## 3 Materials and Methods
### In Situ Gamma-Ray Spectrometry  
Gamma-ray spectra were acquired in situ in the field with a portable gamma-ray spectrometer (GF Instruments Gamma Surveyor Vario), equipped with a BGO probe (Bi\({}_{4}\)Ge\({}_{3}\)O\({}_{12}\)) with 2018 channels, along with a 51 mm \(\times\) 51 mm detector (103 cm\({}^{3}\)) with probe dimensions of 70 mm and a length of 290 mm (VB6), and a shielded photomultiplier. This equipment enables measurement of energies up to 3 MeV. Measurements were taken by direct probe contact at rock outcrops with variable geometry. We followed the recommendations given by the International Atomic Energy Agency (IAEA), as described by [PERSON] et al. [28] for static measurements (i.e., recording a spectrum in a given point during a specific amount of time). A period of 180 s is recommended by the manufacturer to optimise time and precision. Statistical error uncertainties--estimated to be 6% for K, 30% for U, and 16% for Th--were considered, according to the manufacturer's specifications for low gamma-ray emissions. [[http://www.gfinstruments.cz/index.php?](http://www.gfinstruments.cz/index.php?)]([http://www.gfinstruments.cz/index.php?](http://www.gfinstruments.cz/index.php?)) \begin{table}
\begin{tabular}{l l l l l} \hline
**Sample** & **Latitude** & **Longitude** & **WD** & **Location** \\ \hline P1 & 43°19′31.19′′ S & 8°24′31.6′′ W & I & Back of the Professors′ building in Faculty of Philology \\ P2 & 43°19′37.7′′ S & 8°24′27.48′′ W & I & Slope between Faculties of Philology and Sciences \\ P3 & 43°19′43.98′′ S & 8°24′26.4′′ W & I & Beside Casa del Frances \\ P4 & 43°19′34.81′′ S & 8°24′35.39′′ W & I & Beside Faculty of Sciences \\ P5 & 43°20′87.17′′ S & 8°25′0.23′′ W & I & Beside UDC kindergarten \\ P6 & 43°20′1.72′′ S & 8°24′56.7′′ W & I & Car park of Faculties of Law and Education \\ P7 & 43°19′58.71′′ S & 8°24′30.59′′ W & II & Back of CfCA building \\ P8 & 43°19′33.25′′ S & 8°24′33.09′′ W & II & Southwest of Faculty of Sciences \\ P9 & 43°19′41.17′′ S & 8°24′50.83′′ W & II & Surroundings of Castro de Elviña archaeological site \\ P10 & 43°19′58.71′′ S & 8°24′34.59′′ W & III & Car park of the Schools of Civil Engineering and Informatics \\ P11 & 43°20′12.3′′ S & 8°25′35.99′′ W & II & Path in front of Faculty of Law \\ P12 & 43°20′7.99′′ S & 8°24′54.14′′ W & II & Close to Sports Centre \\ P13 & 43°19′50.19′′ S & 8°24′19.05′′ W & I & Monument to Elviña’s Battle \\ P14 & 43°19′35.55′′ S & 8°24′44.64′′ W & III & Slope intercampus road \\ P15 & 43°19′42.9′′ S & 8°24′37.05′′ W & III & Canedo 15, Elviña \\ P16 & 43°19′52.23′′ S & 8°24′41.22′′ W & V & Slope Central Research Service \\ \hline \end{tabular}
\end{table} 
Table 1: Position and coordinates of the studied outcrops and samples (WGS84). WD: weathering degree. menu=gid&smenu=igr&cont=surveyor_V_&ear=ts (accessed on 11 October 2022). These errors were due to background radiation that mostly originated from three main sources: atmospheric radon and its daughters, cosmic rays, and instrument background radiation. Moreover, the geometry of measured surface must be planar (2\(\pi\)rr geometry), as possible surface roughness of the measured terrain and any nearby topographic features can both cause possible inaccuracies in the measurements. A second measurement was carried out for each studied outcrop using a lead collimator specially designed by the device's manufacturer to reduce the background gamma radiation from surroundings. From these spectra, we can extract estimated contents of K (potassium, mass percentage), eU (uranium equivalents, in mass parts per million (ppm)), and eTh (thorium equivalents, also in ppm). While potassium contents were estimated from the peak of \({}^{40}\)K at 1.461 MeV (energy range 1.366 keV-1.564 keV), there were several radioactive isotopes in the uranium and thorium decay series. Thus, we used \"equivalents\" of uranium and thorium, assuming secular equilibrium.