Mold, Casts, and Steinkerns

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Clockwise from upper left: Mold of a brachiopod, Katahdin Woods and Waters National Monument. Nautiloid steinkern, Mississippi National River and Recreation Area. Lava tree mold, Puʻuhonua O Hōnaunau National Historical Park. Cast of a bird track, Death Valley National Park.

NPS images.

Introduction

The formation of natural molds and casts of organisms, especially of their hard parts, is among the most frequently occurring and diverse fossil preservation processes in the rock record. Mold and cast fossils exist of all the major types of fossil organisms (invertebrates, vertebrates, and plants). Many trace fossils, particularly tracks and burrows, are also either molds or casts.

The formation of molds and casts is a particularly common fossilization process for invertebrates. Molds form around shells and other hard parts when they make three-dimensional impressions in the surrounding sediment. A cast may form if the shell is dissolved and its imprint is filled in with sediment or other minerals. Internal casts of the interior shape of a shell, also called steinkerns, form when internal cavities of shells are infilled. If a shell is later dissolved, a steinkern may be the only evidence left of the fossil.

Molds and casts may also form of vertebrates and plants, although they occur less commonly than other fossilization processes for these types of organisms. Vertebrate and plant fossils are more often formed via permineralization and the making of impressions and compressions.

Trace fossils are also frequently preserved as molds and casts. Trace fossils consist of the evidence of living organisms, but not of the actual plant, animal, or microorganism itself. They typically record behaviors of animals as they are interacting with their environments.

Molds and casts naturally go together. Sometimes both molds and casts of a body fossil or a trace fossil are preserved, creating both a positive and negative record of the shape of a shell or other hard part of an organism, or of a track.

Like other fossilization processes, the formation of molds, casts, and steinkerns may occur in conjunction with other fossilization processes. For example, the minerals in them and/or the surrounding rock may also be replaced or recrystallized.

Definitions

Mold

A three-dimensional impression of all or part of a body fossil or trace fossil. Molds most commonly form around hard parts of animals, particularly of invertebrates, although molds are also known of vertebrate animals, plants, and trace fossils.

Internal Mold

A mold of the inner surface of a fossil shell or other hard part of an organism.

External Mold

A mold of the outside surface of fossil shell or other hard part of an organism.

Mold of a horn coral, Toroweap Formation. Grand Canyon-Parashant National Monument.

NPS photo.

Cast

A replica of an organism or a trace such as a footprint or burrow produced by the infilling of a natural mold. A cast shows the external details of a shell or other hard part. A cast may also form between an internal and external mold if the original shell material dissolves.

Casts of mollusks from Mississippi National River and Recreation Area. Photo by Justin Tweet.

Image courtesy of courtesy of the Cincinnati Museum Center.

Steinkern

A stony three-dimensional cast of the interior of a shell or other hollow organic structure made by sediment infilling prior to the shell being dissolved. Steinkerns may be found within an external mold when they are still in situ within rock.

Steinkern of a bivalve. Cliffhouse Sandstone. Chaco Culture National Historical Park.

NPS photo.

National Park Examples

Many national park areas contain molds, casts, and/or steinkerns of the following types of fossil organisms:

• Invertebrates

• Vertebrates

• Plants

• Trace fossils

Invertebrate Molds, Casts, and Steinkerns

Molds, casts, and steinkerns of invertebrate animals are among the most common types of fossils found in national parks. They are most often found in sedimentary rocks that were deposited in marine environments.

Delaware Water Gap National Recreation Area

Delaware Water Gap National Recreation Area contains several Late Paleozoic formations that preserve a variety of invertebrate fossils including brachiopods, crinoids, and bivalves.

Cast fossils of brachiopods. Delaware Water Gap National Recreation Area.

NPS photo.

Learn More

• Delaware Water Gap NRA—The Delaware Water Gap: A Window Into Earth’s Early Oceans

Devils Tower National Monument

Molds, casts, and steinkerns of bivalves are present in the Redwater Shale Member of the Sundance Formation, the most fossiliferous unit in the monument. It was deposited in an open marine environment in the Jurassic between 167 and 160 million years ago.

^{Aspects of bivalve preservation and exposure in the Redwater Shale Member at Devils Tower. A. Bivalve steinkerns (internal casts) beginning to erode from a block. B. Two depressions left of the scale bar may represent large bivalves that have eroded from the block. C. Bivalve molds exposed in cross-section. D. Bivalves are exposed in planar section in this block.
NPS photo by Justin Tweet.}

Learn More

• Park Paleontology News—Paleontology of Devils Tower National Monument

Guadalupe Mountains National Park

The Capitan Limestone in Guadalupe Mountains National Park contains abundant fossils of brachiopods, nautiloids and other mollusks, crinoids, and other invertebrates, many of which are preserved as molds or casts. The Capitan Limestone is one of the best preserved reefs in the geologic record. It consists of reef complex in the Delaware Basin between 277 and 275 million years ago and is one of the most important geologic records for the middle part (Guadalupian Epoch) of the Permian Period.

