Trace elements contained in rocks, especially those classified as potentially toxic elements (PTEs), can be largely harmful. Knowledge of the geochemical composition of waste is of great importance due to the potential possibility of contamination with these elements in the environment. The paper presents the geochemical characteristics of the sedimentary rocks from the Carboniferous coal-bearing series of the USCB. The present study used data for 120 samples from borehole WSx representing Zaleskie layers and Orzeskie layers within the Mudstone Series (Westphalian A, B). Major oxide concentrations (Al2O3, SiO2, Fe2O3, P2O5, K2O, MgO, CaO, Na2O, K2O, MnO, TiO2, Cr2O3, Ba) were obtained using an X-ray fluorescence spectrometry. The concentration of potentially toxic elements (Be, Sc, V, Cr, Co, Ni, Cu, Zn, As, Rb, Sr, Zr, Mo, Cd, Sn, Sb, Ba, W, Tl, Pb, Bi, Th, and U) was analyzed using inductively-coupled plasma mass spectrometry. As there are no relevant standards for the content of toxic elements in post-mining waste stored in dumps, the concentrations of elements were compared to their share in the Upper Continental Crust. Most elements, such as B, Sc, V, Cr, Ni, Cu, Zn, As, Sb, W, Tl, Pb, Bi, Th, and U had higher mean concentrations than those of the Upper Continental Crust (UC). Concentrations of the analyzed toxic elements in the studied samples did not exceed permissible values for soils, therefore they are not a potential threat to the environment. The results of the Pearson correlation analysis showed differing relationships among the analyzed toxic elements in the studied samples.