Uchida et al. Herit Sci (2020) 8:38 https://doi.org/10.1186/s40494-020-00383-2

RESEARCH ARTICLE Open Access Supply ranges of stone blocks used in masonry bridges and their construction period along the East Royal Road in the , Etsuo Uchida1* , Yuichiro Sakurai1, Rathborith Cheng2, Ichita Shimoda3 and Yu Saito1

Abstract During the period, there were fve Royal Roads linking the capital city of Angkor with provincial principal cities. Seven Temples d’étape, six Fire Shrines, and 25 masonry bridges were constructed along the East Royal Road to of Kompong Svay. We conducted measurements of magnetic susceptibility and chemical composi- tion of laterite blocks and magnetic susceptibility of sandstone blocks used for the construction of the bridges to determine the supply ranges of the stone blocks and the construction ages based on the results obtained in this study and previous studies of Temples d’étape and Fire Shrines. The results suggest that most of the sandstone blocks for the bridge balustrades were supplied from quarries in the southeastern foothills of Kulen Mountain, but that the bridges close to Preah Khan of Kompong Svay have sandstone balustrades supplied from nearby quarries. In contrast, cluster and principal component analyses and t-tests using data for chemical composition and magnetic susceptibil- ity of laterite blocks revealed that there were fve sources of supply. These results elucidated that the supply ranges of laterite blocks were narrower than those of the sandstone blocks. Judging from magnetic susceptibilities, supply ranges, shapes, orientations of bedding planes, and stacking methods of the stone blocks, it was concluded that the construction age of the bridges is highly likely to have been in the early period. Keywords: Khmer monuments, East Royal Road, Bridge, Laterite, Sandstone, Chemical composition, Magnetic susceptibility, Cluster analysis, Principal component analysis

Introduction have been conducted since 1907, especially by the École Te Khmer monuments consist of Hindu or Buddhist Française d’Extrême-Orient (EFEO). temples widely distributed across Cambodia, Tailand, Te buildings of the Angkor monuments were con- , and , constructed mainly from the 9th to structed mainly with sandstone and laterite blocks. the 13th centuries. Among them, the temples distributed Bricks were also used in the relatively old monuments in and around , Cambodia, are now called the constructed in the 9th and 10th centuries. Te sand- Angkor monuments. Te Angkor area was the center of stones used in the Angkor monuments have been studied the Khmer Empire and covers approximately 400 km2. by Saurin [1], Delvert [2], Fusey [3], Uchida et al. [4, 5], Conservation and restoration of the Angkor monuments and Carò and Im [6], and the laterites by Uchida et al. [7]. Tere were fve Royal Roads connecting the capital city

*Correspondence: [email protected] of Angkor to provincial cities: Preah Khan of Kompong 1 Department of Resources and Environmental Engineering, Faculty Svay (East Royal Road) and Sambor Prei Kuk (South- of Science and Engineering, Waseda University, Tokyo 169‑8555, Japan east Royal Road) in Cambodia, Phimai (Northwest Full list of author information is available at the end of the article Royal Road) and Sdok Kok Tom (West Royal Road) in

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Tailand, and Wat Phu in Laos (Northeast Royal Road) using a portable magnetic susceptibility meter on sand- (Fig. 1) [8]. Six Temples d’étape (from west to east, stone blocks used for Temples d’étape and Fire Shrines Prasat Banteay Ampil, Prasat Chrei, Prasat Trapeang along the East Royal Road, and reported on the supply Khnar, Prasat Pram, Prasat Supheap Tbong, and Prasat ranges of sandstone blocks in the Angkor Wat period Trapeang Chambok), seven Fire Shrines (from west to (the late 11th to middle 12th centuries) and the east, Fire Shrine of Preah Khan, Fire Shrine of , period (the late 12th to middle 13th centuries) when Prasat Kansaeng, Prasat Toap Chey, Prasat Ta En, Pra- these buildings along the East Royal Road were con- sat Supheap Cheung, and Prasat Kuk), and 25 masonry structed, respectively. Temples d’étape and Fire Shrines bridges were constructed along the East Royal Road are located about every 14 to 15 km along the East Royal that connects with Preah Khan of Kompong Svay (Pra- Road. Temples d’étape are of an Angkor Wat style con- sat Bakan), which is located approximately 95 km east sisting of a Central Sanctuary, two Libraries, two or three of the Angkor area (Fig. 2). Tere were approximately Gopuras, and an Enclosure wall of similar dimensions (32 17 Fire Shrines built with laterite blocks but no Temples to 33 m wide and 37 to 38 m long), and are only found d’étape along the Northwest Royal Road [9, 10]. No Tem- along the East Royal Road. A Fire Shrine is also called ples d’étape nor Fire Shrines exist along the other Royal Dharmaçālā, Teap Chei, Maison du charité, Gîte d’étape, Roads. Tese facts suggest the importance of the East Guest House or Resthouse [9, 13–21]. Fire Shrines built Royal Road [11] compared with the other Royal Roads. with sandstone blocks consist of a single building 4 to Because many of the constructions along the East Royal 5 m wide and 14 to 15 m long. In this study, non-destruc- Road were built with sandstone and/or laterite blocks, tive magnetic susceptibility measurements and chemical this Royal Road is useful for understanding the supply composition analyses were carried out for laterite and ranges of the sandstone and laterite blocks. Uchida et al. sandstone blocks used in the construction of the bridges [12] conducted magnetic susceptibility measurements along the East Royal Road, to clarify their supply ranges.

