This Old Land of Cambridge

This Old Land of Cambridge

By George Ehrenfried

Adapted from a lecture presented at the Mt. Auburn Branch of the Cambridge Public Library

Distributed by the Cambridge Conservation Commission, April 1991

Once upon a time there was a lawsuit, in which a geologist was called to testify as an expert witness. After he had given his testimony, the opposing lawyer got to work to try to wreck it. The first thing he said was, "Don't eminent geologists consider professor so-and-so an authority on this subject?" Our friend the witness was no dumbbell; he saw that the lawyer was trying to sneak up on him and catch him off base. Here is what he answered: "No eminent geologist ever considers any other geologist an authority on any subject."

This comment tells us something about how the science of geology works. In particular, about how geologists disagree with each other, and how often their ideas can change. Today you may read something that has been in all the college textbooks since Cal Coolidge was president, and tomorrow someone may dig a hole down in Antarctica, or even right here in Cambridge, and find something that proves it's all wrong. This is not just a rhetorical speculation. Something like this actually happened, only nine years ago, and I was lucky enough to get in on the ground floor of the story. Let me tell you about it.

I am sure all of you have looked down across Boston from Great Blue Hill to the south, or from Prospect Hill in Waltham to the west, or from the long downhill slope called Mile Hill on Route 2 in Belmont to the northwest, or from Pine Hill in Middlesex Fells to the north. Under the tall buildings, the general layout is a large low-lying, nearly flat area, surrounded on three sides by much higher country. This is what geologists call the Boston Basin. The Harvard Museum used to have a big terrain model of the whole area, and it showed a sharp dividing line between the low, flat Boston Basin, and the more rugged highlands to the north, in Arlington, Woburn, and Medford. All of Cambridge is well within the boundary of the basin.

How did the basin get that way? We can blame it on the rocks that underlie the area. If you look at the rocks in the highlands outside, you'll find a lot of schists and gneisses and granites. These rocks have been cooked, squashed, and pickled, several miles deep in the earth, until they have become so tough that they can take a real beating from the weather and not suffer very much. The rocks inside the basin are very different. They are sandstones, mudstones, and puddingstones, with a few spots of volcanic ash and lava here and there. Very little has happened to them ever since the beds were laid down in bodies of water. Only enough to stick the particles together, so the stuff acts like stone instead of loose sand or clay. These kinds of rocks are nowhere near as durable as the stuff up in the highlands, and they wear down much faster. During the millions of years they have been sitting here, the weather has done a big job on them, so that the land is much lower than it is outside the basin.

How long have the basin rocks been here? That's a tough problem. To get the answer, geologists try to find fossils. If you can find them and identify them, you fit them into the great scale of fossil ages that has taken a couple of centuries to work out, and this correlation gives you a pretty good idea of their age. For a long time, many people even tried to find fossils in the basin rocks. No luck. Professor Mather even used to tell his Harvard geology students, "If you find a fossil, I promise to give you an A in the course, no matter what you do the rest of the term." He never had to pay off. The only thing they had to go on was a very faint impression of what seemed to be the bark of a scale-tree, a fossil tree that you find all over the place in the coal mines in Pennsylvania. This fossil turned up around 1900, so for around 80 years all the books have been saying that these rocks under our feet dated from what they call the Pennsylvanian age, which was about 300 million years ago. The statement went from book to book, until most people forgot how weak the evidence was, and it was considered just as sure as one and one make two.

In March 1982 I went to a lecture on evolution at Harvard. I just happened to find myself sitting beside Professor Elso Barghoorn, a world-famous biologist who specialized in little tiny animals which you can't even see without a microscope. After the lecture, he and I got talking together for a while about some of his work that I had read about. Finally he told me something like this: "We are working on another hot project now. The MBTA people let us go down into the tunnel they're boring for the Red Line extension in North Cambridge, and we brought out some of the mudstone that they're drilling in. I and some of my graduate students have been studying the stuff with an electron microscope, and we found some one-celled plants in there, only about a third of a thousandth of an inch in size. These plants became extinct about 600 million years ago, so we know the rocks have to be at least that old. Nobody knows about this yet, except the group who has been working on it with me, and our paper on the subject is going to be published in Science magazine in about a month."

