Living Observatory Field Guides

A Five Part Primer on Primer on the Geology of Southeastern Massachusetts

Part One: At the Bottom of it All

Imagine Plymouth County as a place shaped by dramatic events over millions of years!
First, there's the bedrock foundation, formed way back in the Precambrian era which spans from 4.6 billion years ago to about 538.8 million years ago and during the Paleozoic era of 541- 252 million years ago. This time is marked by the origin of life, development of our atmosphere and the formation of continents. This bedrock, made of crystalline igneous and metamorphic rocks in the north and south, and meta-sedimentary rocks in the central Narragansett Basin, influences everything from soil type to water flow.

Check out the map showing the bedrock that lies beneath us. The fancy name of this bedrock formation is the Milford-Dedham Zone. It is hard to imagine that the Earth we walk and drive over contains evidence that our present day landmass was once in contact with northwest Africa. Notice that there is evidence of volcanic origins of rock as well as the creation of iron ore which becomes extremely important in the settler’s economy. 

The solid bedrock that lies beneath the glacial deposits that are described in the next section, are buried 200-700 feet below sea level. The bedrock consists of  igneous (volcanic) and metamorphic (changed by heat and pressure) rocks, like granite, gneiss, schist, and slate. (MLRAs website). The closest exposed bedrock can be found in Marshfield and to the west at Fort Phoenix at the mouth of New Bedford Harbor. There at Fort Phoenix, the smoothed granite headland bears the tool marks and polish left by the glacier. (Schwarzman, 2002)

Part Two: The Ice Age

Plymouth County's geology is a fascinating story of ancient rocks, powerful glaciers, and the ongoing forces that shape our world. Let's dig deeper into the awesome power of glaciers and how they sculpted the landscape of Plymouth County! (Schwarzman, 2002)
We will start at what is known by scientists, as the Pleistocene Epoch or the Ice Age. The glacier that left its evidence in Plymouth, was the last advance of the ice sheet that repeatedly flowed across and melted off North America during this time. (NCSS) This was a period between 1,800,000 to 10,000 years ago. Geologically speaking, believe it or not, this is very recent in our geologic history. About 75,000 years ago, a fluctuation to a cold climate began the process that shaped Plymouth and south to Cape Cod and the Islands. 

For fun, let’s imagine some of the spectacular wildlife that was wandering the land and sea.  Keep in mind there were a dozen or so ice ages during this epoch and these ice ages proved to be especially hard on the megafauna mammals. The largest ones most likely could not find enough food to sustain their populations. Whales and other familiar sea mammals and fish were swimming in our seas. Although the Megalodon, the giant shark, went extinct during this time, many other species of sharks continue to flourish and do so to this day. Conditions were especially severe in North and South America and Eurasia, where the late Pleistocene witnessed the extinction of Smilodon (the Saber-Toothed Tiger), the Woolly Mammoth, the Giant Short-Faced Bear, Glyptodon (the Giant Armadillo), and Megatherium (the Giant Sloth). Camels disappeared from North America, as did horses, which were only reintroduced to this continent during historical times, by Spanish settlers.

Back to the geology. Think of a pile of ice that was centered east of Hudson Bay (map?)that accumulated as much as 2 miles thick and spread in all directions from its center. What a bull-dozing capacity it had! It grinded over mountains, carrying and collecting rocks in all sizes from house-size blocks to rocks so fine they are called “rock flour”. This ice moved across Canada and about 25,000 years ago, into what we now call New England. 

These icy giants advanced and retreated across the land, leaving an indelible mark. Here's a closer look at their handiwork in Plymouth County:

[Image: glacier work]

When the temperatures warmed up, the ice melted and retreated north. This process happened at least four times during the Pleistocene. Many scientists believe we're currently in a warm period between ice ages, and that glaciers might return someday!

The most recent ice age was called the Wisconsinan glaciation. During this time, a huge ice sheet called the Laurentide ice sheet covered all of southern New England, reaching its furthest point about 25,000 years ago. The islands of Nantucket, Martha's Vineyard, Block Island (in Rhode Island), and Long Island (in New York) are actually the "terminal moraines," or piles of rock and dirt left at the furthest edge of this glacier. By about 14,000 years ago, the glacier started to retreat, and by 13,000 years ago, it had completely left New England.

Think of glaciers not as static blocks of ice, but as colossal, slow-moving bulldozers.  Over thousands of years, more and more ice piled on top and as this happened, the ice on the bottom spread wider and wider. As the glacier advanced south, it met warmer weather and the southern edge of the glacier began to melt. Keep in mind, although this edge started to melt, more and more snow was piled on top in the north and this accumulation of snow continued to push the glacier south. 