Fig. 1 Map showing the fve Royal Roads radiating from the Angkor region Uchida et al. Herit Sci (2020) 8:38 Page 3 of 16 ​ k.mcfa.gov.kh/ ). Supply ranges of the Distribution of masonry bridges, Temples d’étape, and Fire Shrines along the East Royal Road. Numbers of structures are based on CISARK ( http://cisar Shrines are Numbers of structures Road. along the East Royal and Fire d’étape, Temples Distribution of masonry bridges, 2 Fig. stone blocks are shown with arrows shown blocks are stone Uchida et al. Herit Sci (2020) 8:38 Page 4 of 16

In addition, combining the results obtained in this study Tailand) of the Late Jurassic to Early Cretaceous [22], with those obtained previously by Uchida et al. [12] for and was one of the most commonly used construction the sandstone blocks in Temples d’étape and Fire Shrines, materials of the Angkor monuments. Te sandstone was we investigate the construction age of the bridges along supplied from the southeastern foothills of Kulen Moun- the East Royal Road. tain, approximately 30 km northeast of the Angkor mon- Tere are 25 bridges built mainly of laterite blocks uments [12]. Sandstone blocks were transported to the along the East Royal Road. Sandstone blocks were used Angkor area using canals and rivers [12]. Tis is almost for balustrades. Te bridges have a corbel arch structure, the shortest route from the quarry sites to the Angkor in which laterite blocks are ofset to meet at the apex area, with a total distance of 34 km. of the archway. Te sizes of the bridges vary, but many bridges are 10 to 30 m in length and 6 to 10 m in width. Previous studies Te largest bridge along the East Royal Road is Spean Ta Bridge construction ages along the East Royal Road Ong (no. 319, according to the Carte Interactive des Sites Judging from the decorative style of the sandstone bal- Archéologiques Khmers (CISARK; http://cisar​k.mcfa. ustrades, Boisselier [23] considered that all the bridges gov.kh/), which was constructed over the Chikreng River along the East Royal Road were built during the reigns at 65 m in length and 13 m in width (Fig. 3a). Several of Suryavarman I (1002–1049 CE) and Suryavarman II small bridges have no names, and several have the same (1113–ca.1150 CE), that is, in the Khleang period and the name, “Spean Khmeng” (Fig. 2). According to Uchida Angkor Wat period, respectively. Groslier [16] suggested et al. [7], two types of laterite can be distinguished in the that the Naga balustrades were added by Jayavarman VII Angkor monuments: porous laterite and pisolitic laterite. (1182–ca.1218 CE), and the bridges themselves predated Both types of laterite consist mainly of the same minerals: that, except for a few. Bruguier [24] suggested that previ- goethite, hematite, kaolinite, and quartz. However, difer- ously existing wooden bridges were burned by the Cham ences in the content of minor elements such as arsenic in 1177 CE and then rebuilt with stone blocks by Jaya- (As), antimony (Sb), vanadium (V), and strontium (Sr) varman VII. Based on a combination of node-networks, were found between them. Porous laterite is relatively epigraphy, and architectural information, Hendrickson rich in Sr, As, and Sb, but is depleted in V. Te laterite [8] inferred that the region along the East Royal Road was blocks used in the bridges in general show an intermedi- of great importance through the 12th century, and that ate texture between porous and pisolitic. the road system, including the bridges, was completed Sandstone blocks were used for the balustrades of perhaps centuries before Jayavarman VII. the bridges, although many have been lost. Both ends Decorations in various places, such as short pillars on of some balustrades were decorated with multi-headed the balustrades, are certainly similar to the Bayon style. Naga fgures. Te sandstone is gray to yellowish brown In the Bayon period, it was common for Garuda to be (feldspathic arenite) [4]. Tis sandstone occurs in the Red engraved on the front of multi-headed Naga fgures, but Terrain Formation (called the Phu Kradung Formation in this composition is not found on the bridges along the

Fig. 3 Photographs of bridges along the East Royal Road. a Spean Ta Ong (no. 319) and b Spean Khvav (no. 1599) Uchida et al. Herit Sci (2020) 8:38 Page 5 of 16

