Since structure B1 was located immediately under the northwestern corner of the planned northwestern building (administration), and since it appeared deep and partially hollow, RNR-RC decided to shift its location. The new foundation trench was relocated at 140cm from the expected outer wall limit of the underground structure, that is, about 320cm from the center of B1 (RNR-RC / Helvetas Bhutan 1999:6, plan and section). This relocation enabled the RNR-RC team to go on with the building work and to preserve B1 for further investigation.

Morphology and state of preservation

A first inspection of the B1 remains made it clear that an important part of the construction had been lost. Beside the stones removed by the mechanical shovel, an important number of stones were subsequently detached during the numerous accesses to the inner part.

The morphology of the structure resembles very much to the sketch drawing made for the preliminary report. The inner volume is circular, with a rounded vertical wall made of tightly arranged angular stones of small to medium sizes (Fig. 14). The surface of the bottom is about 5.25m² and the largest perimeter around 8.1m. At the time we accessed the structure for the first time, this inner vertical wall had an overall height of about 100cm before it curved irregularly to a dome-shaped roof. This upper part seemed to be made of bigger stone slabs than the wall, arranged in horizontal to sub-horizontal positions (Fig. 14). The stones of the northwestern quarter of the roofing showed marked centripetal sloping. It was not evident to recognize if these signs of compression were recent, eventually due to the weight of the modern machines driving over the site (caterpillar, truck, mechanical shovel). It is quite possible that the subsidence was progressive through time.

In the western part of the inner wall, a heavy and thick horizontal slab marks the top of what appears as a covered gallery running perpendicularly to the wall in westerly direction (Fig. 14). This feature was discovered during the initial digging works. At another spot of the wall, and a little higher, a small quadrangular hole attracted the attention of the observers.

Information gathered about the initial preservation state made it clear that the deep lying inner volume was filled almost to the top with sediment. A hollow space nevertheless subsisted at its summit, under the roofing stones and slabs. Diggings took place soon after the discovery to get rid of part of the inner filling and to gain a better view of the shape. The workers took out about 6.2m² of silty to clayey sediment mixed with stones. This removed filling was intelligently set aside to be eventually screened at a later stage. The level reached was about 205cm below the initial terraced surface and does not represent any floor. The highest undisturbed roofing stones were lying about 30cm below the terraced surface.

If we observe the arrangement of the stones found in B1, we can infer that there is a clear partition between angular and rounded stones. The rounded boulders and stones are very rarely used as construction material for the inner wall, and never for the roofing, although they are of local origin (from the 70m-terrace riser). Moreover, we can still see more rounded stones in the bottom filling (digging surface) inside of B1. As to the angular stones, they are not of local origin, as a general survey in the immediate vicinity could confirm. The builders must have selected them as the adapted building material for their plan and brought them from an outcrop that is not located yet. From this, we can conclude that the stones which the RNR-RC team removed from B1 early after the discovery are following the same scheme: angular stones mostly came from the dismantled dome roof whereas rounded stones came from the excavated filling (Tab. 2).

Tab. 2: Table showing the number and types of stones removed from the structure B1.
Angular stones come from the dismantled roofing and rounded stones were located in the filling.

Included in the removed filling, the workers found a certain number of charcoal pieces and well preserved wood fragments. Part were sampled and handed over to us before we started for Bhutan. These samples were later added to those recovered during the campaign.

The RNR-RC team also discovered a few worked objects, including at least a fragment of a grinding stone, a rim fragment of a clayware recipient, and what appeared as a wood shingle. We shall describe these objects later.

Excavating B1

To understand the way the structure was built, we felt necessary to have a view in the deposits covering and surrounding the structure laterally. To achieve this, and since the base of the structure was very deep under the actual surface (at a minimum of 205cm), we decided to open an excavation sector in a restricted portion. We chose one quarter located to the west, so that it would include the underground gallery. The sloping of the stone slabs in the western part of the reaming roofing made it clear that those layers of stones had to be removed as fast as possible. Their positions represented a real danger for the worker eventually excavating the remaining filling of B1.

We set the size of the excavation sector to 16m² labeled F-I along a X-axis in northeast-southwest direction and 11-14 along the Y-axis in northwest-southeast direction. The square meter F14 is located approximately above the deepest part of B1. The angle of the X-axis is 148 degrees from the North direction.

