In an eschatological mood, New England poet Robert Frost once mused about whether the world would end in fire or ice. Without taking sides on that question, it’s fair to say that both fire and ice played equally dramatic roles in the beginnings of New England. Fire came first, in the form of molten hot lava that bubbled up violently from the earth’s core starting some half a billion years ago. At that time, most of what would become New England was underwater, just off the leading edge of a proto-continent known as Laurentia.

As the tectonic plate that held Laurentia moved slowly eastward, it folded under its neighbor and melted, causing an upwelling of magma beneath the surface of the ocean. That upwelling formed a chain of island peaks off the coast of the continent. Eventually, the land mass of Laurentia crashed into these islands during the Ordovician Era around 440 million years ago, pushing them up into what is now the Taconic Mountains of far western Massachusetts and southwestern Vermont.

About a hundred million years later, in the Devonian Era, Laurentia crashed into a subcontinent called Avalonia to the south, rolling over the smaller landmass to create more upwelling of magma. At the same time, the continent collided with its neighboring continent Baltica—the precursor to Europe—causing the ocean floor between them to buckle and fold back over the continent. The combination created the Berkshire Hills, which have backbones of volcanic rock topped with older “basement rock” of gneiss and quartz that once sat at the floor of the sea. The ocean, meanwhile, was squeezed out over Avalonia to create a vast delta of sedimentary rock that now forms the bulk of Eastern Connecticut, Rhode Island, and Eastern Massachusetts. A bit later, during the Triassic period, a great fault opened up in the middle of the region, creating a 100-mile-long rift valley that would later become the Connecticut River.

At this time, all of the world’s continents briefly joined together in a giant landmass called Pangaea. The commingling didn’t last long, however. By the Jurassic Era, 200 million years ago, the continents were again on the move, and North America and Europe split up to create the Atlantic Ocean. Around the same time, a field of volcanoes opened up in the area of New Hampshire, spewing hot magma in plutons and ring dikes to form the massive granite peaks of the White Mountains. The Whites are the youngest mountains in the region, and the last evidence of volcanic activity, which ended about 130 million years ago. New England’s fiery birth was followed by a long period of erosion and settling before fire handed off its job to ice, and the last great ice age began.

Temperatures began to cool gradually about a million years ago. By the Pleistocene era, some 80,000 years ago (a mere hiccup in geological time), a massive ice sheet began to build up over Canada, more than a mile thick in places. As it did, the sheer weight of the ice caused it to flow southward in a huge glacier, leveling the earth, gouging out valleys, and breaking off mountaintops as it flowed. The farthest glacier reached all the way down to New York City, depositing millions of tons of rocks in a terminal moraine that forms Long Island, Block Island, Martha’s Vineyard, and Nantucket. (Since so much of the earth’s water was tied up in ice, sea levels were lower, and all of those islands were once mountains.) A more intense, but less far-reaching, glacier left a second terminal moraine around 20,000 years ago to form the northern spur of Long Island, along with Cape Cod and the Elizabeth Islands.

While the terminal moraines are the most visible result of the glaciers, all of New England was definitively shaped by the ice, which rolled and ebbed across the region for thousands of years. Mountains were pushed over and broken, so that even today in the Whites, the northern slopes offer more gradual ascents for hikers, while the steep southern faces present grueling challenges for rock climbers. Mountaintops and boulders, meanwhile, were picked up by the advancing ice sheet, and often deposited miles away from their origins. In some places, these huge glacial boulders, also known as “erratics,” have become local landmarks, such as the 5,000-ton Madison Boulder in New Hampshire’s lake region, the largest erratic in New England.

The glaciers changed the land in other ways as well. As it advanced, the ice sheet pushed away the softer substrate, exposing the harder, immovable granite. In many places, lone mountains called “monadnocks” remain to lord over the surrounding plains. The most famous of these is Mount Monadnock in southern New Hampshire, whose views as far as Boston on a clear day make an irresistible magnet for hikers. In Rhode Island, the advancing ice gouged away the softer substrate in Narragansett Bay to expose granite islands, including the island on which Newport is located and from which the state gets its name.

The last of the glaciers retreated by 15,000 years ago. In its wake, however, it closed off the Connecticut River Valley and filled it with meltwater to create a huge inland lake named Lake Hitchcock. For more than 2,000 years, the lake stretched 200 miles from Connecticut to Vermont’s Northeast Kingdom. (It was probably quite a sight, colored the striking azure blue of the glacial lakes that now grace Canada and Patagonia.) The layers of silt deposited by the drying lake helped create the rich, loamy soil of Massachusetts’ Pioneer Valley—which stands in marked contrast to the rocky, glacial till of the rest of New England. Another glacial lake, Lake Vermont, formed to the west of New England, and drained into modern-day Lake Champlain.

Other land features marked by glaciers include long straight grooves in the earth known as “glacial scarring”; teardrop shaped hills called “drumlins,” which are especially common in Rhode Island and Southeastern Massachusetts; and round depressions known as “kettle holes,” which were caused by standing water from glacial melt drilling down into the earth, and often serve as local swimming holes. Global warming notwithstanding, some geologists surmise that our present period is just a brief interlude between ice ages—and Frost might get his answer when the glaciers roll down from Canada again.