When the paper came out, it was a real bombshell all over the geology community. Not simply because the Roxbury conglomerate, and the Cambridge slate, and the Brighton lava, had suddenly jumped from the comfortable middle-age of the 300 million-year-old Pennsylvanian to the dimly-known Methuselah antiquity of the 600 million-year-old Pre-Cambrian. More than this, it challenged everyone's trust in many of the other long-accepted holy-cow beliefs that had been floating around. When a new idea is proposed nowadays, they don't dare call it hogwash and brush it off. They have to listen-- maybe there's something in it.

Now let's turn to what these Cambridge and Boston rocks look like. Dr. Lawrence LaForge, the Tufts professor who, in his day, knew more than anyone else about Boston geology, used to say that there isn't a single natural bedrock outcrop anywhere in Cambridge. To find solid rock you have to go straight down, for a long way. Finally you come to some rather unglamorous-looking grey stuff, which is, in essence, petrified clay. In some places outside Cambridge you can see it at the surface. The best outcrop I know of is on the Newton Campus of Boston College, just off Centre Street in Newton. In this location it splits very easily into thin layers. Under Cambridge it doesn't want to split, but simply breaks into irregular chunks. This stuff used to be called the Cambridge slate, but now they often call it argillite, which is a fancy name for stone made of clay. In some places in it you can find neat little cube-shaped crystals of pyrite, or fool's gold, or little cubic holes where the crystals used to be. This pyrite tells us that the clay was deposited in deep and stagnant water, where the oxygen in the air never penetrated to clean out the old rotting plant material that settled to the bottom.

According to Dr. Lynn Margulis, of the University of Massachusetts in Amherst, if you had been able to land on the earth's surface back in the ancient pre-Cambrian period, when this clay was still soft, you would not have noticed a trace of any living thing. No matter which way you looked, you would see only rock and sand and dirty water. But if you got down close and looked at it with a high-powered magnifying glass or microscope, you would find that every square inch of surface would be teeming with life. There was probably almost as much life on earth then as there is now, but the individuals were a lot smaller. Some of these tiny plants or animals had the power to extract the small amounts of sulfur and iron in the water, and build it into their own bodies. This sort of thing is still happening in many places. The Saugus Iron-works, which operated up near Lynn in the 1600's, used iron ore dug out of bogs, which was formed in pretty much the same way, but in very recent times.

In the old Cambridge argillite, the iron and sulfur combined to make the little brassy-looking cubes which sometimes have rusted completely away, leaving behind nothing but holes. You can find some of these cubic holes in an outcrop on a hillside behind a firehouse in Somerville. When they were building a high-rise on Commonwealth Avenue in Brighton I found some of the fool's-gold crystals in the freshly broken rock from the foundation hole.

Another kind of Boston Basin rock is sandstone. This is what you get when grains of sand are cemented together. The outside walls of the Story Chapel in Mt. Auburn Cemetery are made of sandstone, which come from the town of Potsdam, way up in the corner of New York state. The people up there are very proud of their quarries. Not long ago, in a Red Line subway train, I saw a young fellow with a T-shirt on that had a big basketball printed on its front, and the words "Potsdam Sandstoners" running in a circle around it. If you have ever taken that famous boat ride down the Ausable Chasm, up near Lake Champlain, you were riding between high vertical walls of this same Potsdam sandstone. This rock is pretty old also, but not so old as the Boston rocks. It dates from the Cambrian age-- around 500 million years ago. The arrangement of stones in this wall is what stone masons call ashlar. The stones are all rectangular, but of varying sizes and shapes, and all the seams between them are horizontal or vertical. There are two other common arrangements. One of them is like a brick wall, with the stones all just alike, and laid in neat rows. The other uses stones that are broken into all kinds of irregular shapes, which are put in any way they can be made to fit. This arrangement is called random rubble.