The excavation properly started on April 15^th. During the preceding day, a yard foreman, accompanied by four workers, took the responsibility to build a shed over the location B1, thus protecting the excavation from the rainfall.

At first, we took off the upper layer of soil, which does not contain any stones or inclusions. We proceeded with crude tools at the beginning, for the 20-30cm near the top. Then we continued with subsequent horizontal scrapings, using finer tools. At the end of the first day, we reached the summit of the stone deposits. The next day, the excavation surface was extended in southwestern direction. The stones that appeared the first day already indicated that we had to do with an overall stoned surface bigger than expected. This impression was even more acute after the extension of the excavation, when we discovered the massiveness of the stone accumulation disposed to the west of the underground structure.

We cleaned the sediment lying over and around the slabs forming the dome-shaped roof. Some of those slabs attain very impressive dimensions (up to 0.5 m²) and correlative weights (certainly up to 130kg). During the third day, we obtained a regular level lying about 70cm below the vegetational surface, which could be recognized during the cleaning of the profiles. This grassy surface is presently compressed under the soils accumulated during the terracing works (see also Fig. 16).

Since mapping of such stone structures with scale drawings is very time consuming, we decided to document the obtained surface with photographs taken vertically, square-meter by square-meter, and then to translate them in drawings back in Switzerland. After we proceeded to this photographic documentation, we realized a rough sketch drawing containing the altitude measurements of the stones. Fig. 15 represents the redrawn surface of the first excavation level showing the horizontal distribution of the stones.

The drawing of the square-meters F-H/11-14 shows clearly the extension of the stony area in western direction, exactly in the same direction as the axis of the underground gallery. This extension measures a little more than 1m in width. Since the gallery was obstructed to the top with sediment at about 90cm from its junction to the inner volume of B1, it was impossible to establish its overall length. At this point, the excavation was not extended enough to enable a measurement of the gallery, but we could at least see that it is running further in southwesterly direction, at least as far as the squares J/12-13. It is thus possible to estimate its minimum length to 270cm, which is already considerable.

In this context, it is important to state that RNR-RC came to the idea that the structure B1 and the wall remains B2 were linked somehow through this gallery. They dug a 2m by 2m test pit, about 2m deep, with the hope of finding the gallery between the two initially discovered locations. No stone deposit appeared. As the yard engineer informed us about the approximate spot of this test pit, we had to recognize that the location was not chosen in the exact direction of the gallery. It was a few meters too far to the north. Nevertheless, this negative result indicated at least, that no massive structural remains were found in this tested area between B1 and B2.

At the end of the documentation of the surface obtained on B1, we proceeded to take away all the upper roofing stones, which were completely delineated (squares F-H/12-14). We numbered 1-14 all the stones bigger than 20cm, so it will be possible to replace them if the reconstitution of the structure is needed in the future.

At the end of this step, we cleaned again the sediment lying around the underlying stones, until a clear surface was obtained again. All the documentation steps were repeated until the delineated stones were again ready be taken away around the center of the hole in the roofing.

At first, we also intended both excavation layers to bring us toward an acceptable solution regarding the danger of a roof collapse. We had to recognize that the process for each layer was very much time consuming, even with simplified documentation steps. Unfortunately, it is not feasible to achieve the work at a faster pace with our small group. Overall, the mass of stones to remove in this quarter to dismantle the roof is about 0.8m³ (2m wide ring, 40cm thick), which represent about 2000kg of stone material! We must admit that we did not reach the depth we expected at the beginning.

To have information about the integration of B1 in the sedimentary deposits, we documented briefly one profile (Fig. 16).

Fig. 17 clearly show that there is a flat deposit of stones separating layers 4 and 6 in the western profile of the excavated sector. The repartition of this deposit seems to be concentrated to the west of B1 (squares I/11-14). At some point of the excavation process, it seems likely that a few stones belonging to this deposit were removed by the workers by inadvertence, especially between the roofing of B1 and the western profile. Nevertheless, the clear line of stones to be seen in the profile gives us a good indication of the vertical placement of this deposit, which can be seen as a rough pavement around the subterranean structure. The entrance of the underground gallery will certainly be found in this paved area.

Abandonment signs

Since most of the initial filling of B1 had already been removed by the RNR-RC team, we decided to concentrate on the surrounding and on the architectural features of the underground structure. In our mind, detailed excavation work inside of the structure had to wait until a few conditions could be achieved, especially the reduction of the risk of collapsing.