External mold of a nautiloid. Capitan Limestone, Guadalupe Mountains National Park.

Identification Guide to the Fossils of the Guadalupe Mountains by  Mary Carol Coleman and Cameron Coleman. http://npshistory.com/publications/gumo/fossil-guide.pdf.

Learn More

• Guadalupe Mountains NP—Geologic Formations

Katahdin Woods and Waters National Monument

The well-preserved late Silurian and Early Devonian fossils found in Katahdin Woods and Waters National Monument were specifically mentioned in the proclamation that established the national monument in 2016. Many of these fossils are molds. Molds of crinoid columns appear as distinctive ring-like impressions and some molds of brachiopods have a distinctive orange color.

Crinoid mold fossils at Katahdin Woods and Waters National Monument.

NPS photo.

Learn More

• Park Paleontology News—Paleontology of Katahdin Woods and Waters National Monument

Mississippi National River and Recreation Area

Molds, casts, and steinkerns of many different types of invertebrate fossils are present in Mississippi National River and Recreation Area. Upper Cambrian and Ordovician marine units contain brachiopods, horn corals, mollusks, trilobites, and bryozoans. Many of these fossils and the surrounding bedrock have also undergone replacement by dolomite.

Dolomitized steinkern from Mississippi National River and Recreation Area.

NPS photo.

Vertebrate Molds and Casts

Overall, molds or casts of vertebrate fossils are generally rare, although they have been documented in national parks. Like other molds, they are three-dimensional impressions of an animal’s hard parts such as bone.

Salinas Pueblo Missions National Monument

The first known vertebrate fossil from Salinas Pueblo Missions National Monument was preserved as a mold in a loose block of sandstone that came from beds of the Permian Yeso Group. The fossil consists of both sides of the mold of part of the pelvis, vertebra, and parts of the lower legs of a tetrapod.

Early Permian reptile discovered in Salinas Pueblo Missions National Monument. Both sides, known as the part and counterpoint of the mold are present. Float originating from the Arroyo de Alamillo Formation.

NPS photo.

Learn More

• Park Paleontology News—Paleontology Intern Profile - Emily Thorpe

Plant Molds

The formation of molds and casts is not a typical way that plant fossils are preserved. The most common way that woody tissues are preserved is through permineralization and/or replacement. When leaves are preserved, it is usually through the making of compressions and impressions, including carbonization.

But sometimes molds of wood or leaves are found in the fossil record. A common way for plant mold fossils to form is when trees become engulfed in lava flows.

^{Sometimes molds of fossil wood may be found. This is a small mold of a woody twig from the Chinle Formation in Capitol Reef National Park, Utah.
Photo by Allyson Mathis.}

Lava Tree Molds

Overall, it is very rare to find fossils in volcanic rocks, but tree molds do occur with some regularity in some lava flows. Basaltic lava flows are fluid and can travel great distances from their vents. As they cover the landscape, they may engulf trees and even forests.Although molten lava is very hot, the surface of a flow quenches quickly when it comes into contact with a tree, making a mold of the exterior surface of the wood. The tree itself is eventually incinerated by the lava flow, leaving a hollow mold.Tree molds may either be oriented vertically when they were formed around standing trunks, or horizontally when the lava flow either encountered downed wood or knocked over standing trees.Lava trees are a distinctive type of mold standing above the surrounding lava flows. Many basaltic lava flows inflate during emplacement, meaning that their solidified surface rises as the interior of the flow continues to be supplied by lava. This solidified surface may later subside once fluid lava has drained away, leaving behind a lava tree rising above the lava flow surface.Lava tree molds and lava trees are fragile surface features on lava flows, which over time are removed through weathering and erosion. Therefore, they typically are only found on young lava flows, a few thousand years old at most.Lava tree molds are known in five units of the national park system:

• Craters of the Moon National Monument & Preserve, Idaho

• El Malpais National Monument, New Mexico

• Hawaiʻi Volcanoes National Park, Hawaiʻi

• Lava Beds National Monument, California

• Puʻuhonua O Hōnaunau National Historical Park, Hawaiʻi

Hawaiʻi Volcanoes National Park

Both lava tree molds and lava trees are present in Hawaiʻi Volcanoes National Park. The Tree Molds area near the summit of the Kīlauea shield volcano contains many tree molds and lava trees.

^{Lava Trees. Havaii Volcanoes National Park.
NPS photo.}

Learn More

• NPS—Lava Tree Mold Fossils

• NPS—Basaltic Lava Flows

• NPS—Lava Flow Surface Features

• Hawaiʻi Volcanoes—Lava Trees and Tree Molds

• Hawaiʻi Volcanoes—Tree Molds

Craters of the Moon National Monument

The 2-mile (3.2-km) round trip Tree Molds Trail in Craters of Moon National Monument leads to an area of the Blue Dragon Flow with both standing and horizontal tree molds. Many of these tree molds show fine details such as bark impressions. The Blue Dragon Flow is one of the youngest in the monument and was erupted about 2,100 years before present.

