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Home > Explore! > Bedrock Geology > Field Localities > Jamies Pond

Geology of Jamies Pond
Augusta, Hallowell, and Manchester, Maine

Jamies Pond entrance sign
Figure 1

Jamies Pond
Figure 2		 Jamies Pond is the centerpiece of a 550-acre wildlife management area maintained by the Maine Department of Inland Fisheries and Wildlife. The pond's past history as a water supply for the City of Hallowell is partially responsible for its preservation in such a pristine state so close to populated areas. Well-maintained trails provide the visitor easy access to a variety of habitats that are home for abundant wildlife. A hand-carry boat launch encourages boaters and fishermen to enjoy the clear waters of the pond (Figure 2).

The trails and lakeshore also afford excellent opportunities to investigate the fascinating geology of the area. The colors on the geologic map (Figure 3) represent the different rock units exposed in the Jamies Pond area.

geologic map
Figure 3		 Waterville Formation
Figure 4

The oldest unit in the area is the Waterville Formation (Figure 4) shown in dark green on the map (also symbol Sw on the map). It consists mostly of schist in thin beds that are usually no more than a inch or so thick. The schist has much mica in it - mostly dark biotite and light muscovite. In places, the beds contain very fine layers of quartz. The best place to see schists of the Waterville Formation is in the area between Meadowhill Road and the northwest shore of the pond.

marble unit
Figure 5		 Within the Waterville Formation is a distinct unit of thinly bedded marble (Figure 5), shown in blue on the map (symbol Swl). The best place to see this rock is on the small island near the middle of the lake, which is nearly rimmed with exposures. Thin beds of marble, which have coarse grains of calcite are separated from each other by thin beds of mica schist. Because of their composition, the marble beds weather more easily and are usually recessed in an outcrop, while the schist is more resistant and stands up, giving these exposures a characteristic ribbed texture.

Marking the boundary of the Waterville Formation with the next youngest unit, the Vassalboro Formation, we often find a thin unit of dark gray schist (Figure 6 and Figure 7) that is full of sulfide minerals like pyrite (shown in orange on the map, symbol Swr). On fresh surfaces, the metallic sulfide minerals sparkle like gold in the sunlight, but remember, this is fool's gold! Left to the elements, the sulfide minerals rapidly deteriorate into rust, which gives most outcrops of this unit a distinctive rusty appearance. The best place to see this unit is near the boat launch at the north end of the pond. There, a small stream that flows into the pond just behind the former pump station exposes this schist just a few yards upstream. Look for the rusty layers.

layers of rusty schist
Figure 6		 hand specimen of rusty schist
Figure 7		 Vassalboro formation
Figure 8

Next in our sequence of units is the Vassalboro Formation (light green, symbol Sv) (Figure 8). This unit generally has thick beds that are a mixture of quartz, feldspar, and biotite grains, giving the layers a distinct salt-and-pepper texture. These quartz-rich beds are usually separated one from another by thin beds of mica schist. This unit is not very well exposed in the Jamies Pond area, but you can see some of the beds a few hundred yards up the stream from the pump house, beyond the rusty schist and a large wall of granite. Better exposures of the Vassalboro Formation are at the western approach to the third bridge north of Augusta.

The youngest rock unit in the area is granite (pink on the map, symbol Dg) (Figure 9 and Figure 10). In the Jamies Pond area, the best exposures are on the western shore of the pond, but there are other smaller exposures scattered about many of the woods trails, and at the small former dam just north of the pond. The granite is a fine-grained mixture of gray quartz, white feldspar, and dark brown biotite mica, in which all the grains are nearly equal in size. Molten magma was the original source for this rock which subsequently cooled at depth. The exposures at Jamies Pond are part of a large body of granite, the Hallowell granite, which includes the former quarries at Granite Hill, and the new "quarries" behind the shops at Augusta's Marketplace shopping center.

granite outcrop
Figure 9		 joints in granite
Figure 10		 glacial erratic
Figure 11

Many millions of years of erosion followed the development of the bedrock units such that rocks formed at depth in the crust (such as granite) are now exposed at the surface. The most significant episode of erosion to affect this area was that of glacial ice, which at its peak was many thousands of feet thick. Evidence for this episode is the basin of the pond itself which was gouged out by the ice, and glacial erratics around the area. Erratics (Figure 11) are boulders of one type of rock that do not match the type of rock in the solid ledge underneath, and must have been transported from a different area by glacial ice.

More information on Jamies Pond:

Web site by Robert G. Marvinney

Originally published on the web as the June 2007 Site of the Month.

Last updated on December 27, 2007