The bulk of material found in the structure has already been described (clayey silt sediment, angular and round stones, wood remains). Since there is no natural agent that could have brought these elements inside of the underground structure, excepting for the clay elements filtering through the roofing, the only explanation for their presence is that they have been brought in by humans. Thus, these elements become part of an intentional filling. The reason is very certainly linked with the abandonment of the structure and of the site; people who were using the location prevented it from being reused too easily. This strategy worked well, since the underground structure remained filled until it was discovered today.

Somehow, the initial ground of B1 could shed some light on the function of the structure. We spent only one day starting to look more precisely at the arrangement of remains in the deep part of the structure. Difficult light conditions, added to the fact that this inner space is limited, that we had to stand directly on the deposits, and that we had no time to easily build a documentation grid, brought us to the final decision that the excavation of the deposit should be postponed provisionally. In general, we can state that the remaining filling is limited in volume and that the real functional bottom of the structure is probably at less than 50cm from the actual pitted surface. The documentation of this part should be achieved during a future campaign.

Archaeological material

Until the top of the stone deposit of B1 was cleared, no significant artifact had been found in the excavation. As we were eager to find datable material, we looked especially for remains of charcoal. The result was meager until we came across what appeared first as a layer with sediment smeared by cinders, grayish in color, about 35-35cm large. The northwestern profile of the sector exactly cut this feature, which we recognized as a small fireplace, or at least as a cinder dejection from a nearby fireplace (Fig. 18). The top of this deposit is located 72cm below the modern grass surface, under what appears as an irregularly spaced stone surface or layer running along the line F-G/10-11 in the stratigraphic cut, thus about at the same altitude as the stone pavement observed in the western cut. We took samples from this fireplace for dating purposes. The remaining charcoal and wood pieces that were sampled for dating are also grouped in Tab. 3. Their provenience was mainly the deep part of the underground structure.

Tab. 3: Table of wood and charcoal samples from B1.

In the square G14, we found small fragments looking like oxidized iron. The morphology of the objects gives no indication of their origin. Further analyses will eventually tell us the exact composition of these objects.

Overall, the number of artifacts from B1 is extremely meager and the amount of chronotypological information to gain from it is almost none. Here follows the artifacts list (Tab. 4), including the objects found during the RNR-RC diggings. The provenience is mentioned precisely when it is known. Among the material from B1, we also have some objects which were probably brought in by humans, but which are no artifacts by themselves (Tab. 5).

Tab. 4: Artifacts found in and around structure B1 (locations marked with * indicate objects found during the RNR-RC diggings).

Tab. 5: Miscellaneous objects found in and around structure B1.

The information, chronological or historical, given by the archaeological material is poor, and at least not sufficient to date the structure in which it was discovered. The flat mill- or grindstone nb. 7 was used secondary as building material. At least, this indicates that the structure was built during a phase during which agriculture was widespread. The secondary use of this well shaped and functional object also shows that the builders were not particularly interested in preserving it. Maybe this type of fixed grindstone was not in use anymore at this time, or the builder had other tools to gain flour, or maybe they had no intention to grew or process crop at all at this location. This could even indicate that the location around the structure changed its economic status, eventually from intensive crop raising to a military functionality. The fragmented millstone nb. 1 found within the filling of the underground structure indicates that crop processing occurred in the vicinity of B1 some time before the filling was achieved. It is yet unclear if this object was part of a hand-turned mill or of a water mill similar to the one used up to this day in the region (Fig. 23).

The presence of a shingle in the filling shows that some kind of roof-covered architecture existed in the vicinity of B1. It could even be that such a building was directly linked to the underground structure, but the shingle could also have been gained from a ruined house or shed. Since another long piece of wood has been discovered near the bottom of the filling of B1, surrounded with traces of fire, it could be that shingles were used as firewood at a nearby spot, just before B1 had been filled. If this hypothesis is correct, then this again could indicate that other buildings existed at proximity, and also eventually that the area immediately around B1 was not so densely forested than now, or, in other words, that it was easier to collect shingles than to cut firewood! Another possibility is that underground structure B1 was complemented with a  yet undiscovered shingle-roofed building.

