From Ocean to Outcrop: How Ancient Reefs Transformed the Land

     Between 250 and 300 million years ago, the Permian Sea covered a vast area, including much of what is now Texas and New Mexico. This warm, shallow, equatorial sea teemed with life and was home to organisms that built massive carbonate deposits.These deposits would eventually reshape the landscape and give rise to the fascinating geological formations we see today. Many of the stops highlighted in this blog lie within the Permian Basin, which spans roughly 100,000 square miles, including the Delaware and Midland sub-basins. However, deposits from the Permian Sea also influenced landscapes farther west, such as the Tularosa Basin. These ancient deposits left behind the cliffs, caves, and springs that we will explore along the Permian Highway, forming the geological backbone of the region. In this blog, "Permian Highway" references the route my class traveled as we explored these geological formations in September 2025.  

Map of the regional extent of the Permian Basin. Basemap sources: National Geographic Society and i-cubed (2013); Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, and the GIS User Community. Additional data: PermianBasin_Boundary_Structural_Tectonic. Map created by Brittney A. Hawley in ArcGIS Pro 

    At the heart of this ancient seascape was the Capitan Reef, a massive barrier reef that stretched for hundreds of miles along the edge of the Delaware Basin. Built primarily by sponges, algae, and other reef-forming organisms, the Capitan Reef was the backbone of the Permian marine ecosystem. Over millions of years, these organisms deposited thick layers of limestone and other carbonates, which eventually lithified into the towering cliffs and ridges we see today in the Guadalupe Mountains. As the Permian Sea retreated and tectonic forces uplifted the region, the reef transformed from a thriving underwater structure into exposed rock formations, eventually creating caves and springs. It was truly a journey from ocean to outcrop. Exploring the Capitan Reef allows us to read the ancient story of the Permian Sea written in stone, from fossilized reefs to modern landforms.

View of El Capitan Peak viewed from the Salt flats viewing point. 

 

The influence of the Capitan Reef and the Permian Sea extends far beyond the cliffs of the Guadalupe Mountains. As you travel along the Permian Highway, you can see this legacy in a variety of landscapes and geological features. At Carlsbad Caverns, the limestone deposited by the ancient reef was sculpted over millions of years into vast underground chambers and stalactite formations. Rattlesnake Springs and Bull Springs owe their waters to aquifers flowing through these same carbonate rocks. Further west, White Sands National Park tells a slightly different story, where gypsum left behind by evaporating Permian seas creates a striking, otherworldly landscape. Even along the Permian Trail, the ancient reef’s fossilized remnants allow visitors to walk through the very structures built by marine organisms hundreds of millions of years ago. Together, these sites form a continuous geological story: a journey that begins beneath the waves of the Permian Sea and unfolds across modern deserts, mountains, and caves. 

By tracing the remnants of the Permian Sea and the Capitan Reef, we begin to understand how ancient oceans shaped the landscapes we explore today. In the posts that follow, The Permian Highway describes our geological field trip across these remarkable formations, revealing the geological processes and ancient life that created them. The adventure I'll unfold before you takes a slightly different route than than the one we traveled, as our plans changed according to the weather. This is something you'll want to keep in mind if you travel to the same locations. Rainy days make for muddy paths... and rental vehicles you might have to pay a cleaning fee for. 

All photographs are original and taken by Brittney Hawley unless otherwise noted 

Map of the stops taken along the journey. Basemap sources: National Geographic Society and i-cubed (2013); Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, and the GIS User Community. Additional data: PermianBasin_Boundary_Structural_Tectonic. Map created by Brittney A. Hawley in ArcGIS Pro 

Day One: From El Paso to Carlsbad — First Encounters with the Capitan Reef

Map of the stops for day one. Basemap sources: National Geographic Society and i-cubed (2013); Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, and the GIS User Community. Additional data: PermianBasin_Boundary_Structural_Tectonic. Map created by Brittney A. Hawley in ArcGIS Pro 

Have you ever driven past a van pulled over on the side of the highway, and noticed a group of people gathered around a rock outcrop pointing and taking notes? You were probably looking at geologists studying the roadcuts. When we study rocks, it’s not enough to observe the weathered surface; we have to look deeper. Breaking open a rock reveals its mineralogy, textures, and clues about its history. We notice bedding, stratigraphic relationships, and structural features like folds, faults, joints, and intrusions. What kinds of rocks are present? Are there clasts, and if so, are they angular or rounded? Each of these details tells part of the story of how the rocks formed and were later altered.

