Late glacial and Holocene history of the Penobscot River in the Penobscot Lowland, Maine

When the Laurentide ice sheet retreated rapidly (similar to 150m/a) across the Penobscot Lowland between similar to 16 and similar to 15ka, the area was isostatically depressed and became inundated by the sea. Silt and clay were deposited, but no significant moraines or deltas were formed. The Penobscot River was reborn at similar to 14ka when ice retreated onto land in the upper reaches of the river's East Branch. As isostatic rebound exceeded sea level rise from melting ice, the river extended itself southward. Between similar to 13.4 and 12.8ka, it established a course across marine clay and underlying glacial till in the Lowland. Its gradient was low as differential rebound had not begun. Discharge, however, was higher and the river transported and deposited outwash gravel. During the cold, dry Younger Dryas, similar to 11ka, eolian sand began to accumulate in dunes in the Lowland. Some of this sand, along with fluvial sediment from the headwaters, was redistributed into terraces along gentler stretches of the river and into a paleodelta in Penobscot Bay. Eolian activity continued to similar to 8ka and aggradation in terraces until similar to 6ka. The climate became wetter and warmer after similar to 6ka, the dunes were stabilized by vegetation, the river began to downcut, and braiding became less intense. Pauses in the downcutting are reflected in discontinuous strath terraces. In due course, the river re-encountered the old outwash gravels, marine clay, glacial till, and, in a few places, bedrock. Its profile is now stepped, with gentle, gravel-bedded reaches between bedrock ribs that form rapids.