"Relatively thick glacial sediments form a nearly continuous cover over the rolling landscape of central Sweden. Here sand and gravel is quarried for construction material, but the miners must work around huge erratic boulders. Grytan near Lake Storsjön, Sweden. Photo date 6/87, © J.S. Aber."
The Baltic Sea formed as The Ancylus Lake about 9000 years ago with the retreat of the Wechselian glacier. So I infer that its seabed was subglacial ground during the first phase of that retreat.