Introduction: A Brief Geologic history of the North Shore
During our excursions throughout the North Shore you'll note that the rocks are far different than those
seen in western New Hampshire. We are in fact on the opposite side of the Iapetus Ocean. Whoever
thought that an ocean could be traversed in a rental van! Read the summary below before lab.
Eastern Massachusetts is composed of several lithotectonic terranes (fig. 1) accreted to North America
during the Paleozoic Era. The bulk of the rocks underlying the North Shore are between 650-340 million
years old, and reflect a long history of volcanism and faulting related to both collision and rifting.
Younger Mesozoic dikes, related to initial rifting and formation of the Atlantic Ocean, are a minor
component of the overall intrusive package.
The Proterozoic rocks that underlie portions of Boston and the North Shore are part of the Avalon Terrane,
a microcontinent that collided with North America during the Paleozoic. These ancient rocks include the
600+ ma Dedham granodiorite and Lynn Volcanics exposed locally on Marblehead Neck and in Lynn.
Avalonia was an arc-bearing microcontinent, either attached to, or located near Proto-Africa (Gondwana)
during the Precambrian. Throughout the early Paleozoic Avalonia drifted towards North America
(Laurentia) as the intervening Iapetus Ocean subducted between the two continental masses. Collisions
with North America occurred during the Ordovician Taconic orogeny and again during the Devonian
Acadian orogeny. Sediments of the Iapetus Ocean were deformed, metamorphosed and folded, creating the
Appalachian Mountains. The Putnam-Nashoba Terrane and Newbury Volcanics (figs. 1 and 2) are
regarded as remnants of the accretionary and volcanic arc complex formed along the eastern subduction
zone.
Figure 1. Lithotectonic terranes of Eastern MA. These fault-bound terranes lie between the Northern
Border and Clinton Newbury faults. Forest River and Lynn Woods, which straddles the Walden Pond fault,
lie in the SE Proterozoic and Mid-Paleozoic Zones.
Figure 2. Tectonic model of the North Shore area during the Silurian. Drawn NW-SE across the region
from the Clinton-Newbury Fault (Putnam-Nashoba) to the Northern Border Fault (Late-Proterozoic zone).
The bimodal intrusive suite of the Mid-Paleozoic Belt are interpreted as forming during a stage of backarc
spreading (See Hon and Hanson, 2004, and references therein)
As the floor of the Iapetus Ocean subducted eastward backarc spreading initiated melting east of the
volcanic arc. The event created the Mid-Paleozoic Plutonic Suite that split and intruded the Proterozoic
rocks of the Avalon Platform. Rocks of this suite included the bimodal intrusions of the Cape Ann Granite,
Salem Granodiorite (Forest River Park), and the Peabody Granite (Lynn Woods).
The last pulse of Appalachian orogenesis occurred when Gondwana collided with southeastern North
America during the Permian Period and then moved slightly northward along strike-slip faults. This third
event, known as the Alleghenian orogeny, produced the fold belts and mountains of the southern
Appalachians and metamorphosed rocks in southern-most New England. These three orogenic events were
part of a series of worldwide collisions that culminated in the formation of Pangea during the Permian
Period. Pangea remained intact for approximately 100 million years when it started to rift apart.
During and following their formation the Appalachian Mountains were subject to tectonic and erosional
unroofing. The metamorphic core of this deeply eroded belt is exposed in the schists and gneisses of New
Hampshire, and central-western Massachusetts. (We will see these when we climb Mount Monadnock.)
The last tectonic pulse to shape the area occurred during the Late Jurassic through Cretaceous periods
(about 150-80 ma) when Pangea rifted apart and the modern passive margin of North America evolved.
Erosion continues, interrupted periodically by brief periods of deposition related brief periods of rising sea
level and glacial-related depositional events.
Geomorphology of Forest River Park.
The geomorphology of Salem is characterized by a rocky knock and lochan terrain, created by regional
scouring of fractured rock by continental glaciers. The rocky landscape, etched by glaciation, is typical of
many northern latitude glaciated coasts. Although the marine record indicates more than ten glacial events
during the ice age, only two events are clearly recorded in the Boston-North Shore area. Evidence of prior
events is typically removed by the erosive power of events that follow.