Our first day in the field took us from the El Paso airport toward Carlsbad, New Mexico, with three roadside stops along the way. Each provided an opportunity to stretch our legs, shake off the travel fatigue, and begin exploring the region’s remarkable geology. The first stop was at the Salt Flat Bolson, where we had an excellent view of the Delaware and Guadalupe Mountains. This salt basin represents a graben, a down-dropped block of crust bounded by normal faults. Faulting played an integral role in the formation of the basin’s distinctive white dunes, creating a closed depression into which nearby rivers drained. With no outlet for the water, evaporation increased salinity, leading to the deposition of thick beds of gypsum and halite.

View encapsulating how the flats meet the mountains

During the Pleistocene Epoch, cooler and wetter climatic conditions led to the formation of a shallow, ephemeral lake within the basin. Seasonal precipitation and runoff from the surrounding highlands periodically filled the depression, further concentrating the salinity as the lake water evaporated. Over time, these repeated cycles of flooding and desiccation increased the accumulation of evaporite minerals. The resulting landscape preserves a record of both tectonic activity from the Miocene and climatic oscillations from the Pleistocene. This illustrates the dynamic interplay between tectonics, hydrology, and climate in shaping desert basins of the American Southwest. Standing there, with the wind sweeping across the broad expanse of the Salt Flats and the Guadalupe Mountains rising in the distance, it was easy to imagine the shifting lakes and ancient shorelines that once occupied this now arid landscape. (National Park Service, 2022)

The second stop along the way was a roadcut exposing the Bone Spring Limestone, the oldest formation exposed in the Delaware and Guadalupe mountains. This dark, organic-rich limestone was deposited as thin, successive beds of bituminous or cherty limestone interbedded with calcareous shales. In the photo below, you’ll notice that some bedding layers protrude more prominently than others. These more resistant layers are composed of cherty limestone, which weathers more slowly due to its siliceous mineral composition. The outcrop is littered with rock fragments, and once you find a piece of bituminous limestone and strike it to expose a fresh surface, you may notice a strong, tar-like odor. This distinctive smell is caused by the high organic content and indicates deposition in a quiet, anoxic marine setting, conditions ideal for preserving organic matter.

Roadcut exposure of the Bone Spring Limestone showing thin-bedded, organic-rich carbonate layers interbedded with calcareous shale. Note the differential erosion; cherty limestone beds protrude due to their resistance to weathering, while softer shale layers have receded.

At this same outcrop, one rock fragment revealed a fracture filled with white calcite which is a common diagenetic feature in carbonate systems. These calcite veins form when calcium-rich fluids migrate through cracks in the rock, and precipitate minerals as the fluids cool or chemically interact with the surrounding material. Their presence suggests post-depositional tectonic activity and fluid movement within the formation. Importantly, this feature contributes to secondary porosity, which plays a key role in reservoir capacity. Limestone is an excellent host rock for hydrocarbon deposits, and oil can become trapped along fractures and within the pore spaces created by these fracture-filling crystals. In formations like the Bone Spring Limestone, such diagenetic features can significantly influence the rock’s ability to store and transmit fluids, making them critical in oil and gas exploration. (King, 1948)

A hand sample collected from the Bone Spring Limestone outcrop shows a fracture filled with white calcite.

The final stop before reaching our hotel in Carlsbad was the El Capitan viewpoint, offering a breathtaking look at the ancient, buried reef system that defines the Guadalupe Mountains. From this vantage point, the scale and geological significance of the Capitan Reef become strikingly clear as its towering limestone cliffs are remnants of a thriving Permian marine ecosystem. This marks the end of day one, but the journey continues. In the next post, I’ll explore the formations of the Guadalupe Mountains in more detail, including highlights from day two of the trip along the Permian Trail, where the story of deep time and reef evolution unfolds even further.

