The slow, deliberate migration of sand dunes in the Gran Desierto de Altar, just east of where the Colorado River once met the Gulf of California, offers a tangible link to Earth’s ancient past. Observing these dunes, with sand cascading down their slopes, mirrors the processes that formed geological formations like the Coconino sandstone in the Grand Canyon, where sweeping diagonal lines reveal wind patterns from 280 million years ago, embodying the geological principle that "the present is the key to the past." However, the rock record also chronicles immense, planet-altering catastrophes, such as the "Big Five" mass extinction events that fundamentally reshaped life on Earth, leaving indelible marks on its evolutionary trajectory. Understanding this complex interplay of gradual change and abrupt upheaval, as documented in the geological strata, provides profound insights into our current world and potential futures, necessitating a grasp of concepts like "deep time" to comprehend how formations spanning vast areas could have formed over millennia. Venturing into this geological timescale, a journey through millions and billions of years, empowers us to better understand the consequences of our contemporary actions. Three seminal books offer compelling gateways into this profound exploration of Earth’s ancient history.

John McPhee, a celebrated writer for The New Yorker, popularized the term "deep time" in his 1981 work, Basin and Range, a foundational text in Earth science literature. A lightly revised edition is available within his 1999 Pulitzer Prize-winning anthology, Annals of the Former World. Though originally published during the Reagan administration and later updated in the Clinton era, with some data refined and concepts now widely accepted, McPhee’s narratives retain remarkable relevance. His intention was to illuminate North America’s billions of years of history by focusing on the most compelling geological events and landscapes as interpreted by scientists from the rocks themselves. This was achieved through his immersive approach, accompanying geologists on a cross-continental journey along Interstate 80, from New Jersey to Nevada.

In the western reaches of the continent, particularly in Utah and Nevada, McPhee delves into the formation of mountain ranges separated by expansive valleys, a geological pattern known as basins and ranges. A geologist guiding him explains this process: "Faulting produced this basin," followed by "Sediments filled it in." This seemingly simple description belies a complex geological history that, while younger than the age of dinosaurs, still stretches back millions of years. McPhee explores how the North American continent is undergoing a process of being "literally pulled to pieces" between the Rocky Mountains and the Sierra Nevada, a phenomenon not entirely unprecedented. A geologist points out that this continental rifting, a tearing of the Earth’s crust, occurred approximately 200 million years ago when the supercontinent Pangea began to break apart, eventually allowing the Atlantic Ocean to form. This raises the intriguing question of whether future generations in Nevada might one day gaze across a new sea towards California.

McPhee’s narrative is characterized by insightful digressions, such as a visit to an abandoned silver mine in Nevada. Following a precarious road overlooking a valley, which McPhee notes held significant cultural importance for the Paiute people, akin to the Black Hills for the Sioux, he learns about the economic realities of 19th-century mining. Despite extracting the richest silver veins, miners left behind substantial quantities of ore, potentially worth millions, as discarded waste.

McPhee also masterfully contrasts human perception of time with the vastness of geological epochs. He observes that "People think in five generations — two ahead, two behind — with heavy concentration on the one in the middle," highlighting our inherent difficulty in comprehending the immense scale of millions of years, even as geologists meticulously measure it. McPhee’s book serves as an accessible introduction to geology, enabling readers to appreciate how scientists can "inhabit scenes that no one ever saw… archipelagos of aching beauty rising in volcanic violence to settle down quietly and then forever disappear — almost disappear."

Science journalist Laura Poppick, in her book Strata: Stories from Deep Time, published in July 2025, embarks on a similar exploration of Earth’s history, exhibiting the same meticulous attention to detail as McPhee, yet with a distinct narrative approach. Her work, woven with contemplative prose, organizes deep time into chronological themes: air, ice, mud, and heat.

Rocks dating back two to three billion years hold crucial clues about the initial emergence and evolution of oxygen in our atmosphere. Poppick travels to Minnesota to examine iron-rich rocks from a period of global anoxia, a time when Earth’s atmosphere contained virtually no free oxygen. The existence of these iron deposits indicates a prolonged era, spanning roughly half of Earth’s history, before oxygen became a significant component of the atmosphere. The composition of the ancient air laid the groundwork for life as we know it, enabling the formation of iron, a material fundamental to our modern infrastructure, from "steel cars and kitchen appliances and medical devices and airplanes," as Poppick aptly puts it.

