of the deterioration processes that affect the monoliths of the Monquirá archaeological park El Infiernito are presented. This work will serve as a base for later conservation and restoration activities.
Decay Phenomena
The exposed monoliths present significant problems due to the environmental conditions (Öcal et al. 2009) and show diverse, partly exposure-specific forms of weathering (Fig. 2). Approximately 200 monoliths and stone columns were used to visually assess and characterize the weathering behavior.
Figure 2: Decay phenomena on the monoliths. a) biological colonization and sanding, b) crust formation and salt efflorescence, c) alveolization, d) scaling.
The tropical climate at the ~2000 m a. s. l. El Infiernito area is characterized by two rainy and two dry seasons. The rainiest months are May and October with up to 200 mm, whereas January is the driest month with 40 mm followed by July with 90 mm. In addition to the rain, in the mostly semiarid microclimate, air humidity plays an important role as a water supplier. The relative air humidity fluctuates between 70–85 % depending on the season. Direct water absorption during rain events or capillary suction of surface and soil water, control the humidity regime. The monthly average temperatures are above 20 °C throughout the year, with lowest mean temperatures of 10 °C and highest mean temperatures of 22 °C and can rise to over 30 °C. Especially during the long dry and cloudless seasons, the temperatures on the monoliths’ surface can reach high values.
The monoliths show a large variety of different damage types, such as relief, back-weathering, bursting, rounding, covings and alveolization, which predominantly occurs. Neighboring areas of the stone surface weather back to different degrees, and create a moving relief, e. g. by weathering parallel to the layer or by the loss of components. Often honeycomb-like structures are formed, which are typical for alveolar weathering (Fig. 2c). The causes can be of a very different nature. These can include salt weathering and wind erosion, as well as structural and lithological peculiarities.
The deposition of dirt and dust particles in the pore space leads to a gradual compaction of the rock surface, and may favor biological growth on some monolith surfaces. Gray to brownish crusts (Fig. 2b) can be found on the stone surface as coherent layers, millimeter to centimeters thick. Visually, the limit between the natural patina and the damage pattern crust cannot always be clearly determined. Biological colonization by green algae, lichens and higher plants was observed on all monoliths (Fig. 2a). Loosely adhering whitish salt efflorescence is an indication of the build-up of harmful salts enriched in the stone (Fig. 2b).
Sanding, flaking, peeling (Fig. 2a, d), crumbling and exfoliation as well as transitional forms are evident. In addition to the relief formation, sanding 211is the second most common form of weathering with clearly varying intensities. Like sanding, scaling and flaking from plate-like areas parallel to the surface can lead to total loss. These phenomena are strongly connected to salt crystallization and expansional behavior, as may be fostered by strong sunshine during the day and followed by cold nights. Stress on the rocks’ fabric leads to the formation of cracks. However, the most noticeable damages to the stone surfaces are large, surface-parallel scales. Sometimes, nearly catastrophic damage events may occur on the exposed monoliths, like on Monolith V-0163, which was mapped in 2007 (Fig. 3 and 4), and was considerably damaged with a huge loss of material 12 years later (Fig. 3). In this example, material inhomogeneities like nearly invisible mica- and clay-layers inside the rock may have been an important primary damage factor. Scaling visibly occurs in connection with increased salt pollution, especially in the splash water area. Exposure-related scale formations are clearly recognizable. While on some monoliths only discoloration is observed, others show heavy break outs and material loss (Fig. 3).
Figure 3: Monolith V-0163. Critical condition and strong deterioration between 2007 and 2019.
Figure 4: Damage mapping of Monolith V-0163 in 2007.
Rock material and its characterization
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)
