Geology and Topography of Mashapaug Pond
What did Mashapaug Pond look like five years ago? Fifty years ago? Five hundred years ago? Five hundred million years ago? How was that terrain different from today? The physical appearance of the pond today has slowly evolved through the dual processes of geologic change and human action.
565 million years ago, the land that we now call Rhode Island was part of a volcanic landmass called Avalonia, located near the Earth’s South Pole. Moving northward through continental draft, it collided with what is now North America. Continuing movement slowly drove Rhode Island from the continental interior to its place today on the northeastern coast of the United States.
Meanwhile, glaciers left impressions in the soil and rocks of Rhode Island. As the glaciers melted, many of those impressions filled up with water to become “kettle ponds.” Mashapaug Pond was only one of the kettle ponds in Providence. Underlying the pond is metasedimentary bedrock that formed when sedimentary rocks made of compressed layers of silt were altered by metamorphism (intense heat or pressure) about 360 million years ago. Over the centuries, Providence’s other freshwater kettle ponds were filled and homes and businesses were built on top.
Mashapaug Pond survived and is one of the city’s last remaining ponds. But it probably looks different than it did before humans began mapping the area around Providence. In many cases, the earliest maps of the region, imprecisely drawn by hand, raise more questions than answers.
In 1750, Mashapaug Pond appeared as a strangely-shaped lump on a local atlas. The odds are good that Mashapaug Pond wasn't shaped that differently from today, but rather that the mapmaker didn’t have access to today’s high-tech measurement and imaging equipment. Scientists can be reasonably confident that the general shape of Mashapaug Pond has stayed the same over the past several centuries because there is no evidence in the metasedimentary bedrock that any large earthquakes have changed the pond's contours.
More recently, however, the pond’s shape and northwest shoreline have been dramatically altered. The straightening of the north shore was a result of human intervention over the course of two years, not geologic processes spanning thousands of centuries. In the early 1960s, Providence pushed through the construction of the Huntington Expressway Industrial Park, designed to house new business and thousands of jobs. But the shoreline on which the new industrial park was to be built sloped down to the pond and rested on a bed of unstable silt. To create a firmer base for building, much of the pond was filled in and the shoreline straightened. At the same time as they were filling in the pond, the industrial park’s developers graded the landscape, flattening most of the hills in the area.
As you examine other maps – of your neighborhood, of Providence, of Rhode Island, of the U.S. – ask yourself what processes could have created the landscape on the map. Did humans or geologic change have a larger role to play?
What are the similarities and differences between human and geologic changes to landscape?
If you could drain all of the water out of Mashapaug, what do you think you would see? Try drawing what the underwater terrain might have looked like.