Picture from El Capitan viewpoint

Day Two: Following the Permian Trail — From Reef Limestone to Tufa Springs

 

Reference map for day two stops. Basemap sources: National Geographic Society and i-cubed (2013); Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, and the GIS User Community. Additional data: PermianBasin_Boundary_Structural_Tectonic. Map created by Brittney A. Hawley in ArcGIS Pro 

Background information: before we begin with the stops along the trail, it is important to cover the different formations of the Guadalupe mountains. During the time of the Permian Sea, differing ocean depths, reef cliffs with steep slopes, and shallow back-reef areas all contributed to different stratigraphic formations. 

Visual of the different stratigraphic formations of the Guadalupe mountains

(Scholle et al 2004)

Let’s start with the oldest layer: the Bone Spring Limestone. We covered this in the first post, but here’s a recap. It formed in a deep marine basin, composed of organic-rich limestone and turbidites. Think of it as the deep-sea foundation of the region, laid down long before reefs began to grow.

Next are the Victorio Peak and Cutoff formations, which sit in the transition zone between the slope and the basin. These thin-bedded limestone, chert, and siltstone layers mark the onset of slope deposition. This is where the ocean floor began to rise, setting the stage for reef-building carbonates.

Following these are the Cherry Canyon, Brushy Canyon, and Bell Canyon formations. These sandstone-rich layers, interbedded with siltstones and limestones, were deposited by submarine fans, which are essentially underwater river deltas spreading sediments farther into the basin. They record the gradual expansion of the depositional environment as the basin filled.

Probably the most famous formation comes next: the Capitan formation. This massive, horseshoe-shaped barrier reef is composed of thick limestone and dolomite, dotted with fossils. It acted like a natural seawall, separating the open ocean from the restricted Delaware Basin. Before it, the Goat Seep Dolomite formed as the early reef precursor. This dolomitized limestone captures the first hints of reef-building activity.

Above the reef, calmer, shallow waters allowed back-reef carbonates to accumulate, forming the Artesia Group (shown in pink in the figure above). These layers include evaporites, dolostones, and sandstones. Think of these as Permian lagoons and tidal flats forming atop and behind the reef.

Finally, we reach the Castile and Salado formations. The Castile formation is the first stop for this day, so we’ll cover it in detail there. The Salado Formation formed in a highly restricted, evaporitic basin, marking the final stage of the Permian Sea’s closure. Thick beds of halite and gypsum here record extreme evaporation conditions. (Scholle 2000)   

Day Three: Into the Depths – Carlsbad Cavern, Parks Ranch Cave System, and Sitting Bull falls

 

Reference map for day three stops. Basemap sources: National Geographic Society and i-cubed (2013); Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, and the GIS User Community. Additional data: PermianBasin_Boundary_Structural_Tectonic. Map created by Brittney A. Hawley in ArcGIS Pro 

First Stop: Carlsbad Cavern 

Descending into Carlsbad Cavern feels like stepping onto another planet. The air grows cooler, the world above fades away, and you're enveloped by a subterranean realm of breathtaking scale and beauty. As someone who hasn't spent much time in caves, everything about this place was spectacular and awe-inspiring. We spent half the day exploring the cavern, and I could have easily spent the rest of the day wandering its passageways. There are countless fascinating geological features, each with its own story.

A closer look at Carlsbad Cavern’s wonders: here, stalagmites rising from the floor meet stalactites hanging from the ceiling, forming striking columns. Popcorn-like calcite textures cover the surfaces, adding a delicate, intricate detail to the formation.

The cave's history is rich and complex. It began with limestone deposition from Capitan Reef, and over time, water moving through folds and fractures dissolved the rock, carving out enormous caverns. Caves develop unique microenvironments, and within these spaces, different geological formations form depending on factors like dissolution, carbon dioxide degassing, convection currents, wind direction, water chemistry, and more.

