Giant's Stairs, Harpswell, ME Maine Geological Survey
Maine Geologic Facts and Localities June, 2019
Giant's Stairs, Harpswell, Maine
43o 43’ 24.6” N, 69o 59’ 43.08” W
Text by Ian Hillenbrand
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 1 Giant's Stairs, Harpswell, ME Maine Geological Survey
Introduction Beautiful rocky ledges near the southern tip of Bailey Island in Harpswell expose several types of bedrock that record hundreds of millions of years of Maine’s geologic history. At Giant’s Stairs, views of bedrock and eastern Casco Bay are easily accessed and enjoyed along an easy half mile loop shoreline path. This 26-acre property was donated to the town in 1910 by Captain Henry and Joanna Sinnett. An additional one-acre parcel was donated to the Harpswell Heritage Land Trust by Adelaide McIntosh.
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 1. Eastern shoreline of Bailey Island, Maine.
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 2 Giant's Stairs, Harpswell, ME Maine Geological Survey
Geologic Setting The majority of the bedrock exposed at Giant's Stairs is assigned to the Cape Elizabeth Formation, part of the Casco Bay Group. The Casco Bay Group is made up of a series of metamorphosed volcanic and sedimentary rocks that were deposited in the Ordovician, between 450 and 470 Million years ago (West and Hussey, 2016). Rocks of the Casco Bay Group are found in a 100-mile-long northeast-trending belt extending from Portland in the south to just south of Bangor in the north (West and Hussey, 2016).
Maine Geological Survey 1971 Hussey, From Figure 2. Portion of geologic map of the Giant’s Stairs area (indicated by red oval) from Hussey (1971).
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 3 Giant's Stairs, Harpswell, ME Maine Geological Survey
The Cape Elizabeth Formation The brown to rusty-weathering layers are schist, while the light-chalky-gray interbedded layers are impure quartzites. These units were originally deposited in an ocean basin as mud and sand. The two types of rock are interbedded, meaning that they were deposited continuously but the sediment source varied over time.
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 3. Cape Elizabeth Formation photographed perpendicular to the foliation.
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 4 Giant's Stairs, Harpswell, ME Maine Geological Survey
The Cape Elizabeth Formation
Figure 4. Cape Elizabeth Formation parallel to the steep foliation.
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 5 Giant's Stairs, Harpswell, ME Maine Geological Survey
Metamorphism and New Minerals During a mountain-building event, the sediments of the Cape Elizabeth Formation were buried deep in the earth, exposing the Formation to high temperatures and pressures that changed it through a process known as metamorphism, and subjecting it to squeezing and shifting forces that deformed the rock. The steeply inclined parallel layers, called foliation by geologists, developed as plate-like minerals such as mica grew perpendicular to the dominant compressive force. Other minerals that grew during metamorphism are garnet, a reddish mineral that often looks round or spherical, and staurolite, a dark-brown mineral that often grows as a cross. Garnet is relatively common here but staurolite is much rarer. These minerals grow under certain combinations of pressure and temperature which geologists use to understand the conditions these rocks were exposed to during metamorphism.
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 5. Garnet minerals within the Cape Elizabeth Formation.
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 6 Giant's Stairs, Harpswell, ME Maine Geological Survey
A Boudinaged Amphibolite One layer of the Cape Elizabeth Formation, about 10 inches in width and 100 feet in length, is different from the others. It is medium to dark gray in color and has little quartz and no staurolite or garnet. This layer is an amphibolite, a metamorphic rock that contains a lot of the dark-colored mineral amphibole. It likely represents a layer of basalt from volcanic systems that were active during the time that the Cape Elizabeth Formation was being deposited. As the whole package of rocks was deformed, the darker layer was stretched and pulled apart like taffy. The schist of the Cape Elizabeth Formation flowed in to fill the gaps. The resulting shape is called a boudin, after the French word for sausage, since the rock now looks like a series of sausages encased in rock.
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 6. Boudins of amphibolite within the Cape Elizabeth Formation.
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 7 Giant's Stairs, Harpswell, ME Maine Geological Survey
A Boudinaged Amphibolite
Figure 7. Detailed geologic map of Giant’s Stairs showing the boudinaged amphibolite
layer from Swanson (2016). From Swanson Swanson (2016) From
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 8 Giant's Stairs, Harpswell, ME Maine Geological Survey
Folds Tightly folded layers of rock are common and are readily apparent when looking down on the outcrop. The easiest folds to find are in the gray impure quartzite layers, which are often highly contorted. These folds (which a geologist would classify as isoclinal, upright folds) were created during a regional mountain building event and then stretched thin by later forces associated with shearing.
