The method of roundness calculation based on the use of outcrop photos, the CorelDraw software and the PlotCalc plug-in was tested for the case of conglomerates from the Neoarchean molassoid complex of the Koikary area (Central Karelia). According to the results, the roundness of the debris depends on their composition and varies across the section of the molassoid complex. For the clasts of komatiite-basalt and andesitic composition the degree of roundness is the lowest, whereas for plagioporphyrygranites it is the highest. A lateral series of the potential sources of the clastic material is proposed. The mathematical calculation of roundness values is shown to be efficient, so that information can be obtained on the remoteness of the sources of clastic material needed for lithological and geodynamic studies.