By: Paul R. Martin -- Inca walls, Inca stone masonry, Inka walls, Inka stone walls, Inca stone walls

August 23, 1987

After having studied pictures of Inca stone walls, and after being inspired by an idea proposed by Joseph Davidovits, I have come up with what I believe is a new proposal for the method of construction for the Inca walls.

Generally, the method is that most, if not all, the blocks in the walls are made of concrete which were cast in their relative position in the walls using previously cast neighboring blocks as part of the forms. After the casting was completed, the walls were disassembled, beginning with the upper courses, and reassembled, inverted, at the final building site. The entire wall was inverted so that the top course in the casting process became the bottom course in the final wall. Given that the entire wall was transported, this inversion would require the minimum labor to accomplish since each block could be removed from the casting site and taken to the building site and placed directly into its final position. If the wall were not inverted, the upper- courses which would have to be removed first from the cast wall, would have to be stored in an intermediate location until the final wall was built up to the height to receive them.

This idea is suggested by the drawings labeled Fig. 1. through Fig. 5>. These are taken from photographs appearing in "Scientific American" and the book "Machu Picchu". Since these are pictures of final walls, it is easiest to see the proposed construction method by observing them upside-down. For that reason, the block identification numbers on the figures are written upside-down. Each identifiable block is given a unique identification number.

The key feature that caught my eye and gave me the most trouble in accepting Protzen's explanation ("Scientific American" Feb. 1986 pp. 94-105) is what I call "Utah" blocks. These blocks (e.g. 4, 10, 13, 21, 22, 35 in Fig. 2) resemble the shape of the state of Utah in that they have a right-angled notch cut out of one corner. If Protzen's explanation is correct, it would seem extremely difficult to pound out the sharp inside corner. It would seem to be much simpler to pound off the corner of the matching stone. For example, it doesn't seem reasonable to me that the masons would pound out the inside corner of block 22 (see Fig. 2.) to receive the corner of block 36, when they could simply pound off the outside corner of block number 36 to fit.

Another interesting feature of the Utah blocks is that in almost all cases, the corner is cut out of the top of the block, and seldom from the bottom. Block 328 in Fig. 3 is one of the few counter-examples.

To show how this method might have been used, look at Fig. 2 so that the block identification numbers are right-side up. I will give a possible sequence of construction and as I go, I will point out how the pattern of blocks supports my idea.

To start with, suppose that all the blocks to the left of, and including, block 83 are already completed, as well as the course below blocks 82 through 87 and also block 88. With this starting point, block 87 is cast using a form for its left side and block 88 as the form for its right side. Next, block 86 is cast with a form on either side; the right form sitting on top of block 87. Next, 85 is cast using only a form an its left side. Finally, block 84 is poured with no need for any forms because blocks 83 and 85 have already been cast.

For the next course, block 70 is cast using two forms followed by 71, 72, and 73 using one form each. At the same time, block 75 is cast using two forms followed by 74 and 76.

Since information on the structure is unavailable, assume that the course to the right of 76 is completed.

For the next course, block 61 is cast first using two forms. The remaining blocks in this course are cast in sequence going both directions from block 61 using a single form for each block.

The irregular shape of the top of block 57 is somewhat of an anomaly. It is possible that some blocks, such as 57, were not cast but were quarried stones set in among the cast ones. Another possibility is that 42 is a quarried stone which was set on top of 57 before 57 was set causing 57 to take on the shape of the bottom of 42.

The general method of casting a course, then, is to start with one or more blocks using two forms, and then working out from these "starting" blocks using one form for each block until they meet. The last block at the "meeting" is cast with no forms. This way, the starting blocks end up being the tallest blocks of the course and the blocks get successively shorter as you move from the starting blocks to the meeting blocks. The meeting blocks, then, are usually the shortest blacks in the course.

Using this general method, the next course begins with blocks 42, 47, 50, and 54. Block 53 is an example of block at the "meeting" for which no form is necessary. Block 52 presents the same anomaly as block 57. The gap between 45 and 46 could have been filled with no form, however since it would have made such a thin block, it looks like the builders decided to skip it for this course and fill it in for the next course. The same thing happened an the previous course between blocks 73 and 74.

Since the information an the next, course (24 through 39) is incomplete, it is unclear exactly what sequence was followed. It is clear, however that 36 is a starting block, and it looks like 32 is a meeting block.

In the next course above, it looks like 16, 18, and either 21 or 22 were the starting blocks. Block 17 is a typical block cast an top of a meeting block, or a meeting gap, which results in two concave corners. Other examples of this are 59, 72, 205, and 407.

For the next course, it looks like 8 and 12 were the starting blocks and 9 is the meeting block between them. For the top course, it looks like 3 and 6 were the starting blocks, although 6 may be a quarried stone.

In this figure, I think the sequence of blocks 36, 22, 13, and 5 is interesting in that it contains three nested Utah blocks. I think it is a result of this particular place on the wall being a place where workmen cast a starting block for each new course (54, 36, 22, 12, and 3).

Turning to Figure 4 the same general method explains the pattern. The tops of blocks 224 and 226, however, present the same anomaly as discussed earlier. A possible explanation for 224 is that the concrete was beginning to set before it was poured. This would account for the convex shape. The left side of 226 could have been formed by a form of that shape, although it is unlikely that the top of 225 would have met at exactly the point of the bend in the side of 226.

The same general method of construction applies to the remaining figures, so I will comment only an peculiarities present in these examples.

Figure3 presents the same anomaly in block 326 as we saw in 57, 224, and 226. Any of the previous explanations for this anomaly could apply.

Block 328 is a rare example of an upside-down Utah block. It is possible that 320 is a quarried stone and was set on top of 329 before 328 was cast. It is also possible that 328's shape could have been made by forms much as we cast concrete steps today but the relative rarity of this pattern and the absence of a reason to do so make it seem unlikely.

Figure 5 shows an interesting variation in block 412. It is a combination of a meeting block between 416 and 417) and a starting block. Even though this is unusual, it is not inconsistent with the general method described.

Other pictures of Inca walls that I have studied seem to follow the same pattern. It would be interesting to chemically analyze the blocks to determine if some or all of them could have been made of concrete.