by James W. Skehan, S.J. and Catherine W. Barton
The City of Newton lies at an elevation ranging from 300 feet above sea level in the southeastern part of the city to 10 feet in the northeastern part.
Geologically Newton lies within the so-called Boston Basin (Fig. 1), which in turn is a tiny structure in the Appalachian Mountain chain. We refer to these as "mountains" in the geological sense even though the high mountain landscape (as high as the Swiss Alps today) which once characterized this region has long since been beveled by erosional processes.
The Boston Basin is bounded on the north by the Lynn Fells or Upland, on the south by the Blue "Hills and on the west by the higher terrain of Needham, Waltham, Wellesley and Weston. The boundary between the rocks to the north and west is a northerly and westerly dipping thrust fault called the Boston Border Fault (Figs. 1 and 2). The Blue Hills are bounded on the north by a high angle reverse fault. The net effect of such faulting is to have moved the older rocks to a position on top of and therefore to a higher elevation than the younger rock sequence of the Boston Basin.
The rocks of Newton belong essentially to a single sequence of volcanic and sedimentary deposits formed during Devonian time or about 350 to 400 million years ago. The sedimentary rocks consist of the Roxbury Conglomerate or "Puddingstone" and the Cambridge Argillite or Slate (Fig. 3). The Brighton Volcanics are distributed throughout the approximately 5000 foot thick sequence of sediments and represent a variety of volcanic products. The youngest rocks of the bedrock sequence are dike rocks, which are earthquake fissure fillings which may be as young as 150 million years. Elsewhere in the Boston Basin these rocks rest upon the Mattapan Volcanics. Just outside the Boston Basin region there are rocks as old as approximately 600 million years, the Dedham Granodiorite. This igneous rock has not been encountered within the Boston Basin. It is likely however that the sedimentary and volcanic rocks just referred to have been built upon this older foundation. Deep drill holes in Newton therefore would be likely to penetrate the Mattapan Volcanics and the Pre-Cambrian foundation of Dedham Granodiorite.
We may visualize the deposits of the Boston area some 350 to 400 million years ago as consisting of gravels, sands and muds which were being deposited beneath the ocean just east of a chain of high mountains which then occupied the New England area.
The Roxbury Conglomerate was deposited at the foot of high mountains by streams which washed sediment down the mountains and out to sea. Generally speaking heavier cobbles and pebbles rapidly settled out of the water and came to rest relatively close to the mountains of east central Massachusetts.
Some structures in the Roxbury Conglomerate suggest that at least part of the "Puddingstone" deposits represent submarine slides which moved down the gentle ocean bottom slope as submarine landslides coming to rest in deeper water. Such landslides were possibly triggered by earthquake activity associated with volcanism and related faulting activity. Such slide masses would be capable of producing a good deal of erosional scouring of the ocean floor and would produce slump structures such as are seen in Figure 8.
Sand and clay deposits of the Cambridge Slate undoubtedly represent those finer grained materials which were able to be transported greater distances. As one might expect, the Slate and the Roxbury Conglomerate consequently show a close association (Fig. 2). The interfingering of the Slate and the Roxbury Conglomerate suggests alternations in the rhythm of transportation of particles. of varying sizes during the long periods of time in which these deposits were formed.
While the sediments were being deposited, volcanic activity was taking place chiefly in that part of the basin of deposition where the conglomerate was being deposited. Also volcanic ash deposits are found in the slate of Cambridge and Somerville. The surface distribution of slate, conglomerate and volcanics in the Newton area is shown in Figure 3. A modern analogue of the Boston Basin may be the Tyrrhenian Sea which is the basin of deposition for coarse to fine gravels washed from the alpine mountains of the North. Nearby, volcanic activity is apparent on the Italian mainland and on the island, Ischia, offshore.
The folding and faulting of the Boston Basin rocks resulted from mountain building forces, which pushed the alpine mountains of east-central Massachusetts in an easterly direction. This thrusting produced faults such as the Boston Border Fault and folds such as the Shawmut anticline of Newton (Fig. 2). These structures were developed when blocks of the earth's crust were squeezed together.