Coastal Ledges of Kittery Formation, Granite, and Basalt, East Point Sanctuary, Biddeford Pool

Introduction

Three major rock types dominate the bedrock of southern Maine: broad units of metamorphosed sedimentary rock, large masses of intrusive igneous rock, and thin dikes of basalt. At East Point Sanctuary in Biddeford Pool, all three of these bedrock types can be found together. The geologic relationships that can be demonstrated in this small area are representative of a large part of York County.

Location and Access

The Maine Audubon Society maintains a shore path at the East Point Sanctuary in Biddeford Pool. For directions and important information about visiting the site, go to the Saco Bay Trails web site. In particular, note that parking is very limited, pets are not allowed, and the privacy of neighborhood residents should be respected.

Regional Bedrock Geology

The recent bedrock geologic map of southern Maine (Hussey, Bothner, and Thompson, 2008 - 12.6 Mb pdf) shows that the Kittery Formation is the bedrock unit for a region along the Maine coast all the way from the New Hampshire line to Saco. The only significant interruption is a large body of granite, called the Biddeford Granite, which intrudes the Kittery Formation in a large area of Kennebunkport and Biddeford.

Figure 1

Figure 1 shows a portion of the bedrock map for the Biddeford area, including Biddeford Pool. The large main body of Biddeford Granite is roughly oval-shaped, approximately 6 miles by 10 miles across. Another small body of Biddeford Granite occurs along the coast east of Biddeford Pool, south of East Point. The bedrock at the point itself belongs to the Kittery Formation. In addition, dikes of basalt up to a few feet thick, are common, a few of which are represented on the map by red symbols. Many dikes of basalt intrude both the granite and the rocks of the Kittery Formation, as we shall see.

What to See Here

Kittery Formation. The Kittery Formation is a thick sequence of layered sedimentary rocks that have been metamorphosed, or changed by heat and pressure. The layers, or beds, were originally sediments such as sand, silt, and mud, deposited in an ancient ocean basin (not the Atlantic), probably during the Silurian Period of geologic time. During metamorphism, the mineral composition and texture of the rock were changed, although the sedimentary layering is still well preserved. The most representative rock of the Kittery Formation is granofels, or metamorphosed sandstone, in medium to thick beds (Figure 2). The beds are distinguished from one another by having different colors, composition, and weathering character, among other things. The fresh rock is a bluish-gray, lavender-gray, or greenish-gray color. Individual mineral grains are very small, giving the rock a finely speckled appearance.

Figure 2

Figure 3

Granite. The Biddeford Granite is a light gray to white, uniform rock (Figure 3). It may be cut by sets of cross-cutting fractures, but it is not layered. Weathered surfaces develop a pale brown to pinkish-brown hue. The rock is composed of mineral grains about ¹⁄₈ to ¹⁄₄ inch across, much larger than the mineral grains in the Kittery Formation. Most of the rock consists of the light-colored minerals quartz, plagioclase feldspar, and alkali feldspar. A few flakes of black mica (biotite) are scattered through it. As with all granite, this rock formed from a molten mass that melted somewhere at depth, intruded into the overlying rocks while still molten, and solidified while still underground. This process produced a rock with interlocked mineral grains of uniform size.

Basalt Dikes. Though they constitute only a small volume of the bedrock, the vertical sheets of basalt are easy to spot. They have a dark rusty-brown weathered surface, straight sides, and break into angular blocks (Figure 4). The rock itself is a dark gray to black igneous rock that has very small grains, entirely visible only under magnification. In some places, the rock contains scattered larger grains of feldspar (Figure 5). Each vertical sheet, called a dike, formed when magma filled a fracture as it was opening.

Figure 4

Figure 5

Structural Features and Cross-cutting Relationships. The main structural features of the bedrock are the bedding in the Kittery Formation, the fractures in the granite, and the long, straight dikes (Figure 6). In addition, minor faults cut the rock in a few places. These structural features control properties of the rock such as weathering and groundwater flow. Cross-cutting relationships demonstrate the geologic history by which the various features formed. The oldest rock is the Kittery Formation. After the layers had formed, they were deformed by folding (Figure 7). The Biddeford Granite intruded later, as demonstrated by fragments of Kittery Formation enclosed by the granite (Figure 8). Finally, the basalt dikes intruded into both the Kittery Formation and the granite Figure 9).

