This dataset contains grain sizes of bright surface clasts extracted digitally from very high resolution aerial imagery for the coastal alluvial fan Guanillos (GUA) in northern Chile (21.97°S, 70.17°W). The largest diameters (a-axes) of bright clasts were derived by global thresholding of RGB orthomosaics (OM) constructed based on surveys using an unmanned aerial system (UAS). Depending on the spatial resolution of the OMs, the minimal detectable grain size is 20 to 22 mm. Restricted to detailed areas of interest (AOI), the gravelometric data is attributed to the four alluvial fan surface generations, or morphostratigraphic units, Q1 to Q4. Results are stored in a .csv table.