Northeastern U. S. Geology Gallery

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To go directly to a particular state shown in this gallery, click on its name below:

- Maine

- Massachusetts

- New York

- Rhode Island

- Vermont

14. Green and brown algae tenaciously cling to the rocky shore of Narragansett Island. The Chlorophyta, the green-algae phylum of Kingdom Protista, have included the precursors of true plants.The more ancient rocks of Rhode Island were once a part of Avalonia, a microcontinent that collided with ancestral New England during the Devonian Period, about 380 million years ago.

4. Near Wellfleet. This is the modern analog to some of the fossilized ripple marks seen in my other geology galleries (see Tour Photos page, no. 7, and Wisconsin Geology Gallery, nos. 5 & 21). These ripple marks, like most, are oriented perpendicular to the prevailing current directions.

5. A sandscape at the National Seashore. This isolated hummock in a very windy locale illustrates how the roots of beach grasses and oher vegetation can play a crucial role in retarding eolian erosion.

6. One of the most magnificent views in southern New England is that from atop Mount Sugarloaf, near Sunderland. Here the Connecticut River flows south toward the traprock ridges visible on the horizon. This peaceful and soul-restoring setting was once a volcanically active rift zone that formed during the breakup of the supercontinent Pangea and the birth of the Atlantic Ocean, some 200 million years ago.

7. One of many roadcuts in the early Jurassic redbeds along Interstate 91, in the Massachusetts stretch of the Connecticut River Valley. These rock formations began as sediments that washed down from the ancient highlands surrounding the rift zone. In a few places, the redbeds contain dinosaur footprints, which were first assumed to be the prints of large birds instead. These sedimentary rocks share the valley's geologic stage with basalt, an extrusive igneous rock that formed from lava flows.

8. A closeup of a Connecticut River Valley outcrop along Interstate 91. Note the pebble conglomerate on top of the finer-grained sandstone.

10. A scene from the Shawangunk Mountains, above New Paltz. Here Silurian-Period conglomerate, affected by the Acadian mountain-building event about 350 million years ago. now dips toward the northwest and forms a dramatic eastward-facing escarpment.

13. A closeup of the Shawangunk Conglomerate. This resistant sedimetary rock is largely composed of quartz pebbles set in a silicious (silica-based) matrix. In other words, it's tough stuff.

12. This small stream in the Shawangunk Mountains has taken the path of least resistance, through what was originally a narrow joint (fissure) in the conglomerate.

9. Life as a geologic agent. This street tree in the Park Slope neighborhood of Brooklyn may look beleaguered, but in this hostile environment it has managed to outlive many of its human abusers. Further, it demonstrates a form of organic weathering: how the patient and insistent pressure of growing tree roots can heave and split rock -- or, in this case, sidewalk and curbstones. Some thoughtful soul has applied Sakrete to the miniature fault scarps created by the pavement heaving. By lowering the slope angles of these mini-scarps, passersby are less likely to trip and injure themselves. Or so goes the theory.

15. A glacially abraded outcrop of the Purgatory Conglomerate, on the shore of Narragansett Bay. This formation dates from the late Pennsylvanian Period, about 285 million years ago. At that point in geologic history, the supercontinent of Pangea was forming, and in this neighborhood northwest Africa was colliding with North America.

16. A close look at the Purgatory Conglomerate, at the same location as no. 12. Note that the pebbles it contains are stretched along the left-right axis. This formation is actually a metaconglomerate -- a metamorphosed con-glomerate -- that was subjected to tectonic forces in the Earth's crust, after the rock was formed.

2. At the easternmost point of the United States. Scraggily taiga conifers struggle to survive on a wind-blasted outcrop of dark igneous rock. The Maine seacoast, with its many bays, inlets, capes, and coves, is a "drowned coast," where the rise in sea level since the last glacial maximum has created a craggy, deeply indented shoreline.

1. On the harsh and unforgiving shore of the North Atlantic, at Acadia National Park's Schoodic Point. The light-tinted Devonian-Period granite is interrupted by a linear dike of dark-colored diabase. The latter rock type is also known as dolerite. At this spot, at least, it seems that surf has eroded the diabase much more that it has the surrounding granite. Dikes form when molten rock infiltrates a crack or fissure in older host rock and then cools and solidifies.

11. The scarp face of the Shawangunk Mountains in the same area. The layers of weathered conglomerate clearly dip down into the hillside.

17. At a gateway to the Green Mountains. A view of the West River near the Scott Covered Bridge, in Townshend. The Green Mountains, like the Massachusetts Berkshires to the south, are the eroded roots of the Acadian Moun-tains, which rose some 385 million years ago when a volcanic island arc (the remains of which may still be seen in western New Hampshire) collided with the ancient east coast of North America.

3. In the same beautiful locale. Silurian-Period Quoddy Formation pelite -- a type of metamorphosed shale -- is here exposed under the snow-mantled vegetation.