You don't have to go out to northern New York for sandstone. There is a great deal of it in the Connecticut Valley, almost all the way from Long Island Sound up to the Massachusetts-New Hampshire line. It used to be quarried in huge quantities, mostly in Portland, Connecticut, and Longmeadow, Mass. You can see it all over Boston and Cambridge, especially in the so-called "brownstone" houses along Commonwealth Avenue in the Back Bay. When carefully selected and properly used, it can be very durable. As often happens, some fast-buck quarrymen and builders cut corners by using low-grade stock and laying it vertically instead of horizontally. This resulted in bad flaking and spalling-off on the surface, so brownstone went out of style, and the quarries are sitting idle. The most recent major project that used Longmeadow sandstone was a group of buildings at New York University, which were built in the early 60's. There is a little Connecticut Valley sandstone in Mt. Auburn Cemetery, but not very much. Some of it is in very poor condition.

This sandstone is much younger than the other, only around 180 million years old. It dates from the age of the dinosaurs, and if you ever go down to the Dinosaur State Park in Rocky Hill, Connecticut, you can see hundreds of dinosaur footprints criss-crossing back and forth across beds from this period. Here in the Boston Basin one of the best places to see the sandstone is along Beacon Street in Newton Centre, and in the Webster Conservation Area just to the south. You can see (Figure 1) from the way these layers twist and turn, that the beds of sand went through some pretty complicated disturbances, after they had settled down and before they were cemented into stone.

The third of the Boston Basin sedimentary rocks is puddingstone, or what geologists call Roxbury conglomerate. It is made out of rounded cobbles and pebbles mixed in with finer-grained sand. This monument (Figure 2), which is also in the cemetery, shows the effect quite well. There are several ways to make conglomerate, and they all involve violent activity. You can have an ocean beach, where heavy waves crash against the shore year after year, tumbling the corners off pieces of rock until they are worn down to rounded shapes. You can have a glacier, which contains rock dust that acts like sandpaper, grinding the pieces of loose rock under the ice. Or you can have a rushing mountain stream, fast enough to carry pieces of rock from the high passes to the valleys or lakes far below, and long enough so the pieces have plenty of time to get worn off smooth along the way. Whatever way you choose, you have a lot going on. That old pre-Cambrian landscape could not have been just a quiet flat plain. It had to have heavy surf, moving ice-fields, or high mountains-- maybe all three.

There is a great deal of conglomerate around Boston. You find it mostly in Roxbury, Brookline, and Newton. One of the best places is in the woods near Hammond Pond, behind the Chestnut Hill shopping center. Many buildings in Boston and Cambridge, especially churches, are made of it.

The last of the major varieties of those old pre-Cambrian rocks around here is the volcanic lava. Many people are very surprised and perhaps a little worried, when I tell them that there used to be volcanoes around here. It seems almost unbelievable, in this peaceful land, that there could have been craters throwing white-hot ash and volcanic bombs miles up into the air, or spreading liquid lava over the countryside. I don't know whether there were volcanoes right here under Cambridge. As I mentioned before, there are no surface outcrops here, so all we know is what we happen to learn when holes are drilled for foundations or tunnels. We have to look in other places nearby. Let's go across the river to Brighton. There's a great deal of lava there. So much that this rock formation is called the Brighton volcanics. There is a park, halfway between Commonwealth Avenue and Union Square in Allston, called the Ringer Playground. It has a lot of patches of bedrock, some of them sticking way up out of the tops of the hills. Most of this stuff is lava. It looks greyish or greyish-green, with wandering streaks and spots here and there. Some of the spots are brighter green with white centers. When the lava was fresh, these spots were bubbles, probably filled with steam. Later on, after it had frozen, hot solutions containing various chemicals seeped slowly through the pores in the rock, and deposited some of the chemicals in these holes.

