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“Geology of the Blackstone Valley” Attracts Almost 100 Attendees

Feb 25, 2021 06:00AM ● By Pamela Johnson
by Marjorie Turner Hollman, Contributing Writer

You know you are on to something when nearly one hundred people stop what they are doing on a weekday evening and show up for yet one more Zoom program. The Blackstone River Valley National Heritage Corridor (BHC), along with the Blackstone River Valley National Historical Park, hosted a skills workshop on the geology of the Blackstone Valley. The presentation was open to the public, and apparently a lot of the public was interested.

BHC volunteer Mark Dennen (pictured above), an adjunct professor at CCRI and environmental scientist for the Rhode Island Department of Environmental Management (RIDEM), began his presentation by saying, “Every rock tells a story.” He then launched into a broad overview of plate tectonics, describing how shifting plates on the earth’s surface are slowly but constantly sliding away from, or toward, one another (at about the rate at which fingernails grow, to help his audience grasp the concept).

Dennen brought his audience along through the millennia, describing the time of the Ice Age, when the entire world was a frozen ball. We learned of a time when glaciers covered much of the earth. When the glaciers began melting, rivers formed at the face of the glacier, dumping sediments from the glaciers into rivers, and these sediments eventually became compressed and turned into rock, creating marble, limestone, quartzite, and sandstones.

He then moved to more specific illustrations of how New England was formed. At one time, the eastern edge of the North American continent was located somewhere around the New York border with Massachusetts. Much of New England was “plastered on” to the rest of the North American continent from an island arc that originated off the coast of Africa. When this island arc collided with the North American continent, the collision formed huge mountains comparable in height to the present-day Himalayan Mountains. These mountains that were pushed upward, the “proto Appalachians,” have essentially disappeared, eroded to what are the present-day Appalachians.

Pointing to specific areas of the Blackstone Valley, Dennen explained that where faults between plates ripped apart, the ground sank, creating “down-dropped grabens” that filled with sediment. Plant fossils exist in these locations of sedimentary rock. However, other areas that are volcanic rock have no fossils. They do, however, offer other delights, such as geodes, crystalline structures that gave Diamond Hill State Park in Cumberland, RI, its name. (Participants in the presentation reminded viewers that removing rocks from Diamond Hill, or other state parks, is prohibited.)

We learned that New England had a small coal industry, specifically in Lincoln, RI. Dennen reminded his audience that coal starts with peat, which becomes lignite, then bituminous rock, then anthracite—the hardest coal. Dennen and others in the presentation recalled a cave-in that occurred in 1981 in Lincoln, RI, near the Lusitania Club, at the site of an old anthracite coal mine.

Looking at one of the few existing locks still intact from the massive Blackstone Canal infrastructure, the Millville lock, built in the 1820s, Dennen noted that the rocks used to create these locks are metamorphic rocks, that is, rocks transformed from one type of rock to another under extremely high pressure. Dennen explained that metamorphic rock is extremely dense, not crumbly—ideal building stone.

Glaciers had a profound effect on the land throughout New England, and there is plenty of evidence of glaciers to be found throughout the Blackstone Valley in woodlands and area ponds. Glacial erratics, that is, boulders left behind by retreating glaciers, are common in the Blackstone Valley.

Cumberlandite, the state rock of Rhode Island, is volcanic rock, unique to the area of northern Cumberland, where it is found near the Wrentham line. The southern boundary of the glacier’s track can be traced by the presence of pieces of Cumberlandite found on places like Block Island, off the Rhode Island coast.

The presentation was recorded and, for those interested in learning more about the background of how the ground where we stand was formed, it’s available at Dennen kept the presentation simple, reminding us at the end that the rocks around us have many stories to tell.




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