Most caves form through the action of carbonic acid, but Carlsbad is unique: its passages were carved primarily by sulfuric acid. Hydrogen sulfide from nearby oil and gas deposits combined with groundwater and rainwater to produce this strong acid, which traveled along fracture planes, dissolving the limestone more aggressively than carbonic acid could. This process is why Carlsbad Cavern is so massive and also why it hosts impressive gypsum deposits. (National Park Service, 2025) Chemically: 

H₂SO₄ (Sulfuric acid) + CaCO₃ (Calcium carbonate/limestone) → CaSO₄·2H₂O (Gypsum) + CO₂ (Carbon dioxide)

Carlsbad Cavern is a vast network of over 100 limestone caves and passageways. One of the most remarkable features is the Big Room, the largest accessible cave chamber in North America. It measures approximately 4,000 feet long, 625 feet wide, and 255 feet high at its tallest point, and the floor space spans about 8.2 acres (Earthdate, 2023). 

I’ll take you on a geologic overview, sharing photos I took along the walk to the Big Room, along with geological context and history.

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Before even entering the cavern, take some time to explore the outcrops around the parking lot. Here, you can see an interesting teepee structure, a secondary diagenetic feature formed when water pushed upward, causing the limestone layers to buckle and fold. You’ll also notice small spherical carbonate grains called pisoids embedded in the rock. These form in shallow marine environments, where concentric layers of calcium carbonate build up around a nucleus, such as a shell fragment or a grain of sand. (Burger, 2007, p. 7)

 

A teepee structure in the limestone outcrop outside
 Carlsbad Caverns Visitor Center. Water pushed upward,
 buckling the layers and leaving this pseudo-anticline.

Spherical concentric concreting layers surrounding a nucleus, called a pisoid. These are about one cm in diameter 

The descent into the cave follows steep switchbacks. Once inside, take a moment to notice the smooth surface of the ceiling. This flatness is due to the Tansill Formation, which contains numerous clay layers interbedded with siltstone and dolomite. Over time, the weaker clay layers give way more easily than the surrounding rock, producing the cave’s smooth surfaces. In some areas, you can even see mud cracks preserved in the mudstone layers of the ceiling, which are polygonal patterns formed when the ancient lagoon environment repeatedly wet and dried. Shining your flashlight along the walls, you’ll also spot small gypsum deposits glinting in the light. (Burger, 2007, p. 8-9)

The natural entrance to Carlsbad Cavern cuts through the Capitan Reef limestone, exposing stratified Tansill Formation layers. 

A look at the cave ceiling showing polygonal mud cracks

Walking toward the Big Room, you’ll notice that many of the stalactites are asymmetrical. The mineral buildup occurs on the side facing the incoming airflow, the upwind side. This happens because moving air enhances carbon dioxide degassing from the thin film of water on the stalactite’s surface, which in turn promotes calcite precipitation and gradual growth in that direction. 

Asymmetrical stalacites forming on the upwind side

Along the trail, you’ll encounter a feature known as boneyard, a highly eroded limestone surface that resembles a honeycomb or sponge. This texture formed through sulfuric acid dissolution when hydrogen sulfide rising from deeper reservoirs mixed with groundwater. The resulting sulfuric acid aggressively dissolved the limestone of the Capitan Reef, enlarging fractures and bedding planes. Over time, this process created the irregular cavities and sharp ridges characteristic of the boneyard, representing one of the earliest stages of cave formation in Carlsbad Cavern. (Burger, 2007, p. 22)

Intricate honeycomb structure with many dissolved passages called boneyard

In the Big Room, one of the most eye-catching formations is the Lion’s Tail, named for its long, slender shape capped with knobby cave popcorn. This feature forms through convective air currents circulating within the cavern. The movement of air within the cave is driven by temperature and density differences between the large cave entrance and the underground chambers.  These convection currents move moisture and dissolved carbon dioxide throughout the cavern, influencing where calcite dissolves and re-precipitates. As the dry air moves down, it increases evaporation and allows the precipitation of aragonite and calcite popcorn. The warm air moving up and out of the cave carries moisture, and dissolves aragonite and calite. This is why you'll see a line on features where minerals have precipitated on the bottom, but the top will be smooth. (Burger, 2007, p. 24)  

Lions tail formation characterized by it smooth surface at the top and cave popcorn texture at the bottom