The Brighton volcanics aren't the only ones around. There are several others, called the Lynn volcanics, the Mattapan volcanics, and so on. Not all of them are inside the Boston Basin, however. The Lynn volcanics are especially interesting, because you have seen a lot of it around, whether you know it or not. There is a huge crushed stone quarry on Route 1, just north of Everett, called the Rowe quarry. The stone is purplish or reddish, and it is used all over Boston, for paving roads and parking lots, and for spreading over areas where they want to prevent weeds from growing. One place you can see it in Cambridge is at the corner of Craigie and Berkeley Streets, where in some places the whole road looks reddish.

Some people are puzzled because the Lynn lava has hardly ever been used as a decorative stone. For instance, the Smithsonian Institution annual report, about a hundred years ago, said the rock has "exceptional beauty, presenting colors as red as jasper, through all shades of pink, gray, and even black, often beautifully variegated in a variety of colors. The rock acquires a beautiful polish, and the fact that it has not ere this come into more general use is a sad comment upon the taste of our wealthier citizens."

Where was Cambridge, back in the pre-Cambrian, when all these local rocks were being formed? That question drags in another story something like my first one. This time it's about a whole slew of geologists who ended up with egg on their faces because they refused to pay attention to a non-geologist outsider named Wegener, who tried to tell them, back in the 20's, that continents can move around. It took about forty years for them to have to admit that he was right. The general opinion now is that, 600 million years ago, Cambridge was part of Africa, and the shoreline of the old Atlantic Ocean was about 30 miles west of here. Maybe I shouldn't say shoreline-- I really mean the edge of the continental shelf, the steep underwater slope that actually defines the boundary of the continent. If you follow Route 495, from where it starts somewhere near Newburyport, southwest to Worcester, then walk due south from Worcester through Connecticut all the way to Long Island Sound, you will be tracing out the approximate edge of the old African continent. The old North American continent started on the other side of the line and went on to the west. There was a lot of deep water between the two continents. Gradually, they slid together. The ocean got narrower and narrower, until finally it got completely squeezed out, and the Americas and Africa became parts of the big super-continent they call Pangea.

A long time later, Pangea started to break up again. But it didn't crack in the same place as before. The crack came farther east. This patch of land we are on did not go back with Africa, but get left behind attached to North America. So that's why we are in the western hemisphere now, instead of the eastern hemisphere.

Do you know where Avalon is? I don't mean the mythical island in the King Arthur legends. I mean a real place. It is a little peninsula that dangles down like an earring from the southeast corner of Newfoundland. I was there five years ago and I felt right at home. You see, Avalon is another part of this same continental sliver that defected from Africa to North America. The middle part of the sliver is mostly under the shallow waters off the coast of Maine, and this whole long, thin piece of land is called the Avalonian Terrane. The rift that became the modern Atlantic Ocean first opened up around 180 million years ago, and the ocean is still getting wider, at a rate of about an inch a year.

The most exciting period in New England's geological history was when the old Atlantic was squeezed out and North America collided with Africa. This was about 380 million years ago, and it is called the Acadian disturbance. All kinds of things were going on. Great mountain ranges were slowly heaved up. Large areas of land emerged from the shallow coastal seas. Huge masses of magma came up from the depths, and froze into granite. Volcanoes erupted here and there. Horizontal beds were tilted until they stood right up on edge. Especially important was the metamorphism of the older rocks. This means that most of them were heated way up until they were almost melted, were stretched, compressed, and twisted like taffy, and were bathed in superheated concentrated chemical solutions. By the time the disturbance was over, the rocks had been changed so radically that geologists, in some cases, don't have the least idea what they had been like beforehand. Naturally, any fossils in them were destroyed, so that it gets extremely hard to figure out how old they are. For some reason, the Boston Basin, and a few other small areas, led charmed lives, and escaped the metamorphism. I don't happen to have seen any really good explanation for why this is so.

All of this was going on in what biologists call the "Age of Fishes". There wasn't much animal life big enough to see on land yet, but there were all kinds of creatures, big and little, in the water: little animals, like these seafans or bryozoa (Figure 3), that you can find all over Cambridge if you look with a magnifying glass at the Indiana limestone at M.I.T., Widener Library at Harvard, the storefront separators in Brattle Square, and many other places; and big animals, like this ammonite (Figure 4), a squid relative that coiled up like a snail, which you can see modeled, in its actual size, on Chestnut Avenue in Mt. Auburn Cemetery.