Refer to the map (Figure 4 p.12 Schwarzman, 2002). Look closely. You might be imagining a 2 mile high pile of ice with a 2000 mile wide advancing straight edge but in fact it had a 3 lobed  edge. From the study of current glaciers, scientists know that glaciers don’t usually move forward all at once. Those areas that move faster from “ice streams” that may have created large curving ice bulges or “lobes”. There was the Buzzards Bay lobe, the Cape Cod Bay lobe and the South Channel lobe. These lobes traversed different land to get to Plymouth and south carrying with them the rocks and soils of Plymouth today. 

As the glacier retreated, you would not have recognized the area. It was all dry land. Plymouth would have been more than 100 miles inland from what was then the coast. Since so much water was trapped in ice sheets, the world’s sea level was about 400 feet lower. This means that any area today that is less than 400 feet under water was dry land. You could have walked out on a gently sloping dry plain, crossed a few valleys, to the places now known as Buzzards Bay, Cape Cod Bay, Nantucket and Vineyard Sounds, and George’s Bank. Cape Cod, the Elizabeth Islands and Martha’s Vineyard and Nantucket did not exist at this time.

As glaciers crept southward, the sheer weight and movement of the ice, often with rocks and debris frozen within it, acted like a giant file called a rasp. They scoured and eroded the underlying bedrock. This process is called glacial abrasion, and it polished rock surfaces. You might even find glacial striations – scratches and grooves left on the rocks, indicating the direction the ice was moving. The glaciers also plucked away loose rocks and sediment, a process known as glacial plucking, further shaping the terrain.

Part Three: What the Glaciers Left Behind

As the glacier melted and moved away, it left behind three main types of deposits in Plymouth County. Here is an outline of the main types of deposits. Notice that geologists give soil types specific names. If interested, use the QR codes to find out more about our soil types. In addition, the glaciers were great carvers of the land. 

1. Glacial Till: This is a mix of all different sizes of rocks, sand, and clay, dumped directly by the moving glacier. It's unsorted and unlayered.

• Dense Till: This till is very compact and doesn't let water pass through easily. Soils formed from this often have "perched" water tables, meaning water collects closer to the surface. Examples of soils in Plymouth County include Paxton, Montauk, Woodbridge, and Brockton. 
• Ablation Till: This till is looser and sandier. It was deposited as the ice melted in place. Soils here include Canton, Gloucester, and Plymouth series.
• Soils made from glacial till often have many rocks and gravel near the surface and deeper down.

1. Glacial Outwash (Glaciofluvial Sediments): As the glacier melted, huge amounts of water created rivers and temporary lakes. These moving waters deposited layers of sand and gravel, which are called outwash.
   • These areas are important for recharging underground water supplies (aquifers) and are often good for farming. They also contain valuable sand and gravel resources.
   • Examples of outwash soils in Plymouth County include Hinckley, Carver, Sudbury, and Scarboro.
2. Glaciolacustrine (Lake Floor Deposits): These are fine-textured materials (like silts and clays) that settled at the bottom of ancient glacial lakes that have since dried up or filled in.
   • These soils, like Scio, Eldridge, and Raynham, tend to hold water well and can have seasonal high water tables.

Coastal Plain Deposits: The southeastern part of Plymouth County is considered part of the Coastal Plain. This area has very deep bedrock, hundreds of feet below sea level. 

In case you are interested and have a chance to travel north of Plymouth, you may find evidence of the Atlantic Coastal Plain. Prior to the glaciers bulldozing through this region, there existed the Atlantic Coastal Plain.  At the end of the Tertiary period (65-1.5 million years ago), the Atlantic Coastal Plain stretched along all of New England. However, the glaciers later deformed and removed much of these soft sediments. Today, original Coastal Plain deposits are only found in the Marshfield-Duxbury area of Massachusetts. Other Coastal Plain deposits found in the Marshfield area and islands were pushed up by the glaciers. These deposits include various colored silts and clays, and sometimes sandy deposits with shells. Glacial deposits usually cover these older Coastal Plain layers.The Marshfield Hills, in the northeastern part of Plymouth County, are irregular hills made of various glacial deposits. Underneath these glacial deposits, in some places, are dark gray silts and clays called the Marshfield Hills Formation. These are very compact, dark, and contain carbon and iron sulfides.
Drumlins: These are streamlined, elongated hills formed by glacial ice acting on underlying till. They typically have a gentle slope on the side facing the advancing ice and a steeper slope on the down-ice side. 