Approximately 540 million years ago, the planet experienced the Cambrian explosion, a pivotal period when nearly all major animal groups present today first appeared. Paleontologists meticulously study fossils preserved in rocks to trace this evolutionary burst and the extent of life obliterated by subsequent mass extinction events.

Poppick details two of the five major mass extinctions: one occurring 250 million years ago, and the next, roughly 50 million years later. Unlike the asteroid impact that famously wiped out the dinosaurs, these earlier cataclysms appear to have been triggered by massive volcanic eruptions in geologically unfortunate locations. Poppick explains that "The magma that welled up from the mantle sat directly beneath massive reservoirs of oil, gas and coal." As this magma ascended to the surface, it ignited these fossil fuels, releasing not only carbon dioxide but also toxic hydrocarbons like butanes and benzenes, along with ozone-depleting gases. This narrative of past environmental devastation offers a poignant parallel, potentially aiding our understanding of a warming future and "how we might find a way out."

Scientific models suggest that the Mesozoic Era, often referred to as the "endless summer" when dinosaurs reigned, was significantly warmer than today, by an estimated 14 to 25 degrees Celsius (25 to 45 degrees Fahrenheit). Poppick investigates how fauna adapted to these extreme "hothouse" conditions. She joins scientists on an expedition to a specialized site in Wyoming, searching for evidence of the largest terrestrial animals that ever lived—the long-tailed, long-necked sauropods, including iconic species like Diplodocus, Brontosaurus, and Apatosaurus.

The scientists’ focus extends beyond mere fossilized bones; they are deeply interested in the ecosystems that sustained these colossal herbivores and how these environments, along with their inhabitants, transformed over time. Their research centers on the Morrison Formation, a sequence of sedimentary rocks stretching from New Mexico to Montana, which has yielded more dinosaur fossils than any other formation on the continent. The deposition of these rocks spanned approximately nine million years, thus preserving nine million years of dinosaurian history. Poppick notes, "By way of comparison, just twelve million years or so of evolution produced humans, gorillas and chimps from the same common ancestor."

By analyzing the distinct layers within the Morrison Formation, scientists endeavor to understand how sauropods and other dinosaurs thrived in the Jurassic warmth. Poppick writes, "As we inch closer to a clearer picture (of that time), we deepen the intimacy with which we know Earth and its capacity to withstand heat."

To truly immerse oneself in the world of the Morrison Formation, however, a visit to Riley Black’s book, When the Earth Was Green, published in February 2025, is recommended. Black, a science writer and paleontologist, artfully blends scientific data with evocative prose, allowing readers to experience what ancient ecosystems might have felt like. Each chapter is presented as a vignette, supplemented by an appendix that details "what we think we know, what we might guess at, and what simply struck (the writer’s) fancy."

In When the Earth Was Green, Black transports readers to Utah 150 million years ago, moving beyond the Jurassic remnants studied by Poppick in Wyoming. The narrative follows an Apatosaurus as it forages in a lush woodland, consuming carpets of horsetail and an ancestor of the modern ginkgo tree. The dinosaur’s immense size and long, muscular neck allow it to access foliage from the forest floor to the high canopy, providing a significant advantage against predators. Black emphasizes that the creature’s very existence is a "testament to the strange nature of her habitat," characterized by towering conifers and a dense undergrowth of ferns and cycads, a rich "salad bar" essential for the sauropods’ colossal growth. This, Black posits, represents an "evolutionary dance between herbivores and plants."

Black’s earlier work, The Last Days of the Dinosaurs, meticulously detailed the fifth of the "Big Five" mass extinctions, chronicling its devastating impact across seconds, minutes, hours, days, and years with poignant detail. In When the Earth Was Green, the focus shifts from catastrophe to the intricate, co-evolutionary relationships between ancient creatures and plants, their intertwined stories preserved as remnants within the rock strata.

McPhee’s writing style offers a lively, road-trip experience akin to traveling with a knowledgeable, albeit eccentric, geologist uncle, punctuated by exclamations like "Shazam!" upon discovering significant rock formations. While not all the geological humor may be immediately apparent, the journey is undeniably captivating. Poppick’s approach invites readers into the scientific process itself, through field trips, research participation, and laboratory visits, revealing the engaging nature of scientific discovery. Black, conversely, plunges readers into almost dreamlike landscapes with her vivid descriptions of long-vanished worlds, employing her imagination and expertise to facilitate an experiential understanding of deep time.

As Poppick aptly observes, "Our planet seems to be telling us to take a look back." Each of these books serves as a guide, leading us through the immense expanse of deep time and prompting us to contemplate our own place within this profound geological narrative.