East Royal Road. Terefore, there is doubt in specifying were not used in the Angkor monuments, and are not this style. Furthermore, Buddha statues were engraved at found in the ancient quarries in the southeastern foot- the center of the hoods of multi-headed Naga, which are hills of Kulen Mountain [12]. Terefore, it is inferred that confrmed on bridge nos. 318, 319, and 1599 (CISARK; sandstone blocks with low magnetic susceptibilities were http://cisar​k.mcfa.gov.kh/). Tey are clearly due to modi- supplied from somewhere near Preah Khan of Kompong fcations in the subsequent iconoclasm. Svay. We confrmed that sandstone with similar low mag- netic susceptibilities is distributed in the southern foot- Magnetic susceptibilities of sandstone blocks used hills (13º 38′ 24″ N, 105º 01′ 11″ E) of Tbeng Mountain, in Temples d’étape, Fire Shrines and Preah Khan approximately 30 km northeast of Preah Khan of Kom- of Kompong Svay along the East Royal Road pong Svay [25]. Te average magnetic susceptibility of Magnetic susceptibility measurements of sandstone sandstone blocks measured in the modern sandstone blocks used in the construction of Temples d’étape and quarry along the road on the east side of Tbeng Mountain −3 Fire Shrines along the East Royal Road were carried out was as low as 0.5 × 10 SI units. Tis value is concordant previously by Uchida et al. [25]. with the magnetic susceptibility of sandstone blocks in Preah Khan of Kompong Svay is located on the east- the later period of Preah Khan of Kompong Svay. Because ern end of the East Royal Road. Sandstone blocks with sandstone blocks with high as well as low magnetic sus- relatively high magnetic susceptibilities, ranging from ceptibilities are found in the Inner and Outer Galleries, −3 1.4 to 2.0 × 10 SI units, and some with low magnetic and because even the sandstone blocks from the Angkor −3 susceptibilities, ranging from 0.3 to 0.6 × 10 SI units, Wat period with high magnetic susceptibilities have rela- −3 were used in this temple complex. Sandstone blocks with tively low values, ranging from 1.2 to 2.0 × 10 SI units, high magnetic susceptibilities were used in Prasat Kat it is presumed that the change from the sandstone quarry Kdei, which is the oldest temple in Preah Khan of Kom- with high magnetic susceptibilities to that with low mag- pong Svay, constructed in the late Khleang period (the netic susceptibilities occurred in the early Angkor Wat early 11th to middle 11th centuries) as suggested by the period [5]. inscription K.161, and also in the buildings in the central Along the East Royal Road, there are six Temples part of Preah Khan of Kompong Svay such as the Cen- d’étape (from west to east, Prasat Banteay Ampil, Prasat tral Sanctuary, Gopuras of the Inner Gallery, and the Chrei, Prasat Toap Chey, Prasat Pram, Prasat Supheap Northern and Southern Libraries. Tis fnding suggests Tbong, and Prasat Trapeang Chambok), which are that sandstone blocks with high magnetic susceptibili- thought to have been constructed in the Angkor Wat ties were used for construction in the early period, and period [25]. Te magnetic susceptibilities of the sand- then sandstone blocks with low magnetic susceptibilities stone blocks in these buildings increased from west to −3 began to be used later. Because the average magnetic sus- east (from 2.84 to 4.48 × 10 SI units) [25]. In contrast, ceptibility of sandstone blocks used in Prasat Kat Kdei is the magnetic susceptibilities of sandstone blocks from −3 1.60 × 10 SI units, and the magnetic susceptibilities of Prasat Trapeang Chambok at the eastern end are low −3 sandstone blocks used in the Angkor monuments from (0.39 × 10 SI units), similar to those with low mag- the same period (the late Khleang period) range from netic susceptibilities in Preah Khan of Kompong Svay. −3 1.1 to 2.4 × 10 SI units (Uchida et al. [5]), it is inferred Te change in magnetic susceptibilities of the sandstone that sandstone blocks with high magnetic susceptibilities blocks from Prasat Banteay Ampil to Prasat Supheap in Preah Khan of Kompong Svay came from quarries in Tbong is similar to that in the magnetic susceptibilities of the southeastern foothills of Kulen Mountain. Te build- sandstone blocks in the early Angkor Wat period [5]. Tis ings in the central area where the high magnetic suscep- fnding suggests that Temples d’étape were constructed tibility sandstone blocks were used are inferred to have eastwards and that sandstone blocks were supplied from been constructed in the Angkor Wat period because the the southeastern foothills of Kulen Mountain (Fig. 8 in cross section of the sandstone blocks is close to square Uchida et al. [25]). In contrast, it is inferred that the sand- in shape, the orientation of the bedding planes is almost stone blocks of Prasat Trapeang Chambok, at the eastern horizontal, and the sandstone blocks were stacked with end, were supplied from a sandstone quarry near Preah coursed ashlar [26, 27]. Because the magnetic susceptibil- Khan of Kompong Svay (Fig. 2). ities of the sandstone blocks from the Angkor Wat period Along the East Royal Road, there are seven Fire Shrines in the Angkor region were initially low and increased (from west to east, Fire Shrine of Ta Prohm, Fire Shrine −3 over time from 1.1 to 5.6 × 10 SI units [5], it is inferred of Preah Khan, Prasat Kansaeng, Prasat Trapeang Khnar, that these buildings were constructed in the early Angkor Prasat Ta En, Prasat Supheap Chueng, and Prasat Kuk) Wat period (Fig. 2). Sandstone blocks with low magnetic which are considered to have been constructed in the −3 susceptibilities ranging from 0.3 to 0.6 × 10 SI units Bayon period [25]. Te two westernmost Fire Shrines Uchida et al. Herit Sci (2020) 8:38 Page 6 of 16

