NBS Blog

From the Archives: Millions of Years in the Making

Revel in the impressive geological legacy of Aquidneck Island with this article written by former NBS Education Coordinator, Hap Morgan, about the formation and history of Hanging Rock and the unique ridges of Paradise Valley.

Within the boundaries of the Norman Bird Sanctuary are 450* acres of some of the most beautiful and diverse geological formations in all of Rhode Island. The unique outcropping of Hanging Rock ridge is a very prominent and famous landmark. Admired by artists, nature lovers and geologists for more than 200 years, Hanging Rock is approximately 250 million years old.

Well over 300 million years before the present, the continents of the world were on a collision course with one another. Sometimes known as continental drift, a theory of plate tectonics has evolved to help explain many of the earth’s mysteries. When continents collided, volcanoes erupted, building new land, and earthquakes trembled, crumbling existing land forms. At this time, mountain building occurred (orogeny). A super continent called Pangea was formed from this collision.

Before the dinosaurs roamed the earth and long before the Atlantic Ocean existed, a large mountain chain lined what is today the East Coast. Once the mountains were formed, however, the ongoing process of erosion began. Large boulders, cobbles, pebbles and sand were worn away and washed into mountain rivers and streams. The ancient water ways emptied into the Narragansett Bay Basin. The sediments were deposited as the currents of the rivers and streams subsided.

Sedimentary rock is formed from layers of sediments piled on top of each other. Millions of years of tectonic activity transformed the sediments into sedimentary rock. The conglomerate of boulders, cobbles, pebbles and sand from the eroding mountains helped to make the Purgatory formation, sometimes called puddingstone. Hanging Rock is part of this formation.

The Rhode Island formation is a sedimentary bedrock that fills the entire Narragansett Bay. Aquidneck Island is part of this Rhode Island formation and so is the Purgatory formation.

When the Purgatory formation was still deep within the earth, strong forces were pushing and pulling the conglomerate bedrock. The strain from all the stress stretched the boulders, cobbles, pebbles and sand of the conglomerate so much that fractures or joints occurred. This powerful activity metamorphosed (changed) the conglomerate. The elongated pebbles are a curious attraction to both the geologist and the non geologist.

Ten to twelve thousand years ago, the last of the giant continental glaciers, called the Wisconsin Ice Sheet, extended over much of North America, including Rhode Island. As the glacier moved forward, it carried along boulders and rocks scraping the land like abrasive sand paper. Soil was removed, exposing Hanging Rock ridge. Close examination of the ridge may reveal glacial scratches. The very end of Hanging Rock was plucked away where the East-West fracture occurred, leaving the shear cliff face. Erosion at the ridge base is from the last 9,000 years. This gives the ridge its distinct overhang and its name.

Less than one hundreds yards west of Hanging Rock ridge are two more ridges, although these are of igneous (fire formed) origin. Geologists believe the magma (melted rock) never reached the surface as lava but cooled deep within the earth, forming a fine-grained rock called a diabase. Weathering and glacial erosion has exposed the two ridges.

The enormous amount of time to create such landforms is mind-boggling. Three hundred million years went into the making of the Bird Sanctuary’s landscape, and it is forty years this year that we celebrate its preservation for all time.

*Note: Post-1990 land surveys have confirmed that The Norman Bird Sanctuary is roughly 300 acres, not 450 acres as was previously understood. 

Note: Hanging Rock Photograph, date and photographer unknown, Wikimapia.
Sketch of Hanging Rock by Richard Grosvenor, 1981. First published in the Spring 1990 Norman Bird Flyer (Volume No. 11, Issue 2).