie downs for roofs. Hiram Bingham noted that stone pegs were also found inside of their homes, which he speculated were for hanging various items including jars of chicha (corn alcohol). If this is a huaca, a religious altar, then they would have made sacrificial offerings of things they valued, a practice that continues today during Pachamama (Mother Earth) celebrations. So perhaps these were used to hang offerings, such as corn, quinoa, and vases of chicha?

“Stone pegs were usually placed between the niches and on a level with the lintels. They made handy hooks for all sorts of purposes. It is quite possible that from them were hung the characteristic water or chicha (beer) jars which had pointed bottoms. Their handles are so placed in the line of the center of gravity as to make it easy to suspend them and, by using a nubbin on the shoulder of the jar, to pour out a drink without having to take the jar down from the peg. The pegs were also convenient for fastening one end of a hand loom, while the weaver sat on the ground with the other end of the loom tied to his or her waist.”

• Hiram Bingham, Lost City of the Incas (1952)

Another possibility is that some of these protrusions also had an astronomical function, as other Inca sites were certainly constructed with those alignments in mind. During the solstice, shadows from the right two pegs are said to cast down into two notches on the ridge of the stone below (Salazar & Salazar, 1996). This doesn’t, however, explain why they are inclined, or the role of the first peg, which doesn’t have a lower notch. It’s also possible too that some of these nubs had dual purposes. For example, while lifting bosses were primarily used to aid in construction, some may have been intentionally left in order to track solar positions during the turning of the seasons. Incan astronomy books claim to have discovered some of these alignments. Skepticism here is appropriate considering that seasonal shadows wouldn’t explain all the unremoved nubs, and occasional solar alignments on a subset of these may be coincidental rather than intentional. Or some of these may have simply been aesthetic, where they enjoyed how their shadows travel across the stone walls as the sun moved across the sky.

Clearing Up Stone-Melting Confusion

As detailed above, we have multiple lines of evidence indicating that the Incas’ stone masons used hammerstones to skillfully shape these stones. That is supported by eye-witness accounts of the early Spanish chroniclers, dozens of hammerstones found at these sites, rock chips found below the walls, and the hammerstone impact marks seen on the rocks themselves.

Despite this evidence, later folklore suggests the Incas possessed plants or acids capable of softening stone. According to this hypothesis, that would hypothetically make it easier to ‘squish’ a stone’s placement into neighboring stones, allowing for ultra tight joints. Jason Colavito recently did a deep dive into the history of this legend (2025). Hiram Bingham was the earliest to mention local folklore of the Incas’ masons using some kind of stone softening liquid:

“The modern Peruvians are very fond of speculating as to the method which the Incas employed to make their stones fit so perfectly. One of the favorite stories is that the Incas knew of a plant whose juices rendered the surface of a block so soft that the marvelous fitting was accomplished by rubbing the stones together for a few moments with this magical plant juice!"

• Hiram Bingham, Across South America (1911)

However, this reference is dated to nearly four centuries after the Spanish conquest, so can it be trusted? The original source of these stories may be related to a much earlier story dating to 1638 about a woodpecker using some kind of herb to dissolve stone, where it was said that prisoners used that same herb to dissolve iron to break out of their chains. But a very similar folktale appeared in earlier classical sources too, so the story may have been brought to the Americas by the Spanish (Froemming, 2006).

A 2017 paper by Helmet Tributsch theorizes that they used an acidic mud to soften the stone. In it, he quotes Cieza de León mentioning "melted gold" being used in place of mortar, suggesting that excerpt is evidence for his proposed acidic stone softening liquid. But rather than this being evidence for stone softening, it might be better understood as referring to the T and O shaped clamps that they used to join their stones together at a few of these sites, including Ollantaytambo and Qoricancha. Molten metal was poured directly into these carved recesses to help lock them in place. The melted gold here may have simply been bronze, which is known to have been used on these clamps.

“Moreover, it is said with certainty that in these buildings of Tambo — or in others that bore the same name, for this was not the only place called Tambo—there was found in a certain part of the royal palace or the Temple of the Sun some melted gold used in place of mortar, which, together with the bitumen they applied, caused the stones to be set and joined firmly one to another.”

• Pedro Cieza de León (1518-1554), The Chronicles of Peru (1553)

There are also some stones at these sites that are alleged to show physical evidence of stone-softening. Rocks appear to have a glassy surface, while others appear almost to be melted. These features can be well explained with a better understanding of the area’s geology.

Limestone, common around Cusco and especially at Sacsayhuaman, is primarily calcium carbonate (CaCO₃), which dissolves in acidic water. This process formed the area’s karst landscape of caves and irregular formations. Industrial-era sulfur pollution has accelerated limestone dissolution. Within just the last century ma