of Ta Prohm and Preah Khan are located in the Ang- In addition, we measured non-destructively magnetic kor region. Similar to the Temples d’étape, the magnetic susceptibilities of laterite blocks of the bridges and sand- susceptibilities of the sandstone blocks used for the Fire stone blocks of the balustrades using a portable mag- Shrines increased from west to east (Fig. 9 in Uchida netic susceptibility meter (ZH instruments SM 30, Brno, et al. [25]). Te magnetic susceptibilities of the sandstone Czech Republic) with a reading surface of 5 cm × 5 cm. blocks used in the Angkor monuments tend to increase Measurements were made on randomly selected 40 fat −3 gradually from 0.7 to 3.1 × 10 SI units in and after the and non-deteriorated laterite block surfaces in total main Bayon period (the end of the 12th to early 13th cen- for each bridge. Although most of the sandstone blocks turies) [5]. It is inferred from this fnding that, similar have disappeared, possibly due to reuse by local people, to the Temples d’étape, the Fire Shrines were also con- measurements were made on up to 20 sandstone blocks structed from west to east, and the sandstone blocks used selected randomly for each bridge. For four bridges, nos. for construction were supplied from the southeastern 2498, 3701, 1600, and 3705 (CISARK; http://cisar​k.mcfa. foothills of Kulen Mountain. However, it is inferred that gov.kh/), no sandstone blocks remained, so we could not −3 the Fire Shrines at Prasat Supheap Cheung (0.36 × 10 measure magnetic susceptibilities. Magnetic susceptibil- −3 SI units) and Prasat Kuk (0.40 × 10 SI units) are made ity measurements were performed on fat surfaces not of low magnetic susceptibility sandstone blocks supplied deteriorated physically and/or chemically, and also not from a quarry near Preah Khan of Kompong Svay (Fig. 2). covered with algae or lichens, and the median values As described previously, at the beginning of the Ang- were obtained. Te magnetic susceptibilities of sandstone kor Wat period sandstone blocks were transported from depend on the content of magnetite ­(Fe3O4), which is a the southeastern foothills of Kulen Mountain to Preah minor mineral in the sandstone. Because magnetite is a Khan of Kompong Svay along the whole length of the heavy mineral compared with major rock-forming min- East Royal Road, around 60 km. In the early Angkor erals such as quartz, feldspar and micas, it concentrates Wat period, sandstone blocks were supplied to Prasat in specifc place. In addition, the content of magnetite in Supheap Tbong from Kulen Mountain, but by the late sandstone depend on source rocks. Tese factors deter- Bayon period, they were only supplied as far as Prasat mine the horizontal and vertical diference in magnetic Ta En (Prasat Pram) (Fig. 2). Tis confrms that the sup- susceptibility of sandstone layer. Conversely, the mag- ply ranges of sandstone blocks from Kulen Mountain netic susceptibilities of laterite are due to large amounts became narrower over time. of goethite (FeO(OH)) and hematite ­(Fe2O3), as well as minor amounts of maghemite ­(Fe2O3). Maghemite is a highly magnetic mineral, like magnetite [29]. Methods Ticknesses were measured for 25 laterite blocks In this study, non-destructive chemical composition of each bridge and the median values were obtained. analyses were carried out on laterite blocks used for Te thicknesses of both sandstone and laterite blocks bridge construction along the East Royal Road using decreased over time in the Angkor monuments, from a portable X-ray fuorescence analyzer equipped with around 45 cm before the Angkor Wat period, to around a silicon drift detector and a 4 W, 50 kV Rh X-ray tube 25 cm in the late Bayon period. Te cross section of stone confguration (Type DP-4000-C, Innov-X Systems Delta blocks was square until the Bapuon period, but changed Premium, Waltham, MA, USA). Te measurements were to rectangular after that [30]. We can see stone blocks carried out in a soil mode for precise analysis of minor with both square and rectangular cross sections in the heavy elements (using three diferent beams, beam 1: Angkor Wat period. In addition, the orientation of bed- 40 kV, 100 μA, beam 2: 40 kV, 100 μA, beam 3: 15 kV, ding planes was random before the period, but 200 μA). Measurement time was 20 s for each beam, and almost horizontal in the Angkor Wat period and later [4]. total measurement time was fxed at 60 s. Before meas- To compare with laterite blocks used for bridge con- urement, the instrument was calibrated using 10 refer- struction, laterite blocks used in temples at Chau Say ence rocks from the Geological Survey of Japan (JA-1, Tevoda, Prasat Suor Prat, Tommanon, Angkor