A few more interesting features include stone lily pads and pool fingers. Stony lily pads are a special type of shelf stone that reflects water levels in the cave pools. When the caves were dry, stalagmites would form on the ground surface. Then, water entered, and floating calcite crystals attached to the stalagmites as the water rose, creating these lily pad structures. Pool fingers are interesting because they have evidence of fossil bacteria. These are slender calcite structures that can be up to 30 cm in length and about 1.5-6 mm in diameter, and grew vertically under shelf stones in water. It is believed that bacteria contributed to the formation of these structures through microbial mediation or passive processes, and is a subject of much scientific debate. (Melim et al., 2001) 

Stone lily pads in an ancient cave pool — calcite shelves that formed as rising water levels allowed floating crystals to attach to submerged stalagmites. These structures record changes in past water levels within Carlsbad Cavern

Slender calcite pool fingers along the shelf stones of a former cave pool. These tube-like formations, up to 30 cm long, likely formed through microbially mediated precipitation of calcium carbonate within still, mineral-rich water (Melim et al., 2001)

These are just a handful of the incredible formations found in Carlsbad Cavern. Before your visit, I highly recommend reading about how these features formed. You’ll recognize them much more easily once you’re underground. On your way out of the cave via the elevator passage, take a moment to look closely at the surrounding walls. If you’re lucky, you might even spot fossil remains, like the trilobite our group found near the exit.

Fossilized trilobite embedded in the limestone wall near the Carlsbad Cavern elevator exit. These marine arthropods lived in the shallow seas that once covered this region during the late Paleozoic era

Day Four: Tracing the Rift – Bottomless Lakes, the Rio Grande, and White Sands

 

Reference map for day four stops. Basemap sources: National Geographic Society and i-cubed (2013); Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, and the GIS User Community. Additional data: PermianBasin_Boundary_Structural_Tectonic. Map created by Brittney A. Hawley in ArcGIS Pro 

Stop 1: Bottomless Lakes State Park

Our last day of the journey begins with Bottomless Lakes State Park. The park sits on the eastern edge of the Pecos River Valley, about 14 miles southeast of Roswell, and includes most of a chain of nine lakes formed through karst processes in the Seven Rivers Formation of the Artesia Group. The lakes were named for their bottomless appearance and for the local cowboys who, according to legend, tied their ropes together yet still couldn’t find the bottom  (Mclemore, 1999; NMBGMR, 2025).

Spring-fed lake at Bottomless Lakes State Park, formed by karst processes in the Pecos River Valley

Each lake occupies a collapse sinkhole created when groundwater dissolved underlying gypsum and limestone, causing the overlying rock to cave in. Where these depressions intersect the water table, spring-fed basins formed, their deep blue and turquoise hues reflecting both depth and mineral content . The surrounding cliffs expose alternating layers of dolomite, gypsum, and red siltstone, showing that these lakes developed within ancient back-reef and sabkha deposits tied to the Permian reef complex farther south (McLemore, 1999).

Even today, the system remains dynamic: groundwater continues to circulate through the subsurface, while high evaporation rates concentrate minerals within the lakes. Bottomless Lakes offers a vivid example of how ancient depositional environments and modern hydrologic processes intersect to create New Mexico’s distinctive karst landscapes.

Stop two: Rio Grande Rift 

On our way toward White Sands, we made a brief stop overlooking part of the Rio Grande Rift, a major continental feature that stretches from southern Colorado through New Mexico into northern Mexico. The rift marks an area where the Earth’s crust is being pulled apart, forming a series of fault-bounded basins separated by uplifted mountain blocks. Extension began around 30 million years ago and continues at a slow rate today, shaping the valleys that now host the Rio Grande River. (USGS, 2023)

From our stop, the broad, flat valley floor and distant fault-scarred uplands offered a clear view of this active tectonic landscape. Although we only paused long enough for photos, the rift provides an important link between the region’s tectonic evolution and surface processes, influencing drainage patterns, groundwater flow, and even the location of volcanic activity throughout central New Mexico. 

View across the Rio Grande Rift toward the Tularosa Basin. The white flats visible at the base of the mountains are the Alkali Flat of White Sands