The conglomerate-sandstone transition (CST) preserved in foreland basins records the stratigraphic equivalent of the gravel to sand transition (GST) which migrates in response to shortening in steady state orogen-foreland basin systems. Two seismic profiles in the southern Junggar foreland basin provide the first seismic record of migrating CSTs and an opportunity to quantify its migration history in response to the growth of the northern Tian Shan. We identify the horizontal positions of the CSTs based on the distinct seismic signature of conglomerate versus sandstone and estimate their ages according to magnetostratigraphic sections. The linear regression analysis reveals that the CSTs migrated northward at 0.4 ± 0.1 mm/yr along 84°E from ∼23 Ma to ∼6 Ma and at 4.0 ± 1.2 mm/yr along 87°E from ∼3.1 Ma to ∼0.7 Ma. The discrepancy relates to variations in crustal shortening along strike of the range. Temporal deviations in the CST positions suggest the second-order impacts of climatic change.