Wat, JA-2, JB-1b, JB-2, JB-3, JG-1a, JG-2, JGb-1, JR-1, and JR-2) and Banteay Samre constructed in the Angkor Wat [28]. Analytical precisions were summarized in Table 1. period, and also Ta Prohm, Angkor Tom, and Banteay Te measurements were made on fat and non-deteri- Kdei constructed in the Bayon period, were subjected orated surfaces of stone blocks not covered with algae to chemical composition analyses, magnetic suscep- or lichens. Te measuring surfaces were cleaned with a tibility measurements, and thickness measurements. brush. For each bridge, measurements were made on fat According to Uchida et al. [7], there were no clear dif- surfaces of 10 diferent laterite blocks selected randomly, ferences in chemical compositions or magnetic suscepti- and the median values were obtained. bilities between the laterite blocks used for the buildings Uchida et al. Herit Sci (2020) 8:38 Page 7 of 16 0.5 29 29.5 36 30 27 31 35 28 30.5 34 32 34.5 29 34 29.5 27.5 28 30 34.5 31 29 34.5 28 36.5 Thickness (cm) 0.55 0.80 0.39 0.80 1.21 0.84 0.75 1.01 0.50 0.44 0.44 0.48 0.50 0.45 0.46 0.52 1.23 1.14 1.28 1.29 0.53 0.44 0.44 0.50 0.001 M.S.* 31 18 46 53 72 32 26 29 60 84 107 183 46 63 58 57 19 19 29 28 62 89 49 61 1.4 Zn (ppm) 38 29 49 26 67 20 26 34 135 166 166 108 45 52 57 27 46 53 53 39 67 44 99 208 1.6 As (ppm) As 1921 1411 1420 1328 2424 1740 2192 2544 1394 1709 2123 1472 840 978 1391 1058 1716 1611 1732 1814 1416 1382 1426 1161 8.6 V (ppm) 1084 555617 450687 376840 315716 535771 328955 466086 585268 422228 472045 559457 323235 244007 235661 301240 316576 490763 493618 515340 497275 494871 400229 517535 409642 Fe (ppm) Fe 1 22 10 33 42 47 30 20 26 62 67 67 37 89 41 40 168 217 160 152 125 141 143 141 170 Sr (ppm) 222 9977 9528 8577 9175 8911 8161 9627 7102 9787 8447 6557 8490 6924 10237 10613 11617 10936 11525 12295 11796 11448 11416 10752 10232 Ti (ppm) Ti 128 95 113 158 200 137 162 138 163 219 275 245 228 265 237 217 110 126 130 141 152 186 131 159 1.4 Zr (ppm) 8 5 7 9 9 9 4 4 12 19 16 14 13 21 14 13 9 7 7 8 8 24 14 8 0.3 Y (ppm) 68 34 39 42 80 65 84 79 60 57 46 87 84 85 93 91 131 94 110 36 287 68 140 163 1.6 Cu (ppm) Cu 208 266 187 187 205 128 190 205 171 215 179 108 102 99 133 124 246 225 230 180 159 167 135 124 7.5 Co (ppm) Co 672 260 424 311 534 370 394 621 454 587 910 470 298 245 489 498 648 589 425 526 230 257 318 252 8.5 Cr (ppm) Cr Kream (no name) Sp. Thmor Thmor Sp. Bay Sp. Khmeng Sp. Sp. Khmeng Sp. Sp. Khmeng Sp. Sp. Khmeng Sp. Sp. Yea Pun Yea Sp. Sp. Toap Chey Toap Sp. Sp. Khmeng Sp. Sp. Ta Ong Ta Sp. Sp. Touch Sp. Sp. Khvav Sp. Sp. Tradaev Sp. Sp. Phum O Phum Sp. Sp. Rontadev Sp. (no name) (no name) (no name) Sp. Khmeng Sp. Sp. O Phdar Sp. Sp. Khmeng Sp. Sp.Khmeng Sp. O Chattrea Sp. Sp. O Chungruk Sp. Monuments 2695 1025 5426 1974 306 3702 3703 2498 318 319 225 1599 3701 1600 3707 3705 2496 2495 7081 7397 213 3706 3708 2494 CISARK no. Median values of chemicalMedian values and thickness magnetic susceptibilities compositions, of laterite blocks used for bridge construction along the East Royal 1 Group D Group Group C Group Group B Group Group A Group Precision Table Road Grouping Uchida et al. Herit Sci (2020) 8:38 Page 8 of 16 35 N/A 31 36 33 24 32.5 40 42 30.5 29.5 Thickness (cm) 0.30 0.53 0.45 0.36 0.35 0.45 0.30 0.38 0.29 0.22 0.30 M.S.* 60 74 28 47 42 58 85 54 28 41 56 Zn (ppm) 134 198 217 176 337 178 275 310 208 326 152 As (ppm) As 763 686 1082 1393 864 851 562 1044 802 604 768 V (ppm) 137270 427471 396912 233858 297714 383051 265561 338433 281670 260241 298551 Fe (ppm) Fe 180 151 156 177 190 128 206 231 132 214 136 Sr (ppm) 4235 6411 7373 7029 5637 6431 4717 6973 6759 5304 6928 Ti (ppm) Ti 117 129 116 141 128 120 111 132 110 124 144 Zr (ppm) 51 15 19 56 19 11 17 19 17 17 12 Y (ppm) 82 83 72 71 66 89 73 52 67 44 57 Cu (ppm) Cu 146 201 199 172 154 168 138 144 125 136 167 Co (ppm) Co 193 357 365 490 381 392 165 472 401 274 346 Cr (ppm) Cr ------(outer enclo (outer sure) - (foun Tevoda dation) (foundation) (inner enclo sure) enclosure) Tevoda (outer (outer Tevoda enclosure) (inner enclo sure) (outer enclo (outer sure) (S2,N3) (outer enclo (outer sure) ung Touch Angkor Wat Angkor Wat Chau Say Chau Say Thommanon Banteay Ta Prohm (inner Prohm Ta Chau Say Chau Say Banteay Samre Samre Banteay Banteay Samre Samre Banteay Prasat Suor Prat Suor Prat Prasat Sp. Thmor Thmor Sp. Ste Monuments SI units) − 3 (10 ments Angkor monu - 466 CISARK no. (continued) Group E Group 1 Table Grouping west east to positions from to corresponds bottom to of the bridges from sequence The bold rectangular frames. by surrounded the bridges are group used to susceptibility data and magnetic Chemical composition *Magnetic susceptibility ­ Uchida et al. Herit Sci (2020) 8:38 Page 9 of 16