After the big show, things quieted down. There were some other small disturbances, but none of them around here were anything like the Acadian. Then, around 60 or 80 million years ago, the disturbances in New England quit completely. Practically nothing happened, except the land rose, and wore down, and rose again, and wore down again, over and over. This happened for such a long time that, in some places, as much as ten miles of thickness have been removed from the land and washed into the sea.

Gradually the land began to look more like the way it looks like now. Every time the land rose, the streams and rivers and frost action wore down the soft rocks rapidly, and the hard and tough rocks slowly. The old mountains, which had been built during the Acadian disturbance, were completely removed, and only the complicated rock structures that had been way down under them remained. The new mountains, which were slowly appearing, owed their existence, most of the time, to the fact that their rocks were more durable than the rocks of the areas around them. The land was no longer being molded by earth forces from underneath, but now by the more gentle, but just as effective, weathering mechanisms acting from above. There were plants and bushes and trees that look a lot like they do now. The big dinosaurs became extinct, but there were insects, birds, and mammals, and eventually mammals like wolves, tigers, mammoths, and small horses, evolved.

The White Mountain highlands, mostly made of tough rock called Littleton Schist, began to stand out, as did the Green Mountain highlands, also made of durable schist and gneiss. The Connecticut Valley, underlain by soft sandstone and shale, was being worn way down, as well as the lowlands of central New Hampshire, where the bedrock was a very easily eroded rock called Winnepesaukee quartz diorite. There were almost no lakes or ponds, for there were no natural dams to hold them. There were very few places where you could see smooth bedrock on the surface, for the soil had had millions of years to establish itself to great thickness.

In a general way, we can say that the New England landscape was in an equilibrium condition, which could have gone on this way for a long time, except for a big change in the weather, which finally led to a whole new array of forces acting on the land.

I hope you are willing to believe me when I tell you about the great continental ice-fields that came down from the north and covered the whole land, down as far as Nantucket and Martha's Vineyard and Long Island. There are still people who don't believe in such a thing. I ran into one them on top of Tumbledown Mountain in Maine a few years ago. When I showed him the glacial scratches on the polished rock surface, and when I showed him the U-shaped valley, and when we looked at the pretty little tarn pond sitting in a cup-shaped depression in the hard rock, he always said, 'That was made by Noah's flood." He hadn't hiked in the northwest, where you can see hundreds of beautiful tarns, just down the mountainsides from what's left of the glaciers that made them. When he saw us filling our plastic bags with ripe juicy mountain cranberries, he was just as emphatic in declaring that those things were deadly poison, and would surely kill us if we made sauce out of them! The sauce tasted real good.

Even the rest of us, who accept the idea of the continental glacier, are sometimes rather confused about the details. I keep running into people who ask such questions as, "Did the glacier make the puddingstone?" or, "Did the glacier make Mount Monadnock?" It is true that I mentioned the possibility that a glacier may have helped to round off the pebbles in the puddingstone, but if so, that was way back over 600 million years ago. The continental glacier that we usually hear about was only 20 thousand years ago, hardly an eye-blink of time compared with the uncertain older one. These misconceptions about glaciation history sometimes have led to very strange behavior.

The weirdest of all of these stories is about a man up in Lynn. He lived around the time of the Civil War, when the glacial theory, as it was called back then, was still new. This man had heard that a certain famous pirate, back in the 1700's, had a dry-land base of operations hidden in the wildest part of the large hilly wooded area which is now called Lynn Woods Reservation. Somehow or other, he decided that there was a big treasure buried at a certain spot. He went there, and discovered that this spot was at the top of a hill, completely covered with bare bedrock. This didn't faze him a bit. He had read about the glacier, but he didn't know when it happened and what it could and couldn't do. He figured that AFTER the pirates had buried their loot, the glacier had come along, and had built a whole hill of solid rock on top of it. This man spent the next twenty years, working with his son, digging and blasting a tunnel which slants deeply down into the hill. It's still there, and it's called Dungeon Rock. You can go into it, if you feel like it, and if you don't worry about the darkness, slippery footing, and risk of bumping your head. I don't guarantee the accuracy of the details of this story, but this is the way I heard it.