In Plymouth County, there are three main types of drumlin till:

• Dark-colored, fine-textured, very firm till from the Boston Basin (Newport and Pittstown soils).
• Olive-colored, loamy, very dense till in the northeast and central parts of the county (Paxton and Woodbridge soils).
• Light-colored, sandier, slightly looser till with granite fragments, found mainly in the southeastern part of the county (Montauk and Scituate soils).
• These dense tills often have a "densic contact," meaning they are very compact and don't let water through easily, which affects how soils behave.

As the glaciers retreated, they formed end moraines (ridges of deposited material) and "pitted outwash plains." These plains are full of "kettle holes”. When large blocks of ice were left behind in the glacial deposits and eventually melted, they created depressions called kettles. The sand and gravel above them collapsed into the spaces left by the ice. When the buried ice had completely melted, hollows formed. Some of these hollows formed bays that were flooded by the sea. Hollows on higher land formed the freshwater ponds or “kettle lakes” and other wetlands that were filled either by rain or ground water.  Some kettle holes filled with water to become "kettle ponds", while others filled with organic sediments to form bogs. Many of these peat-filled bogs were used for cranberry farming and some still are today. (Schwarzman, 2002)

Glacial Lake Taunton and Jones River Lowland: As the ice melted, it also formed large glacial lakes in low areas. The biggest one in southeastern Massachusetts was Glacial Lake Taunton, which covered parts of Plymouth and Bristol Counties. Fine sediments (silts and clays) settled in this lake, sometimes over 100 feet thick. The Jones River was the outlet for this lake. The clay from this lake has even been mined for making bricks. Soils like Scio, Raynham, Birdsall, and Enosburg are found in this area, and some are excellent for farming. Large swamps, like Hockomock Swamp, also formed in the low areas of Glacial Lake Taunton.

Moraines:  These are ridges of till deposited at the edges or ends of glaciers.Terminal moraines mark the furthest extent of the ice sheet, while recessional moraines are formed during temporary halts in the glacier's retreat.The prominent hills and ridges in some parts of Plymouth County could be remnants of these moraines. (Soil types:The Plymouth, Carver, Barnstable, Canton, and Montauk soils are often found on these moraines.)

The sea eventually reached its current level and the ocean took over the shaping of Plymouth’s coast and Cape Cod to its south. Bluffs were created from waves eroding the sandy hills along the shore. Wind carried eroded sand to the bluffs’ foot to form beaches and barrier spits as well as created dunes. Rainwater that had sunk deep into the sandy ground and moved underground to the south east, began to find sea water filling the space between sand grains all along the shores. Since freshwater is less dense than sea water, freshwater was forced to the surface creating springs we can observe along our shoreline today. (Schwarzman, 2002).

The glaciers were the master sculptors of Plymouth County. They eroded the bedrock, transported massive amounts of material, and deposited it in various forms, creating the diverse terrain we see today – from the hilly areas potentially formed by moraines to the flatter, sandy outwash plains and the numerous lakes and ponds dotting the landscape. The legacy of the Ice Age is deeply etched into the very story of Plymouth County's geology.

Part Four: Wind-Blown “Mantle” (Eolian Mantle)

Let’s consider how the land looked at this point. The geologic epoch following the glacial period (Pleistocene) is called the Holocene. The Holocene is the current geological epoch, which began roughly 11,700 years ago. It's characterized by a relatively warm and stable climate compared to the preceding ice age, allowing for the development of agriculture and the flourishing of human civilization. While often considered a period of relative stability, the Holocene has also seen significant climate shifts and the rise of human influence on the planet which we will explore in Study Guide 2. 

 Early in the Holocene, strong winds blowing off the remaining glacial ice picked up sand and silt particles and spread them across the bare, glaciated land. This "eolian mantle" created a layer of sand or silt on top of the older glacial deposits. This layer can be anywhere from a few inches to about 40 inches thick. It forms the upper layers of many New England soils. The type of sand or silt in this mantle helps classify and map many soils in Plymouth County. (NCSS p.14) Organic sediments accumulated in wetlands, and rivers deposited alluvium in floodplains, making the soils more fertile. 

Before pioneer plants established themselves, the wind must have carried sand and silt from the hills and out-wash plains and sand-blasted the bare, rocky sand and gravel. As it did so, rocks on the surface were polished. Fine grained material must have been deposited in shallow channels known as swales, leaving boulders exposed on hills and ridges. Although it is not known exactly how long it was after the ice left and the first plants grew in Plymouth, it must have been quite a sight. The wind would have been slowed and the surface of the land protected. The oldest known plant remains are pollen of spruce, pine and Hudsonia, (one type of plant with the common name Pine Barren False-heather) among others from 12,000 year old freshwater peat deposits. Pioneer grasses and lichen that can grow on bare sand were most likely here before these species. (Schwarzman, 2002)

Here is a quick outline of some of the changes in biota (plants and animals) during this time.