constructed in the Angkor Wat period and those built in in the temples along the East Royal Road is not diferent the Bayon period in the Angkor monuments. Tis fnd- from the sandstone in the Angkor monuments in terms ing suggests the strong possibility that they were supplied of chemical composition. In this study, therefore, the from the same quarrying area. However, their source is chemical composition analysis of sandstone used in the still unknown. bridges of the East Royal Road was not conducted. To infer the supply range of laterite blocks used for the bridges, hierarchical cluster and principal component Magnetic susceptibility analyses were conducted. Te median concentrations of Te median magnetic susceptibilities of the laterite −3 each element and magnetic susceptibility (Table 1) stand- blocks in the bridges range from 0.30 to 1.29 × 10 SI ardised to a constant total were used in the cluster and units (Table 1, Fig. 4, and Additional fle 3). Te same principal component analyses (Additional fle 1) [31–33]. groupings as those based on the chemical compositions T-tests were performed to examine the confdence of the were obtained: the median magnetic susceptibilities of deduced groupings of the bridges using chemical compo- the laterite blocks of Group A are low, ranging from 0.44 −3 sitions and magnetic susceptibility of laterite and sand- to 0.53 × 10 SI units; those of Group B are high, rang- −3 stone blocks. R (ver. 3.5.3) was used for the cluster and ing from 1.14 to 1.29 × 10 SI units; those of Group C −3 principal component analyses and t-tests [34]. are low, ranging from 0.44 to 0.52 × 10 SI units; those −3 of Group D range from 0.39 to 1.21 × 10 SI units; and Results the median magnetic susceptibility of the laterite blocks Chemical composition −3 of Group E is as low as 0.30 × 10 SI units (Table 1 and Te major components of laterite used in the Angkor Fig. 4). Te magnetic susceptibilities of laterite blocks in monuments were ­Fe2O3, ­Al2O3, and ­SiO2 [7]. Te port- Group E, closest to the Angkor monuments, are in the able X-ray fuorescence analysis in soil mode revealed same range as those of the laterite blocks in the Angkor 16 elements across all laterite blocks: K, Cr, Co, Cu, Zn, monuments from the Angkor Wat period to the Bayon −3 As, Y, Zr, Ti, Sr, P, S, Ca, Mn, Fe, and V. Silicon (Si) and period (0.22 to 0.53 × 10 SI units), and it is inferred aluminum (Al) were not detectable by the portable X-ray that they were supplied from the same quarrying area. fuorescence analyzer in soil mode. Five elements (K, P, S, Te median magnetic susceptibilities of the sandstone Ca, and Mn) that exhibited large variations or no system- blocks used for the bridge balustrades range from 0.23 −3 atic variations were excluded; then, the bridges were clas- to 3.43 × 10 SI units (Fig. 5 and Additional fle 4). Te sifed into at least fve groups based on contents of the number of sandstone blocks is small and they are widely other 11 elements (Table 1 and Additional fle 2), Group dispersed. No sandstone blocks remained for bridges A to Group E, from east to west (Figs. 2 and 4). Te lat- nos. 2498, 3701, 1600 and 3705 (CISARK; http://cisar​ erite blocks of Group A are relatively depleted in Cr, Co, k.mcfa.gov.kh/). Sandstone blocks of the four bridges Ti, Sr, and V, but are enriched in Cu, Y, and Zn compared nos. 213, 3706, 3708, and 2494 near Preah Khan of Kom- with those of Group B. Te laterite blocks of Group B are pong Svay on the eastern side of the East Royal Road have depleted in Y, Zr, Sr, and Zn, but are enriched in Co and low median magnetic susceptibilities, ranging from 0.23 −3 −3 V compared with those of Group C. Te laterite blocks to 0.51 × 10 SI units, averaging 0.31 × 10 SI units. of Group C are enriched in Y, Sr, and As compared with Sandstone blocks of the bridges on the western side have those of Group D. Te laterite blocks of Group D are higher median magnetic susceptibilities, ranging from −3 −3 depleted in Y, Sr, and As compared with those of Group 0.75 to 3.41 × 10 SI units, averaging 1.73 × 10 SI E. Tese facts suggest that the laterite blocks were sup- units. plied from fve diferent quarrying areas. Only the later- ite blocks used for bridge no. 466 (CISARK; http://cisar​ Thickness of laterite blocks k.mcfa.gov.kh/) in Group E, near the western end of the Te laterite blocks used in construction of the bridges East Royal Road closest to the Angkor monuments, are have rectangular ends, not square ends, and their median similar in chemical composition to the laterite blocks thickness ranges from 27.5 to 36.5 cm (Table 1 and Addi- used for the Angkor monuments from the Angkor Wat tional fle 5). Tese results are almost concordant with period to the Bayon period. It is possible that these those obtained by Uchida and Ando [30] for the laterite blocks came from the same quarrying area. and sandstone blocks used for temples in the Angkor In the Angkor monuments, gray to yellowish brown monuments from the Angkor Wat period to the Bayon sandstone was used as a main construction material, period, not before the Angkor Wat period. but there is no diference in the chemical composition between the temples. Our preliminary surveys have also revealed that the gray to yellowish brown sandstone used Uchida et al. Herit Sci (2020) 8:38 Page 10 of 16

