Romancing the Stone

As a preliminary to a geology section in our developing online nature trail guide, Jonathan Alvarez, a geologist from EA Engineering in Warwick, kindly shared his knowledge of features found at Salter Grove.

It is truly a shame that so many of the exposed rocks have been defaced by juvenile graffiti that obscures a fascinating geological history. Even birds know better than to foul their own nest! Please help us to realize our park’s potential as an outdoor classroom by leaving no trace of your visits.

Jonathan explains how conglomerates, sometimes referred to as puddingstone, are metamorphosed sedimentary rocks. These are part of the Rhode Island Formation and are Pennsylvanian in age (298-323 million years old). These conglomerates formed in a turbulent environment similar to a river bottom where the rounded cobbles and sandy matrix eventually were compressed through burial. During the Pennsylvanian Period, Rocky Mountain-sized ranges occurred east and west of the present Narragansett Bay, and their erosion produced enormous quantities of sediment that filled the space between, including wide river basins similar to the San Joaquin or Sonoma Valleys.

During the Pennsylvanian Period, which was the later portion of the Carboniferous, our park was adjacent to a swamp forest of horsetail (Equisetum) ancestors with bamboo-like trunks up to 40 ft tall. Jonathan is pointing to the fossilized remains of a stem of one of these Calamites. This fossil of a preserved plant from over 300 million years ago has stood the test of time, glaciers and sea level rise to showcase the park’s unique position in regional paleontology. These trunks lay in metamorphosed grey sand with beds of gravel. This type of environment could be found in modern day rivers similar to the Mississippi delta.

We’ve all seen how storms can tear up a coastline or re-channel a river. That’s what happened here, but over 300 million years ago. Above Jonathan’s hand is a band that was deposited by fast-flowing water with enough energy to carry rounded pebbles. The layer below was deposited by slower moving water that could only carry smaller sediment. This unit of rock is likely upside down, a result of two continents that collided with North America between 300 million years ago and today. Notice how unique a situation exists in our park whereby the rounded clasts in a 300 million year old conglomerate are preserved alongside an active beachfront where the same rounded pebbles are being weathered and will eventually be buried in the perpetual rock cycle.

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