The Palmer Hill Glacial-Marine Delta, Whitefield, Maine

Introduction

The most recent continental glacier in New England was the Laurentide Ice Sheet, which covered Maine between about 25,000 and 15,000 years ago. The weight of this ice sheet pushed the Earth's crust downward, enabling the sea to advance across low-lying areas of southern Maine as the glacier retreated. In many places the sea was in contact with or very close to the ice margin, and streams issuing from the melting glacier carried vast quantities of sediment into the ocean. The coarser, heavier sediments (sand and gravel) were immediately dumped on the ocean floor, while plumes of fine mud dispersed farther out to sea. If the ice margin remained in one place long enough, the sediment pile built up to the ocean surface and formed flat-topped deposits called "glacial-marine deltas."

Figure 1

Deltas that formed where the glacier margin stood in the sea are called "ice-contact." In other cases, the meltwater streams flowed across terrain that stood above sea level before entering the ocean. Sand and gravel avalanched down the fronts of deltas, building a series of seaward-sloping layers called "foreset beds." Continued accumulation of the foresets caused the deltas to expand farther out into the ocean. This lateral growth caused the streams on top of the deltas to extend their seaward reach while depositing horizontal gravelly "topset beds" above the foresets. The boundary between topset and foreset beds in each delta is sometimes visible in gravel pits and marks where sea level stood when the delta was deposited (Figure 1).

Over 100 glacial-marine deltas are scattered across southern Maine, providing one of the best records of sea level in late-glacial time (Thompson and others, 1989). They are important sources of sand and gravel aggregate for the construction industry. They also store large amounts of ground water in their deeper portions, and the sandy soils host many of Maine's large commercial blueberry fields. The example described here is the Palmer Hill Delta in Whitefield, which is an especially good example of an ice-contact delta. It has long been a source of sand and gravel, and pit exposures through the years have provided opportunities to study it in detail. Many of the photos below were taken in the 1980's and 90's, so continued expansion of the pits has removed some of the features shown here.

Map 1

The Palmer Hill Delta is located near the southeast corner of the East Pittston quadrangle and overlaps slightly into the neighboring Wiscasset quadrangle as shown in Map 1. The colors and letter symbols on this map differentiate the sediments formed in various glacial environments. Map unit Pmdph is the Palmer Hill Delta, which formed when the glacier margin lay against Palmer Hill to the northeast and an unnamed hill to the southwest. Subglacial meltwater carved an ice tunnel in the gap between the hills, and the water rushing through this tunnel supplied the sediments that built the delta. The tunnel eventually became clogged with gravel, producing a long ridge called an "esker" (map unit Pge) that was revealed when the surrounding ice melted.

Glacial meltwater from the ice tunnel blasted up under pressure at the head of the delta and flowed southeastward across the delta top, carving a series of shallow channels indicated by the blue arrows on the map. Coarse gravels were deposited in these channels as they migrated across the delta top. The remaining sediments, including a lot of mud, were carried all the way to the front of the delta and into the ocean. Some of this material cascaded down the face of the delta, contributing to its seaward expansion, while the mud suspended in the meltwater plume dispersed farther away and became part of the Presumpscot Formation (map unit Pp) deposited on the sea floor.

The following photos show other features associated with the Palmer Hill Delta:

References

Newberg, D. W., 1992, Reconnaissance bedrock geology of the East Pittston quadrangle, Maine: Maine Geological Survey, Open-File Map 92-56, scale 1:24,000.
Map in pdf format - 1.7 Mb

Thompson, W. B., 2009, Surficial geologic map of the East Pittston quadrangle, Maine: Maine Geological Survey, Open-File Map 09-9, scale 1:24,000.
Map in pdf format - 1.9 Mb
Sidebar in pdf format - 641 Kb

Thompson, W. B., Crossen, K. J., Borns, H. W., Jr., and Andersen, B. G., 1989, Glaciomarine deltas of Maine and their relation to late Pleistocene-Holocene crustal movements, in Anderson, W. A., and Borns, H. W., Jr. (editors), Neotectonics of Maine: Studies in seismicity, crustal warping, and sea-level change: Maine Geological Survey, Bulletin 40, p. 43-67.

Text and photos by Woodrow Thompson.

Originally published on the web as the December 2010 Site of the Month.

Last updated on January 5, 2011