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Overview of the Geology of the Savin Hill Section of Boston, Massachusetts

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Overview of the Geology of the Savin Hill Section of Boston, Massachusetts OVERVIEW OF THE GEOLOGY OF THE SAVIN HILL SECTION OF BOSTON, MASSACHUSETTS Summarized below is a brief overview of the geology of the Savin Hill Section of Dorchester, Massachusetts. The presented information is based on my observations and a distillation of the published work of others (Irving Crosby, Clifford Kaye, Laurence LaForge and James Skehan). The geologic history is a generalized summary only; it is not a complete chronological record of the physical processes and landforms present in the area through geologic time. When reading this overview, the reader is encouraged to place your mind or better yet your body at the top of Savin Hill Park, more commonly known as the “Top of the Woods”. The top of Savin Hill Park can be reached by many different routes. For the unfamiliar; travel east on Savin Hill Avenue, take a left onto Caspian Way, walk up to the top or end of Caspian Way and take a right into Savin Hill Park. Approximately 200 feet east of the end of Caspian Way is a bedrock promontory that provides an excellent observation point for the below-discussed features. The bedrock outcrop beneath your feet is sedimentary conglomerate, more specifically, throughout Roxbury and Dorchester this particular formation is known as the Roxbury Conglomerate or Roxbury Puddingstone. It is Savin Hill’s most unique geologic feature. Close observation of the exposed bedrock should identify the source of the name Puddingstone (see photo). The rock is comprised of a fine-grained matrix (the pudding) within which are semi-rounded, variable sized clasts or pieces of igneous rock, mostly granite (the plums or nuts). Savin Hill and much of Dorchester and Roxbury is underlain by Roxbury Puddingstone. The formation extends to a depth of 1,500 to 3,000 feet. For the most part, the bedrock subsurface is covered by a variable depth of soil initially deposited during retreat of the last glacial advance (approximately 12,000 years ago). In areas of elevation or where the overburden soils are absent the bedrock is exposed. In addition to Savin Hill Park, there are numerous outcrops of the Roxbury Puddingstone, including: behind the former Motley School, adjacent to the Ledgeview Apartments, Jones Hill behind the Edward Everett School and Franklin Park and Parker Hill in Roxbury to name a few. Approximately 500 million years ago (late Cambrian – early Ordovician), the bedrock of the area of land subscribed by present day, greater Boston was the Braintree Slate and later the Quincy Granite. Starting about 400 million years ago (mid Devonian), seismic activity caused large cracks or faults in the existing bedrock. Continued seismic pressures overtime caused the landmass between the faults to sink or subside hundreds of feet while the land immediately beyond the faults increased in elevation. This area of subsidence, which is still evident today, is commonly known as the Boston Basin. The northern fault or north boundary of the Boston Basin exists on a line along the southern base of Arlington Heights, Prospect Hill and Middlesex Fells. The southern fault or south boundary of the Boston Basin exists on a line at the northern base of the Blue Hills. On a clear day with the aid of a map you should be able to visualize the approximate southern boundary of the Boston Basin from your observation post above Savin Hill Park. A more dramatic and obvious view of the Boston Basin is provided to travelers as they drive east on Route 2. Upon passing through the Heights of Arlington the extensive lowland of the Boston Basin surrounded by fault blocks to the north and south stretches out below with the skyline of Boston in the distance. Back to late Devonian Time; as the Basin continued to subside below sea level, the encroaching Atlantic Ocean submerged it. Over millions of years, rivers and streams flowing to the floor of the subsided Basin transported and deposited eroded rock fragments and sediments from the surrounding uplands (Arlington Heights, Prospect Hill, Middlesex Fells and the Blue Hills). Overtime the clay, silt, sand and rocks from the surrounding uplands filled the basin with a great thickness and weight. The overlying pressure caused the transported sediments to form into sedimentary rocks. The area of the Basin underlain by present day Cambridge, Somerville and parts of South Boston is underlain predominately by slate, which formed by the