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

Author:

Hooke Roger LeB1,Hanson Paul R2,Belknap Daniel F1,Kelley Alice R1

Affiliation:

1. School of Earth and Climate Sciences and Climate Change Institute, The University of Maine, USA

2. Conservation and Survey Division, School of Natural Resources, University of Nebraska–Lincoln, USA

Abstract

When the Laurentide ice sheet retreated rapidly (~150 m/a) across the Penobscot Lowland between ~16 and ~15 ka, 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 ~14 ka 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 ~13.4 and 12.8 ka, 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, ~11 ka, 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 ~8 ka and aggradation in terraces until ~6 ka. The climate became wetter and warmer after ~6 ka, 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.

Funder

School of Earth and Climate Sciences, University of Maine

Publisher

SAGE Publications

Subject

Paleontology,Earth-Surface Processes,Ecology,Archeology,Global and Planetary Change

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