Radargrams obtained by the Yutu-2 rover reveal that the post-mare deposits at the Chang'E-4 landing site are similar to 45 m thick, consistent with estimations based on orbital observations. Besides obvious ejecta from Finsen, polarized interpretations exist on whether or not the Alder crater is another contributor to the post-mare deposits, although predicted thicknesses of ejecta from all potential source craters are not adequately large compared to observations. We recognize that the rover has been exploring along a shared crater wall of two secondaries from Finsen. Mechanics of secondary impacts suggests that the discontinuous layers of coarse and fine materials observed in the radargram are older than Finsen. Cross-cutting relationships and crater density comparisons show that Alder is older than the mare basalts, and the Orientale basin was the major source for the post-mare materials. Lower-crust materials excavated by Orientale constituted a substantial portion of the surface regolith detected by the rover.