As the climate warmed and glacial ice retreated northward, the landscape underwent significant transformations. Continual weathering and erosion of rock released essential nutrients, simultaneously creating new soils that facilitated the recolonization of plant life. Initially, the areas just south of the glacier were dominated by tundra-like vegetation, though in some instances, spruce forests may have directly abutted the ice. For several thousand years, the landscape was primarily characterized by sedges and dwarf shrubs, with associated animal species following the receding glacier northward.
The post-glacial plant biogeography in southern New England was largely determined by regional temperature and moisture levels. From approximately 14,600 BP until 11,600 BP, spruce was the predominant tree species across the New England landscapes. However, around 11,600 BP, a drier and warmer climatic period led to white pine becoming the dominant species. By about 8,200 BP, a rise in moisture levels saw hemlock, beech, and birch replacing white pine. Hemlock, a species favoring more mesic (moist) conditions, experienced a notable population crash around 5,400 BP. While originally attributed to the first recorded occurrence of a pathogen, more recent evidence suggests that a drier microclimate may have also contributed to this decline. Deciduous species like hickory and chestnut arrived much later in New England, around 6,000 BP and 3,000 BP respectively, likely due to regional temperatures and moisture levels being cooler and lower than present-day conditions.
The ecological shifts also profoundly impacted large mammal populations. Spruce parklands and grassy savanna habitats provided sustenance for these mammals including mastodons. However, these large creatures disappeared rapidly as the glacier continued its retreat and humans advanced across the region, altering the existing faunal landscape.
Fire historically played a major role in shaping the ecosystems of southern New England, particularly the oak-dominated forests in the south, and the barrens and coastal marsh habitats. Several natural historians have concluded that fires set frequently by native peoples, along with naturally occurring fires, were important ecological factors in New England, especially in oak forests and pine plains. In reconstructing pre-European North American fire frequencies, it was estimated that fires occurred approximately every 7 to 12 years in the more fire-prone habitats of the coastal plain, while on plains with hills or low mountains further inland, fire-prone areas burned approximately every 13 to 25 years. In pre-colonial and early colonial periods, the pine barren habitat in Plymouth County was frequently burned. At that time the region was a mosaic of pitch pine-scrub oak barrens with frequent shrubby openings and grasslands. Pitch pine-scrub oak communities need fire to maintain the community structure and diversity. The resinous, waxy cutins in the leaves of many of the plant species found in this community are highly flammable and ignite easily during dry periods. The soils from glacial outwash sands and gravel made New England especially fire-prone areas. It has been reported that fire was a major historic disturbance that shaped the vegetation of coastal Massachusetts, Connecticut, Rhode Island, and New York.

In our study guide on plymouths history [link], the story continues. You will have a chance to explore the Plimouth Patuxet Museum and design a historical journey through videos you create, interviews you have. What was home like for the Native People of Plymouth who thrived for 1000s of years before the European settlers? How had plant and animal life transformed the landscape from the earliest tundra like species to Spruce, Pine, Hudsonia, grasses and lichen? 

Part Five:  Let’s go look for the evidence of our geologic past

While the following description offers a general overview of Plymouth County's geologic features, pinpointing exact, publicly accessible locations for specific moraines and drumlins within the town of Plymouth itself can be a bit challenging without detailed local geological maps or specific park information. However, we can explore some areas within the broader Plymouth County and nearby regions where these features are known to exist, and infer potential occurrences within Plymouth based on the general geological context. 
Moraines:
Moraines are ridges of unsorted glacial till deposited at the edges or ends of glaciers. In the Plymouth region, the most significant morainal features are associated with the terminal moraine of the last glacial advance, which formed the islands of Martha's Vineyard and Nantucket and extends through Cape Cod.

• Ellisville Moraine: While the main body of the terminal moraine lies to the south and east of Plymouth (forming the spine of Cape Cod), a segment known as the Ellisville Moraine extends into the southern part of Plymouth. You might be able to observe the hilly, often rocky terrain characteristic of moraines in the Ellisville area of Plymouth. The Emery Preserve East in Plymouth, mentioned in one of the search results, notes a "challenging hill" that is part of the Davis-Douglas Conservation Area, situated on an outwash plain, but also mentions the "Ellisville Moraine" nearby. Exploring trails in this area might offer glimpses of morainal topography.