Another strange story is about a monument. There are all kinds of monuments. Here in Cambridge, of course, we see mostly monuments to people, historical events, the sites of old buildings, and so forth. But have you ever seen a monument to itself? No kidding-- the actual monument is itself the object being commemorated. You can see it in the little grass-plot in the square near the city hall in Fitchburg. For the last 10,000 years, more or less, this hundred and ten ton boulder was sitting right on top of a high bare hill on the edge of the city, where it could be seen from far away in every direction. As this bronze tablet says, some people started to quarry away that part of the hilltop. The city people raised money, broke the rock into several pieces, lugged them down the hill, stuck them together again, and set it up in the park. There's one thing wrong with the tablet. It says the rock was brought from Mt. Monadnock by the glacier. Well, it happens that this rock is made of out of some stuff called Kinsman quartz monzonite, which has a distinctive appearance that makes it easy to identify if you've seen a lot of it around. It also happens that if you look at Katherine Billings' geological map of Mt. Monadnock, these isn't any kinsman on it. Practically the whole of Monadnock is made of Littleton schist, the same tough and durable rock that holds up Mt. Washington. So, wherever the glacier brought the Rollstone boulder from, it wasn't Monadnock. The quarry on the hilltop, where the boulder used to be, is the source of the granite used in the base of the Minute Man monument in Concord.

What did the glaciers do, right around here? Quite a lot. Before the glaciers there wasn't any Mt. Auburn Hill, or Observatory Hill. There wasn't any Fresh Pond. There wasn't any Spy Pond in Arlington. There weren't any Mystic Lakes in Winchester. Instead, there was a long, deep valley coming down from the north. It started in Wilmington, came down between Burlington and Reading, between Woburn and Stoneham, then right through Arlington Center, where it cut through the wall of the Boston Basin. Then it went across Spy Pond and past the Arthur D. Little buildings. It went under the western part of Fresh Pond, then down Fresh Pond Parkway, under Mt. Auburn Hospital, and out into the Charles.

Before the glaciers, Cambridge looked completely different from the way the colonists found it. If you wanted to walk from the Star Market in western Cambridge to Harvard Square, you would have to go down a steep slope about a hundred feet deep, cross a good-sized river, and climb back up again.

What happened to this valley? It's still there, but it was filled with sand and gravel and clay by the glaciers. If you were digging a well just northwest of Fresh Pond, you could keep going 180 feet before hitting bedrock, but on both sides of the valley you'd hit bottom only about 50 feet down, or even less.

Glaciers have a tremendous capacity for carrying dirt. This small glacier (Figure 5) has scraped off a lot of rubble off the cliffs at its upper end, and carried it down mixed with the ice. As the glacier comes down the valley, the ice melts away but the dirt doesn't. Down at its bottom end, it's practically all dirt. The run-off streams carry the silt down the lower part of the valley. They join the meltwater streams from other glaciers, and they all dump their loads of silt when they get to the sea. This process moves the seashore farther out all the time. In this particular case, the result is a big, flat outwash plain, about 400 square miles in area, with streams wandering all over the place, changing courses every year, bringing sand and gravel and clay down from the mountains to make the outwash plain bigger still. Compared to this, it is just kid stuff for a series of four continental glaciers to fill up our valley here in Cambridge.

The clay which the outwash stream laid down in this valley furnished the raw material for one of Cambridge's biggest industries for over a hundred years. From about 1850 to about 1950 the Bay State Brick Company and its successor, the New England Brick Company, shipped out tons and tons of good red brick every year. These were used in the homes in the Back Bay, the office buildings of downtown Boston, the brick sidewalks of Boston and Cambridge, and many of the dorms and houses of Harvard. The clay pits sometimes went down more than 80 feet, and workers found that the wet mucky material was really diabolical stuff to dig in. They had to watch out for collapses and cave-ins all the time. The final blow that killed the brick business was a big cave-in that completely buried a whole steam shovel and its operator back in 1952.