The presence of a possible whetstone indicates that sharpening of cutting implements occurred at the spot before B1 was filled. The rusted iron remains which we found are also indicators of the use of metal tools or weapons around the underground structure.

The scarcity of pottery remains could show that B1 had not primarily an agricultural or domestic function, but that turned plain pottery was in use before the structure was abandoned. Pottery is one of the thirteen traditional crafts of Bhutan. Its traditional name is jinzo (Ison 1997:112). Today, active potters are very few in the country. The rare information found in literature shows that earthenware recipients were mainly hand-modeled with the coil technique (Ibid.:113) (Fig. 24). We could not find any reference for the use of the turning wheel, but its existence is almost certain.

Time setting

Among the eight charcoal and cinder samples collected during the excavation of B1, we chose two that represented a clear event related to the use of the structure. We sent two samples for dating to the laboratory in Miami (Beta Analytic, Inc., Miami, Florida, USA). Both samples represent the burning of a fire, probably inside of the underground structure, but eventually also in its immediate vicinity (in this later case, the charcoal fragments were brought into the structure during its filling).

The first dated sample is labeled nb. 3 (Tab. 3); its dating received the laboratory code Beta-133587. It has been dated by the conventional radiometric method, whereas the second sample, labeled nb. 4 (Tab. 3), contained insufficient carbon for conventional dating and was thus dated by the AMS method (accelerator mass spectrometry); its laboratory code is Beta-133585. The quantity of carbon extracted from sample 3 was somewhat little, so the laboratory gave it an extended counting time resulting in a shorter time span.

The radiocarbon dating results are summarized here:

Beta-133587: 1220 ± 70 BP (before present = before 1950 AD)

Beta-133585: 1220 ± 40 BP (before present = before 1950 AD)

They cannot be connected to our calendar as directly. It is necessary to proceed to a calibration to calendar years. The calibration results provided by the laboratory are the following:

For Beta-133587:

2 Sigma calibrated result: 665 – 980 years AD cal (95% probability)

1 Sigma calibrated result: 700 – 890 years AD cal (68% probability)

The 2 sigma result means only that there is a probability of 95% that the real calendar age at which the firewood has been cut is located in the year range 665 to 980 AD. The following figure shows the age ranges before and after calibration (Fig. 25).

The probability curve of the calibrated result shows one probability peak (illustrated by a thick vertical line on Fig. 25) at 785 AD cal. This peak is the real center of the probability distribution. The arithmetic center of the range is 823 AD cal. Overall, this radiometric dating result enables us to state that the underground stone structure B1 was probably in use or filled at some time near the beginning of the 9^th century AD cal.

For Beta-133585:

2 Sigma calibrated result: 690 – 895 years AD cal (95% probability)

1 Sigma calibrated result: 765 – 875 years AD cal (68% probability)

Again, the 2 sigma result means only that there is a probability of 95% that the real calendar age of the wood cutting is located in the year range 690 to 895 AD. The following figure shows the age ranges before and after calibration (Fig. 26).

The probability curve of the calibrated result shows one probability peak (illustrated by a thick vertical line on Fig. 26) at 785 AD cal. This peak is the real center of the probability distribution. The arithmetic center of the range is 793 AD cal. Overall, this radiometric dating result enables us to state that the underground stone structure B1 was probably in use or filled at some time near the end of the 8^th century AD cal.

To summarize the time setting given by both samples, we can state that the underground structure B1 was very probably in use or filled at some time between the late 8^th century and the early 9^th century AD cal.

Function of underground stone structure B1

As we can judge from the preceding indications, it is necessary to investigate further to really understand the function of underground structure B1. At least, the hypothesis of the well to collect water, which had been formulated by local people and RNR-RC or Helvetas employees, can be abandoned. The presence of a clear defined access gallery, the shape of the dome-roof, the traces of fire inside of the structure, and the intentional stone filling, all these facts indicate that B1 was designed for some other purpose. And among the other hypotheses, the military function appears as the most probable, although it is still to be proven by more facts. We could easily imagine B1 to serve as a depot, a cache, or as a short-term protective hiding place. The possible connections and relations between the underground structure B1 and the stone wall B2 will eventually shed more light upon the function of the complex.

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Copyright 2000 by Blumer R. and Vial F. 
Copyright 1999 by SLFA Zürich, Switzerland.
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Last updated: 29-05-2001.