• Manomet Bluffs: Located in Plymouth along Cape Cod Bay, Manomet Point features bluffs that are identified as a terminal moraine. This location offers a direct view of a morainal feature and the glacial erratics (large boulders deposited by the glacier) along the shoreline. Check out the link that will introduce you to Manomet Conservation Science’s campus located on Manomet Bluff. 

As quoted from Manomet Conservation Sciences, “Manomet’s Plymouth campus shares a small stretch of coastline with Cape Cod Bay, a thriving ecosystem brimming with marine life. The bluff at Manomet’s headquarters that overlooks Cape Cod Bay is a terminal (or end) moraine, the point of furthest eastward advance of a glacial lobe of the Laurentide Ice Sheet, which reached its maximum advance about 23,000 years ago near Martha’s Vineyard and Nantucket. The large rocks along the shoreline, known as “glacial erratics,” were deposited by the moving glaciers!”
Drumlins:
Drumlins are streamlined, elongated hills formed by glacial ice acting on underlying till. They often occur in "fields" and have a characteristic teardrop shape.

• World's End (Hingham): Just north of Plymouth, in the town of Hingham, the World's End reservation is a classic example of a landscape formed by four prominent drumlins. While not in Plymouth itself, it's a relatively short drive and offers an excellent opportunity to clearly see the shape and form of drumlins.
• Boston Harbor Islands: Further north, the Boston Harbor Islands National Recreation Area contains many well-formed drumlins, now partially submerged as islands. This illustrates how drumlins appear in the broader region shaped by the same glacial events.
• General Upland Areas: Keep an eye out for smooth, elongated hills as you explore the more inland, slightly elevated areas of Plymouth. While not definitively labeled on typical maps, these could be subtle drumlin features. The "rolling landforms consisting of glacial fluvial, lacustrine, and swamp deposits" mentioned in the search results often occur in association with drumlin fields.

Tips for Observation:

• Look for elongated hills with a gentle slope on one side and a steeper slope on the other. This is the classic drumlin shape.
• Morainal areas often have a hummocky, irregular topography with many unsorted rocks and boulders.
• Conservation areas and land trust properties sometimes have interpretive trails or information about the geological features present. Checking local trail maps and descriptions might provide more specific clues.

Water movement
Imagine Plymouth as a giant sponge that was squeezed, carved, and then had water poured all over it by massive ice sheets! That's essentially what the glaciers did, leaving a lasting impact on how water moves, drains, and gets stored in the region. It is this unique geologic history of water, wetlands and groundwater that later was critical to development of the Plymouth cranberry agriculture that became the largest in the country. 

• The Great Carvers: Reshaping the Plumbing: As these colossal ice bulldozers ground their way across the land, they didn't just move rocks; they also dramatically altered the existing river valleys and drainage patterns. They could deepen some valleys, carve out new ones, and even block off old ones with the debris they carried. Think of it like a giant hand dragging across a sandbox, creating new channels and blocking others. This meant that when the ice finally melted, water didn't necessarily flow where it used to. New pathways were created, leading to the formation of the rivers and streams we see today.
• The Debris Pile-Up: Creating Natural Dams and Ponds: Glaciers are messy movers! They carried a huge mix of rocks, sand, and clay called till. When they retreated, they dumped this material in piles called moraines. These moraines acted like natural dams, sometimes blocking valleys and leading to the formation of lakes and ponds. Think of building a dam with sand and pebbles in a stream – the water backs up behind it. Many of Plymouth's picturesque ponds and lakes likely owe their existence to these glacial roadblocks.
• The Sandy Sponge: Building Underground Reservoirs: The meltwater flowing from the glaciers was also a powerful force, carrying vast amounts of sand and gravel. As this water slowed down, it deposited these materials, creating the outwash plains we discussed earlier. These sandy and gravelly deposits act like giant underground sponges, allowing rainwater to easily soak into the ground and be stored as groundwater. This groundwater is a vital source for many of Plymouth's wells and contributes to the baseflow of its rivers and streams, ensuring water keeps flowing even during drier periods.
• The Impermeable Layer: Influencing Drainage: In some areas, the glaciers deposited layers of fine-grained sediment like clay. These layers are much less permeable than sand and gravel, meaning water can't pass through them as easily. These impermeable layers can influence how surface water drains, sometimes leading to the formation of wetlands and slow-draining areas. Think of trying to pour water through a thick layer of mud versus a pile of pebbles – the pebbles let the water through much faster.

So, the next time you see a lake, a winding river, or even a sandy area in Plymouth, remember the powerful role the glaciers played in shaping it. They were the ultimate landscapers, not just carving the land but also fundamentally altering how water moves across its surface, drains away, and gets stored beneath our feet!

In the next page, we will dive into Watersheds from Aquifer to Ocean.