Fig. 4 Box and whisker diagrams for contents of Y, Sr and V, and magnetic susceptibility of laterite blocks used in the bridges along the East Royal Road. Five groups (Group A to Group E) can be distinguished Uchida et al. Herit Sci (2020) 8:38 Page 11 of 16

Fig. 5 Box and whisker diagram for magnetic susceptibilities of sandstone blocks used for the bridges along the East Royal Road

Hierarchical cluster analysis of laterite blocks included in the area of Group D. In both diagrams, the We performed hierarchical cluster analyses using sin- area of Group A is separated completely from the area of gle linkage, complete linkage, group average, centroid, Group B, the area of Group B from the area of Group C, median, and Ward’s methods (Additional fle 6), but the the area of Group C from the area of Group D, and the analysis result by Ward’s method with a squared Euclid- area of Group D from the area of Group E. Te laterite ean distance gave the result most consistent with the blocks of Group A are enriched in Cu; those of Group B hypothesis presented in this paper compared to other and D are enriched in Co, V, and Fe and have higher mag- methods. In other words, Ward’s method classifed the netic susceptibilities; those of Group C are enriched in laterites used in the Angkor Wat to Bayon periods into Zn and Zr; and those of Group E are enriched in Sr, As, one group (Fig. 6), while the other methods split them and Y. Tese facts suggest that the laterite blocks of each into at least two groups. Ward’s method gave almost the group (Group A to Group E) were supplied from diferent same groupings (Group A to Group E) as those obtained quarrying areas. from the chemical composition and magnetic suscepti- Bridge no. 3706 (CISARK; http://cisar​k.mcfa.gov.kh/) bility data shown in Table 1 and Fig. 4. However, bridge was classifed as both Group A and Group C in the prin- no. 318 (CISARK; http://cisar​k.mcfa.gov.kh/) was reclas- cipal components 1 vs. 2 diagram, but was classifed as sifed from Group C (Table 1) to Group E in the cluster Group A, not Group C, in the principal components 1 vs. analysis, and bridge no. 3706 was reclassifed from Group 3 diagram. Bridge no. 318 was classifed as both Group A to Group C. A and Group C in the principal components 1 vs. 2 dia- gram, but was classifed as Group C in the principal com- Principal component analysis of laterite blocks ponents 1 vs. 3 diagram. Tese results are concordant Results of the principal component analysis are shown with the results obtained by grouping based on chemical in Fig. 7 as principal components 1 vs. 2 and principal composition and magnetic susceptibility data in Table 1 components 1 vs. 3 diagrams. In the former, the area of and Fig. 4. Group A classifed in Table 1 and Fig. 4 slightly overlaps Studies on laterite blocks in the Angkor monuments with the area of Group C, but in the latter, the area of [7] revealed that Fe, V and magnetic susceptibility have Group A does not overlap with the area of Group C at a positive correlation to each other, and that Sr, Y and As all. In both diagrams, the area of Group B is completely also have a positive correlation. It is, however, clarifed Uchida et al. Herit Sci (2020) 8:38 Page 12 of 16

Fig. 6 Dendrogram obtained from hierarchical cluster analysis using Ward’s method for the bridges along the East Royal Road, based on median chemical composition and magnetic susceptibility data for laterite blocks listed in Table 1. The median concentrations of each element and magnetic susceptibility standardised to a constant total were used in the analysis that there is a negative correlation between the former those in the bridges 7397 to 466 was extremely small −28 and the latter. Tese results are in a good agreement with (4.45 × 10 ). Tis result suggests a clear diference in those of the principal component analysis. provenance. Te t-tests were also conducted to examine the conf- T‑tests to examine the confdence of the grouping dence of the grouping of the bridges based on Y, V, and of the bridges Sr contents of laterite (Additional fle 1). As a result, p We conducted t-tests to examine the confdence of the values were less than 0.05 with one exception (p value groupings of the bridges based on magnetic suscepti- for V: 0.136 between Group A and Group B) (Table 2). bility of laterite and sandstone blocks (Additional fles Tese results clearly support that laterite blocks of 2 and 3), and p values were calculated. As a result, p Group A and Group B, Group B and Group C, Group C values for magnetic susceptibility of laterite blocks and Group D, and Group D and Group E were supplied between Group A and Group B, Group B and Group C, from diferent quarrying areas, respectively. Group C and Group D, and Group D and Group E, were −18 Discussion all extremely low (less than 2.65 × 10 ) (Table 2). Tese results revealed that laterite blocks of Group Based on the results for magnetic susceptibilities of A and Group B, Group B and Group C, Group C and sandstone blocks and those for chemical compositions, Group D, and Group D and Group E were supplied magnetic susceptibilities and thicknesses of laterite from diferent quarrying areas, respectively. In addi- blocks, presented in the previous chapter, we discuss tion, p value for magnetic susceptibility between the here the stone block supply range and construction sandstone blocks used in the bridges 2494 to 213 and period for the bridges along the East Royal Road. Uchida et al. Herit Sci (2020) 8:38 Page 13 of 16

Fig. 7 Results of the principal component analysis are shown as principal component 1 vs. 2 and principal component 1 vs. 3 diagrams. The median concentrations of each element and magnetic susceptibility standardised to a constant total were used in the analysis

Sandstone blocks units), whereas the four bridges on the east side (nos. Te same tendency as Temples d’étape and Fire Shrines 213, 3706, 3708, and 2494; CISARK; http://cisar​k.mcfa. is observed for the magnetic susceptibilities of sand- gov.kh/) have low median magnetic susceptibilities −3 stone blocks used in the balustrades of the bridges along (0.23 to 0.51 × 10 SI units). Tis fnding suggests that the East Royal Road; that is, median magnetic suscepti- the sandstone blocks for the bridges on the western side −3 bilities on the west side are high (0.75 to 3.41 × 10 SI were supplied from Kulen Mountain, but that those for Uchida et al. Herit Sci (2020) 8:38 Page 14 of 16

Table 2 P values obtained by t-tests to examine the confdence of the grouping of the bridges along the East Royal Road based on magnetic susceptibility and chemical compositions of laterite blocks Groups A and B Groups B and C Groups C and D Groups D and E