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 8. Upright fold of impure quartzite layer in Cape Elizabeth Formation.
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 9 Giant's Stairs, Harpswell, ME Maine Geological Survey
Basaltic Dike The feature that gives Giant's Stairs its name is an eight-foot-wide, east-northeast-trending dark-gray to green, fine-grained igneous rock called a basaltic dike. The rock intruded the Cape Elizabeth Formation as hot magma but cooled quickly as it came into contact with the colder rocks surrounding it. The way that the rock cooled led to the development of crude breaks in the rock called joints. The hydraulic action of ocean waves has plucked out blocks along these joints creating the appearance of stairs leading down to the ocean (Hussey, 2015).
Figure 9. Giant’s Stairs basaltic dike
and eastern view towards Small Point. Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 10 Giant's Stairs, Harpswell, ME Maine Geological Survey
Basaltic Dike
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 10. Looking down on the dike shows the columnar jointing.
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 11 Giant's Stairs, Harpswell, ME Maine Geological Survey
Talc Bearing Ultramafics More unusual igneous rocks are found on the southern end of the park. Here, dark-green, smooth-feeling metamorphosed ultramafic rocks (rocks that contain a lot of iron and magnesium and very little silica), are found parallel to the foliation of the Cape Elizabeth Formation. The mineral talc formed in these rocks during metamorphism and imparts the smooth feel to the surface. Little is known about these rocks and an explanation for their origin remains elusive (West and Hussey, 2016). What is known is that they were quarried from a narrow north-south trending trench where you can still find drill holes. Mining of similar talc-bearing rocks was mentioned in C.T. Jackson’s First Report on the Geology of Maine, though this particular locality was not mentioned. This rock type is “capable of being wrought into excellent slabs for the construction of stoves and fire grates.” (Jackson, 1836)
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 11. Dark green, talc-bearing ultramafic intrusion within the Cape Elizabeth Formation.
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 12 Giant's Stairs, Harpswell, ME Maine Geological Survey
Talc Bearing Ultramafics
Figure 12. Closeup of circled rock, a loose, quarried block showing several drill holes.
Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 13 Giant's Stairs, Harpswell, ME Maine Geological Survey
Geologic History: Deposition of the Cape Elizabeth Formation The Cape Elizabeth Formation was deposited in a basin near a volcanically active island arc about 472-466 million years ago. Silica-rich volcanic rocks erupted from the volcanic arc and were subsequently eroded into the muds and sands of the Cape Elizabeth Formation that were then deposited in a backarc basin (Hussey et al. 2010). At this time, the island arc was separated from proto-North America by an ocean basin, called the Merrimack Basin. Through the late Ordovician to Silurian, thick accumulations of
sediment were deposited and eventually buried the older volcanic arc rocks (West and Hussey, 2016). From Hussey et al. (2010) al. et Hussey From Figure 13. Schematic interpretation of the depositional environment of the Cape Elizabeth Formation from Hussey et al. (2010). Maine Geological Survey, Department of Agriculture, Conservation & Forestry 14 Giant's Stairs, Harpswell, ME Maine Geological Survey
Geologic History: Deposition of the Cape Elizabeth Formation
(2016)
Scotese Modified Modified from Figure 14. Paleogeographic reconstruction of the Late Ordivician. Modified from the PALEOMAP PaleoAtlas for GPlates (Scotese, 2016).
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 15 Giant's Stairs, Harpswell, ME Maine Geological Survey
Geologic History: Collision The ocean between proto-North America (Laurentia) and the island arc (Gander) began to close in the Late Silurian–Early Devonian and with it mountain building began. This event, known as the Acadian Orogeny, first began with the thrusting of the older Casco Bay Group rocks over the younger East Harpswell Group rocks and was followed in the Middle to Late Devonian (roughly 370 million years ago; West and Hussey 2016) by intense folding from compressional forces accompanied by regional low-pressure metamorphism.
Figure 15. Paleogeographic reconstruction of (2016)
the Late Silurian to Early Devonian, as Gander Scotese collides with Laurentia to create the Acadian Orogeny. Modified from the PALEOMAP
PaleoAtlas for GPlates (Scotese, 2016). Modified Modified from
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 16 Giant's Stairs, Harpswell, ME Maine Geological Survey
Geologic History: Shearing Tectonic forces shifted later on from head-on collision of tectonic plates to a setting where the plates were sliding by one another, shearing the rocks and creating brittle strike-slip faults. The strike-slip faulting, often associated with the extensive Norumbega Shear Zone, stretched the rocks in the Giant’s Stairs
parallel to their layers and led to the development of the boudins discussed earlier (Swanson, 2016). From Swanson Swanson (2016) From Figure 16. Shear zones of the Norumbega Shear System, from Swanson (2016). Giant’s Stairs is indicated on the map at stop 3 within the main shear zone (pink).
