Lava flows ⁴⁰Ar/³⁹Ar analysis and glacial geology, Lago Buenos Aires, Santa Cruz Province. Eight basaltic lava flows and two erratic boulders in moraines east of the Andes near Lago Buenos Aires, Santa Cruz province, Argentina (46º30'-47º00' S; 70º30'-71º45' W) and one lava along the Río Gallegos valley (51º50' S, 70º40' W) were analysed using the ⁴⁰Ar/³⁹Ar incremental-heating technique. The ⁴⁰Ar/³⁹Ar results indicate that these samples include: (1) earliest Miocene (24.2 Ma) valley-filling lava flows, (2) late-Miocene (10.0 Ma, 7.4 Ma, and 5.0 Ma) meseta-capping lava flows, the latter two interbedded with glacial till, (3) Pleistocene lavas erupted at 1.17 Ma, 1.02 Ma, 0.76 Ma and interbedded with glacial tills, (4) a very young lava that proved difficult to date, and (5) basaltic erratic boulders that erupted at 117.5 Ma and 16.8 Ma, but which were incorporated into moraines during the late Pleistocene. These age determinations provide new constraints on the timing of Miocene and Pleistocene glacial advances of the southern Andean ice cap. The oldest Cenozoic till recognised in South America underlies a lava capping Meseta de Lago Buenos Aires whose ⁴⁰Ar/³⁹Ar isochron age of 5.04 ± 0.04 Ma is older than a previous K-Ar date by >400 k.y. Glacial till , together with outwash gravels and erratic boulders, that correspond to the greatest extent of ice in Patagonia (the so-called "Greatest Patagonian Glaciation", or GPG) are underlain by a lava dated at 1.168 ± 0.007 Ma and overlain by a lava dated at 1.016 ± 0.005 Ma. Thus the GPG occurred sometime between oxygen isotope stages 30-34. At Lago Buenos Aires, the deposition of at least twelve moraines since the GPG, including one overlain by lava dated at 0.760 ± 0.007 Ma, are consistent, in general terms, with the number and timing of glacial advances predicted by the astronomical time scale based on global marine oxygen isotope stages.