25 33 18 34 Magnetic susceptibility 3.06 10− 4.05 10− 2.65 10− 1.52 10− × 6 × 20 × 19 × 12 Y content 4.61 10− 7.11 10− 1.55 10− 3.76 10− × 4 × 6 × 15 × 33 Sr content 6.96 10− 2.10 10− 2.14 10− 8.04 10− × × × 6 × 16 V content 0.136 0.0151 5.29 10− 2.26 10− × ×

the bridges on the eastern side were supplied from the Mountain and Preah Khan of Kompong Svay, stone quarry near Preah Khan of Kompong Svay (Fig. 2). Te blocks may have been transported over land. Transpor- boundary between high and low magnetic susceptibil- tation of stone blocks by canal would have been difcult ity sandstone blocks is consistent with the boundary for because the altitude is the highest in the area around sandstone blocks used for Temples d’étape, but is dif- bridge no. 7081 (about 90 m above sea level), between ferent from the boundary for sandstone blocks used for Kulen Mountain (about 80 m above sea level in Beng Fire Shrines. Judging from the supply ranges of the sand- Mealea) and Preah Khan of Kompong Svay (about 65 m stone blocks used for the balustrades of the bridges, it is above sea level). Te lowest altitude is near bridge no. 319 inferred that the bridges were constructed from the early (around 40 m above sea level). Tis represents a difer- Angkor Wat period to the early Bayon period. Regarding ence in elevation of around 50 m. As mentioned above, it the magnetic susceptibilities of sandstone blocks used for is inferred that sandstone blocks were transported from the balustrades of the bridges, if we assume construction Kulen Mountain to Preah Khan of Kompong Svay in the occurred in the early Angkor Wat period, contradictions beginning of the Angkor Wat period, so it is reasonable do not arise with magnetic susceptibilities of sandstone to infer that the East Royal Road, as well as the masonry blocks used in the Angkor monuments. Judging from bridges, were already constructed at the time. Te facts these facts, it is highly likely that the balustrades of the that the boundary between Group A and Group B for the bridges along the East Royal Road were constructed in laterite bridges coincides with the supply boundary of the early Angkor Wat period. the sandstone blocks used for the balustrades, and that it also coincides with the supply boundary of the sandstone Laterite blocks blocks used in Temple d’étapes, support the idea that the Based on chemical compositions and magnetic suscepti- bridges along the East Royal Road were constructed in bilities of laterite blocks, the bridges were classifed into the early Angkor period. In addition, the facts that rec- fve groups (Group A to Group E) (Table 1 and Fig. 4). Sr, tangular cross section laterite blocks instead of square V, and Y contents, and magnetic susceptibilities are good ones were used for the bridge construction, that they indicators for grouping the laterite bridges (Tables 1 and were stacked with coursed ashlar, and that the orienta- 2). Analysis revealed that only bridge no. 466 (CISARK; tion of bedding planes is almost horizontal, suggest that http://cisar​k.mcfa.gov.kh/), classifed as Group E, was the bridges were constructed in the early Angkor Wat constructed from the same laterite blocks as those used period, not before the Angkor Wat period [27]. from the Angkor Wat period to the Bayon period. Tis fnding indicates that the laterite blocks used in the con- Conclusions struction of the Angkor monuments were supplied from Sandstone and laterite blocks used in the 25 bridges the area west of bridge no. 2695. Te supply ranges of along the East Royal Road connecting the capital city of the laterite blocks used for the construction of the other Angkor to Preah Khan of Kompong Svay were investi- laterite bridges are also narrower than those of the sand- gated to clarify the supply range of stone blocks and the stone blocks. Tis result suggests that laterite occurs in construction period of the bridges. many areas along the East Royal Road, and that procure- Tere are marked geographical diferences in mag- ment of laterite blocks was easier than that of sandstone netic susceptibilities of sandstone blocks used for the blocks. balustrades of the bridges along the East Royal Road. Sandstone blocks were supplied from the southeastern Based on these magnetic susceptibilities, it was inferred foothills of Kulen Mountain to Preah Khan of Kompong that sandstone blocks in the bridges east of bridge no. Svay in the beginning of the Angkor Wat period, and 213 (CISARK; http://cisar​k.mcfa.gov.kh/) were sup- were transported approximately 60 km. Because canals plied from a sandstone quarry near Preah Khan of have not been confrmed in the area between Kulen Kompong Svay, and that those west of bridge no. 7397 Uchida et al. Herit Sci (2020) 8:38 Page 15 of 16

were supplied from the southeastern foothills of Kulen Funding This research was fnancially supported in part by Grants-in-Aid for Scientifc Mountain. Tis result is concordant with that obtained Research of the Japan Society for the Promotion of Science (Uchida: no. from the sandstone blocks used for the Temple d’étapes 23401001) and Waseda University Grants for Special Research Projects (Uchida: along the East Royal Road [25]. no. 2016B-137). Te laterite blocks, which were the main bridge Availability of data and materials construction materials, are presumed to have been Not applicable. supplied from fve diferent quarrying areas, and the Competing interests supply ranges were much narrower than those of the The authors declare that they have no competing interests. sandstone blocks. Tis fnding indicates that laterite is widely distributed along the East Royal Road, and that Author details 1 Department of Resources and Environmental Engineering, Faculty of Science the procurement of laterite blocks was easier than that and Engineering, Waseda University, Tokyo 169‑8555, Japan. 2 Department of sandstone blocks. However, these ancient laterite of Geology, Ministry of Mines and Energy, Phnom Penh 12210, Cambodia. 3 quarries have not yet been found. World Heritage Studies, Graduate School of Comprehensive Human Sci- ences, University of Tsukuba, Ibaraki 305‑0821, Japan. Judging from the median magnetic susceptibilities, supply ranges, and characteristics of laterite blocks, Received: 19 January 2020 Accepted: 10 April 2020 such as cross sectional shapes and the orientations of bedding planes, the construction of the masonry bridges along the East Royal Road is highly likely to have occurred in the early Angkor Wat period (the late References 1. Saurin E. 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