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 17 Giant's Stairs, Harpswell, ME Maine Geological Survey
Geologic History: Continental Breakup After this shearing, regional stresses again changed direction and the plates began to move away from one another as the North Atlantic ocean began to open (West and Hussey, 2016). This change allowed the intrusion of basalt dikes like the one at Giant’s Stairs and at numerous other localities in Maine (McHone et al. 2014).
Figure 17. Paleogeographic reconstruction of
the Triassic as North America separates from (2016) Africa to form the Atlantic Ocean. The
Harpswell area is indicated by the red ellipse, Scotese arrows indicate approximate direction of rifting. Modified from the PALEOMAP
PaleoAtlas for GPlates (Scotese, 2016). Modified Modified from
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 18 Giant's Stairs, Harpswell, ME Maine Geological Survey
Directions From Route 1, take the exit towards Cooks Corner/ME-24. Go south on ME 24 through Sebascodegan Island and Orrs Island, and use the historic Cribstone Bridge (Locke, 2015) to cross onto Bailey Island. Continue for another 1.5 miles and take Washington Avenue on the left. Approximately 500 feet down the avenue, public parking space is marked for Giant’s Stairs visitors. After parking, walk down the access drive to the shore and follow the path south along the water’s edge.
Figure 18. Google Map of Giant’s Steps parking and trail access.
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 19 Giant's Stairs, Harpswell, ME Maine Geological Survey
References and Additional Information Hussey, A.M. II, 2015, A guide to the geology of southwestern Maine: Peter E. Randall, publisher (available through the Maine Mineral and Gem Museum, Bethel, Maine), 216 p. Hussey, A, M. II, Bothner, W. A., and Aleinikoff, J., 2010, The tectono-stratigraphic framework and evolution of southwestern Maine and southeastern New Hampshire, in Tollo, R. P. Bartholomew, M. J., Hibbard, J. P., and Karabinos, P.M., eds., From Rodinia to Pangea: The Lithotectonic Record of the Appalachian Region: Geological Society of America Memoir 206, p. 205-230. Jackson, Charles Thomas, 1837, First report on the geology of the State of Maine: Smith & Robinson, Printers to the State, Augusta, Maine, 128 p. Accompanied by atlas of 24 plates. https://digitalmaine.com/geo_docs/8 Locke, Daniel B., 2015, The Cribstone Bridge, Harpswell, Maine and its Granite Source Quarries: Maine Geological Survey, Geologic Facts and Localities, Circular GFL-217, 30 p. https://digitalmaine.com/mgs_publications/508 McHone, J.G., Hussey, A.M., II, West, D.P., Jr., and Bailey, D.G., 2014, The Christmas Cove Dyke of coastal Maine, USA, and regional sources of Early Mesozoic flood basalts in northeastern North America: Atlantic Geology, v. 50, p. 66-90. Scotese, C.R., 2016. PALEOMAP PaleoAtlas for GPlates and the PaleoData Plotter Program, PALEOMAP Project, http://www.earthbyte.org/paleomap- paleoatlas-for-gplates/ Swanson, M.T., 2016, Kinematic indicators and ductile strain domains associated with regional shearing: A transect across the Norumbega fault and shear zone system, Pemaquid Point to northern Casco Bay, in H.N. Berry IV and D.P. West, Jr., editors, Guidebook to the Geology of the Maine coast from Maquoit Bay to Muscongus Bay, New England Intercollegiate Geological Conference (108th annual meeting), p. A1-1 – A1-19. https://digitalmaine.com/mgs_publications/20 West, David P., Jr., and Hussey, Arthur M., II, 2016, Middle Ordovician to Early Silurian terranes of the northern Casco Bay region, Maine: in Berry, Henry N., IV, and West, David P., Jr., editors, Guidebook for field trips along the Maine coast from Maquoit Bay to Muscongus Bay: New England Intercollegiate Geological Conference, p. 249-266. Maine Geological Survey Publications. 18. http://digitalmaine.com/mgs_publications/18
Maine Geological Survey, Department of Agriculture, Conservation & Forestry 20