Coastal Ledges, Beaches, and Bluffs. Modern geologic processes continue to shape the coast. Storm waves loosen and move blocks from the bedrock preferentially along structurally weaker zones such as dikes, faults, and fractures. Pocket beaches form in small recesses between rocky headlands (Figure 10). The beaches may be made of sand or cobbles, but whatever the sediment it is moved by waves of sufficient energy up and down the beachface (Figure 11). The bluffs of sediment at the head of the beach and resting on the coastal ledges, are actively being eroded, exposing bare sediment and undercutting sod and plant roots (Figure 12).

Figure 10

Figure 11

Figure 12

Where to Visit

Figure 13

Four areas of geologic interest, labeled Site A through Site D, are indicated on the index map (Figure 13) and on the Google Earth file.

Figure 14

Site A is an area of Biddeford Granite bedrock (Figure 14). This is the best place to inspect the granite. The granite contains fragments of the Kittery Formation, which sank into the molten rock before it solidified. Also, the granite is intruded by basalt dikes, which must be younger.

Figure 15

Site B is an area of Kittery Formation intruded by several basalt dikes (Figure 15). This is the best place to see the dikes. A small body of granite intrudes the Kittery Formation near the north end of the outcrop, near the beach.

Figure 16

Site C is a pocket beach made of smooth, rounded pebbles and cobbles (Figure 16). Most of the cobbles are light gray rock derived from the Kittery Formation, although some stones of granite and other rock types are mixed in. Notice the ridges (berms) at certain elevations, produced by storm waves. The bluff at the head of the beach is eroding, providing sediment to the beach.

Figure 17

Site D is the large outcrop of ledge that extends to the point (Figure 17). It is dominantly Kittery Formation, intruded by a small body of granite toward the south end, and a large basalt dike just below the point of land. Many minor structural features such as folds and faults deform the bedding in the Kittery. The bluff at the point, though not very high, is highly unstable.

Google Earth file

Geologic features of the East Point Sanctuary are presented in an interactive Google Earth file. The file includes polygons showing geologic units and placemarks showing photo localities superimposed on the high-resolution air photo image. You will need to install Google Earth on your computer to view this data. For help using Google Earth go to the Google Earth Help Center.

Geology of the East Point Sanctuary - KML file which opens Google Earth

Using the file. Check the box beside Photo localities to see Placemarks where geologic photos are available. Double-click on the name of a Site to zoom to that site. Double-click on numbered Placemarks for oriented snapshot view. Single-click on numbered Placemarks to see photos and captions. Check the box beside Geology to display colored polygons showing outcrops of granite and Kittery Formation, red lines highlighting basalt dikes, and purple lines highlighting faults and fractures.

References

Hussey, Arthur M., II, Bothner, Wallace A., and Thompson, Peter J., 2008, Bedrock geology of the Kittery 1:100,000 quadrangle, Maine and New Hampshire: Maine Geological Survey, Geologic Map 08-78, scale 1:100,000 (View map - 12.6 Mb pdf; View sidebar - 2.4 Mb pdf). (Paper copies available from MGS for $6. Ordering instructions)

Sinson, David A., Nestor, Rebecca A., Dickson, Stephen M., Kelley, Joseph T., 2002, Coastal bluffs in the Biddeford Pool quadrangle, Maine: Maine Geological Survey, Open-File Map 02-174, scale 1:24,000. (View map - 2.4 Mb pdf; View sidebar - 2.2 Mb pdf) (Paper copies available from MGS for $5. Ordering instructions)

Photos and text by Henry N. Berry IV except Figure 11 and photos at C2 and C3 by Stephen M. Dickson.
Web design, graphics and editing by Susan S. Tolman.
Air photo images downloaded from Google Earth, © 2012 TerraMetrics, Data SIO, NOAA, US Navy, NCA, GEBCO. Imagery date 5/18/2012.

Originally published on the web as the September 2012 Site of the Month.

Last updated on October 5, 2012