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As possible source rocks for the monoliths, four comparable sandstones (S1–S4) were taken in the immediate vicinity of the archaeological park. They likely correspond to four sedimentary geological formations consisting of the Arcabuco, Ritoque, Paja, and Churuvita, which are Late Jurassic to Cretaceous in age. The formations are described in more detail by Etayo-Serna (1968), Patarroyo (2008) and Renzoni (1983). Field observations point towards a higher utilization of S2 and S3 as possible source rocks for the monoliths.

S1 is a very fine-grained, gray sandstone, which appears heterogenous due to centimeter long white bands and dark gray to black colored lithic fragments (Fig. 5a). It shows sublitharenitic composition and a matrix (with argillaceous and organic components) supported fabric (Fig. 6a). Monocrystalline, angular quartz grains with various degrees of undulose extinction make > 75 % of the rock. 212Feldspar is occasionally visible (< 5 %). Quartz and feldspar show average grain sizes of < 0.2 mm. Mostly rounded, sometimes bent and elongated, chert fragments are greater in size and make up about 15 % of the rock. About 2 % phyllosilicates are found, preferably in the white bands of the sandstone. From the petrographical point of view, S1 could belong to the Ritoque Formation, which crops out near Villa de Leyva.

S2 is a whitish and reddish mottled fine-grained quartz arenite (Fig. 5b). Like in S3 and S4, the homogenous fabric is grain supported (bound by silica cement). Angular quartz grains (> 95 %) are partly polycrystalline and between 0.1–0.01 mm in size. About 30 % of the quartz grains are coated by iron oxides (Fig. 6b). Less than 5 % lithic components of very fine grained quartz and argillaceous material, as well as minor amounts (< 1 %) of mica can be found. A quartz arenite layer of the normally silty to pelitic Paja Formation, which actually crops out at the park, could be the possible source of this sandstone.

S3 shows macroscopically and microscopically strong similarities with S2. It has a whitish and reddish speckled appearance (like S2), but additionally contains white and red bands (similar to S1), which depicts a layering (Fig. 5c). The amount of very fine grained lithic fragments is higher than in S2 (> 5 %), the amount of iron oxides appears to be lower (Fig. 6c). Due to the strong similarities to S2, it can be assumed that S3 also originates from the Paja Formation.

S4 is a very fine-grained and very light colored, whitish-grey (with tiny yellowish spots) sandstone of sublitharenitic to quartz arenitic composition (Fig. 5d). The homogenous fabric is grain supported and very porous (Fig. 6d). The sandstone consists of > 90 % subrounded to subangular, polycrystalline quartz, with minor undulose extinction. The average grain size is about 0.01 mm. Less than 5 % fine grained lithic fragments (consisting of almost exclusively quartz), 2 % of opaques and < 1 % of mica can be found. A possible origin could be a very fine grained subgroup of the Arcabuco Formation, cropping out near Villa de Leyva. Although the Churuvita Formation crops out further to the southeast of the park, it cannot be ruled out as a possible source material.

Figure 5: Macroscopic photographs of a) S1, b) S2, c) S3 and d) S4 (size of the photos: 4.5 cm × 4.5 cm).

Figure 6: Thin section photomicrographs of a) S1, b) S2, c) S3, d) S4 in plane-polarized light and 2.5x objective. The blue resin visualizes the differences in pore space.