Single crystalline copper and gold foils are bombarded with 1–5 keV argon ions. Transmission electron micrographs of the bombarded foils show black spots, which are caused by small interstitial agglomerates. Measurements of the spatial distribution of these agglomerates, carried out by means of a special stereo technique, indicate that the average distance of the agglomerates from the bombarded surface is much higher than the calculated range of the Ar ions. From the variation of the depth distribution of the agglomerates with the orientation of the foil and with the ion energy, it is concluded that the interstitial atoms, which form the agglomerates, penetrate into the lattice by means of focusing collisions. Tentative conclusions on the ranges of the focusing collision chains which transport matter (dynamic crowdions) are discussed.