When the Perini Construction Company was building the Alewife subway station, they ran into the same problem. When I visited the construction job site, I saw one of their bulldozers sinking into the quicksand-like clay in the bottom of the excavation. Most of the undercarriage had already disappeared. They weren't really worried about it. They knew they had enough heavy-duty equipment so they could hoist it up out again before it went down too far. This stuff is called "sensitive clay." This means that it looks stiff enough to start with, but as soon as it is disturbed it turns into something like pea soup. The digging problems are horrendous. They never could have got the job done unless someone had invented a brand new digging procedure involving the placement of "slurry walls." After they finished the two concrete sidewalls, the pressure from the outside was so great that they had to put in hundreds of big heavy iron struts to prevent these walls from collapsing inward (Figure 6). Even then, when they measured the force on the struts with strain gauges, they found that the loads were very close to the safe limit of what the struts could stand.

You remember that I wrote, some time ago, that there were almost no lakes or ponds during the long quiet period before the glaciers. If you look at a map of one of the southern states, down where there was no ice sheet, you will find that almost all the lakes are artificial with dams at their ends.

New England is well supplied with natural lakes and ponds. Most of them are either moraine lakes or kettleholes. You will find many moraine lakes in Maine and New Hampshire. The three great lakes of New Hampshire, Winnepesaukee, Squam, and Winnesquam, are examples. A moraine is a mass of dirt that was brought down by a glacier and was dumped there when the ice melted. If it happens to be dumped in a valley, it can dam it up and make a lake. Around Boston, kettleholes are more common. Boston's best known kettlehole pond is Jamaica Pond. When the ice sheet was melting away along its south edge, it often happened that some patches didn't melt so fast as the rest. Imagine a big raft of ice, half a mile across, and two or three hundred feet thick, sitting out there all by itself, while the rest of the ice is five or ten miles to the north, melting away like mad, and generating dozens of streams which spread out across the outwash plain, depositing silt everywhere on it and building up its level higher and higher. After a few years there will be a thick layer of gravel all around the raft. Sometimes the layer gets so thick that it completely buries the raft of ice. Sooner or later, of course, this raft melts away, leaving behind a hole in the bed of the sediment. This hole usually fills with water, and creates a lake or pond, like Jamaica Pond, Walden Pond, Fresh Pond, Halcyon Pond, or Consecration Dell Pond in Mt. Auburn Cemetery.

Mt. Auburn Hill was made by the glacier, too. The experts disagree on how it got there. I prefer the explanation that calls it a kame. A kame is almost the opposite of a kettlehole. One kind of kame is what you get when there is a hole in the ice field, perhaps a few hundred feet across, and sediment collects in it. It doesn't have to be a hole. It can be a deep notch in the front edge of the ice field, also. In the books a kame is usually described as a knobby hill. Kames often occur near kettleholes. The Indian Ridge Path in the cemetery runs along the top of a small esker, a ridge made of sediment that collected in a long narrow crevasse in the ice field, or more commonly, in a tunnel under the ice. There are many long and high eskers in Maine, where the local people call them "horsebacks." Three of the most scenic eskers near Boston are the esker in Ridge Hill Park in Needham, the system of eskers in Great Esker Park in Weymouth, and the esker in Beaver Valley Reservation in Boxborough and Harvard.

Although I have wandered off in various directions from time to time, I have tried, in general, to help you get a feel for the way our land has been shaped, at first by a long sequence of powerful forces acting upward from the interior of the earth, and later by less spectacular but very effective influences acting downward from the air and water and ice above. The land is now beginning to feel a third kind of pressure for change, the activity of human technology. It is changing the landscape hundreds of times more rapidly than most of the past geological or biological forces. No one can prevent the changes, but it is important to try to make sure that, in as many places as possible, the changes are being wisely and carefully managed.