compaction of clay and silt. The area of the Basin underlain by present day Savin Hill and parts of Dorchester and Roxbury is underlain predominately by the Roxbury Puddingstone which formed by the compaction of clay, silt, sand and semi-rounded pebbles and small boulders. The sharp edges of the pebbles and small boulders were rounded off by weathering and abrasions during surface water transport. The fine-grained silt and sand became the matrix for the semi-rounded pebbles and boulders, which are the inclusions in the Roxbury Puddingstone. Approximately 300 million years ago, the igneous rock masses of the surrounding uplands to the north and south began exerting great lateral pressures on the relatively flat lying sedimentary rocks of the Boston Basin. If you apply pressure to the opposite ends of a flat lying piece of paper or cloth the material will bend and fold. In a similar fashion, due to the enormous lateral pressures over time, the sedimentary rocks of the Boston Basin arched up in some areas while folding down in other areas. Relative to Savin Hill; Jones Hill and Savin Hill Park are examples of elevated arches. The elevated sedimentary rocks of the Basin were then subject to increased weathering and erosion. The exposed slate was far more susceptible to erosion relative to the Puddingstone, which is more susceptible to erosion compared to the surrounding hills of igneous uplands. Consequently, the elevated areas of the Boston Basin are underlain not by slate but by Roxbury Puddingstone. To this day, the form of the Boston Basin remains intact owing to the more resistant igneous rock of the surrounding uplands. The last major modification to the Boston Basin and thus Savin Hill took place during the most recent glaciation, the Wisconsin Ice Age, which began approximately 25,000 years ago. The ice advanced over the entire area of New England reaching as far south as Long Island, New York. The south- advancing glacier was hundreds of feet thick and exerted enormous amounts of pressure. The glacier modified points of elevation, scoured out valleys and deposited great volumes of soil and rock over the entire area. When the heavy ice advanced over a hill it smoothed over the northern approach face making it less steep. In contrast, when the ice migrated away from the hill it frequently “plucked” away large fragments of rock from the south face resulting in a steep southern slope. Savin Hill Park is a good example of this process. As you stand on your Savin Hill Park observation point looking south toward Quincy and the Blue Hills, the rock face drops off as a steep cliff right below you (careful!). While behind you, the north slope up from the gully, the lower end of Grampian Way and the kid’s coasting hill to the top of the woods; the angle of the rise is more gentle. Now direct your eyes to the east, resulting in a gorgeous view of Boston Harbor. Observe (binoculars?) the harbor islands: Deer Island, Thompson Island, Spectacle Island, Long Island, George’s Island and Great Brewster Island. These islands are drumlins. The word “drumlin” comes from Ireland where they are a common landscape feature. In general, drumlins have a smoothly rounded, elongate profile and an elliptical ground plan. Drumlins are made of compact glacial till soils deposited and shaped by glacial movement. In addition to the islands, there are over 150 drumlins located in the Boston Basin, including such well known elevations as: Meeting House Hill, Bunker Hill, Orient Heights and Chestnut Hill. The last ice of the Wisconsin Ice Age disappeared from this area about 12,000 years ago. The glacial retreat was a slow process that took thousands of years to complete. As the melting, shrinking glacier retreated to the north it dropped a vast load of sediment and rock. Much of the sediment and rock was transported from the glacier by south flowing, melt-water streams that preferentially deposited the greatest volume and depth of sediment in valleys and lowlands. Except for the exposed, bedrock elevations and the drumlins, the present day surficial landscapes (soils and lakes) are the result of ice wastage or glacier retreat and/or the earth work and development activities of man. The author of the article is Joseph Hobin. Joe grew up in Savin Hill. He holds an AS in Health Science for Quinnipiac College; a BS in Biology from Boston State College and a MS in Geology from Western Michigan University. Currently Joe is the President of Compliance Environmental, Inc. an Environmental Consulting Firm located in Attleboro, Massachusetts (Tel: 508-223-3812) .
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