^{Impressions of fallen trees on the Tree Molds Trail. Craters Of The Moon National Monument & Preserve.}

^{NPS Photo.}

Learn More

• Craters Of The Moon—Hike the Tree Molds Trail

• Craters Of The Moon—Fossils

Trace Fossil Molds and Casts

Molds and casts of trace fossils such as tracks and burrows are found in many national park areas.

Fossil Tracks

Fossil tracks found in national parks include those of dinosaurs and other reptiles, mammals, birds, and invertebrates such as trilobites.

Capitol Reef National Park

The best-known fossil tracks in Capitol Reef National Park are swimming traces in the Early Triassic Moenkopi Formation. These traces were made when tetrapod reptiles were swimming in shallow water in a deltaic environment and their toes touched the ground. The swim smears in Capitol Reef National Park have been preserved as casts that filled in molds in soft sediment that the animals made while swimming.

Track Fossils. Capitol Reef National Park, Utah.

Learn More

• Capitol Reef—Triassic Tracks in the Moenkopi Formation

• Capitol Reef—Fossils

Death Valley National Park

An exceptionally well-preserved track site in Death Valley National Park contains molds and casts of a variety of Pliocene mammal and bird tracks that were formed on the shoreline of a lake. those of birds, horses, camels, elephants, and carnivores. These tracks are the only evidence of many of these organisms in the fossil record.

^{Mold of a small felid (cat) track in lacustrine (lake) deposits in Death Valley National Park.
NPS photo.}

Learn More

• Death Valley—Fossil Track

• Death Valley National Park Paleontology Survey [PDF]

Glen Canyon National Recreation Area

Both casts and mold of tetrapod tracks, including dinosaur tracks, are known in several Mesozoic formations in Glen Canyon National Recreation Area, including the Moenkopi Formation, Chinle Formation, Wingate Sandstone, Kayenta Formation, Navajo Sandstone, Entrada Sandstone, and Morrison Formation. Many important trackways have been discovered due to the falling water levels of Lake Powell exposing outcrops that had been under water.

^{Natural casts of large therapod tracks in the Upper Jurassic Navajo Sandstone in Glen Canyon National Recreation Area.}

Learn More

• Glen Canyon—Mesozoic Fossils

White Sands National Park

White Sands National Park contains one of the most important Pleistocene fossil track sites in the world. Between about 22,860 and 21,130 years ago, humans along with ancient camels, Columbian mammoths, Harlan’s ground sloths, and predators, left the footprints along the shoreline of Lake Otero in the Tularosa Basin. These tracks have been preserved as molds in the gypsum soils. This site has great scientific significance as previously it was thought that humans arrived in North America much later (between about 16,000 and 13,500 years before present).

^{Ice Age human footprints belonging to one adolecent and possibly female teenager. White Sands National Park, New Mexico.
NPS photo.}

Learn More

• White Sands—Fossilized Footprints

• Park Paleontology News—Much More than a Sand Box: Fossil Tracks from the Lakes of the World’s Largest Gypsum Dune Field

• White Sands—Album Fossilized Trackways

• White Sands—Fossilized Footprints Video - B roll

Fossil Burrows

Many organisms, including a variety of invertebrates, mammals, and reptiles, burrow or tunnel into the ground for habitation, for protection against predators, to feed, or for other reasons. These burrows are important records of life and can be found in many units of the National Park System.

Agate Fossil Beds National Monument

Casts of large spiral-shaped burrows are one of the premiere types of fossils known from Agate Fossil Beds National Monument. These fossils are known as Daemonelix or “Devil’s Corkscrew.” The burrows were made by dry-land beavers known as Palaeocastor.

^{A fossil exhibit at Agate Fossil Beds National Monument of a Daemonelix corkscrew.
KoalVlachos, CC BY-SA 4.0 , via Wikimedia Commons.}

Learn More

• Agate Fossil Beds—Mammal Fossils

• Park Paleontology News—Prehistoric Road Trip Helps Expose Deep Histories of some National Parks in New PBS Series

Mississippi National River and Recreation Area

A variety of fossil worm burrow casts are found in Late Cambrian and Ordovician marine units in Mississippi National River and Recreation Area. For example, Phycodes shows the behavior of a worm-like animal probing the seafloor mud for food from a central point, yielding a fossil consisting of a mop-like array of burrows.

^{Trace fossils. Mississippi National River and Recreation Area.
Photo by Justin Tweet.}

Featured Parks

• Agate Fossil Beds National Monument (AGFO), Nebraska—[AGFO Geodiversity Atlas] [AGFO Park Home] [AGFO npshistory.com]

• Capitol Reef National Park (CARE), Utah—[CARE Geodiversity Atlas] [CARE Park Home] [CARE npshistory.com]

• Chaco Culture National Historical Park (CHCU), New Mexico—[CHCU Geodiversity Atlas] [CHCU Park Home] [CHCU npshistory.com]

• Craters of the Moon National Monument and Preserve (CRMO), Idaho—[CRMO Geodiversity Atlas] [CRMO Park Home] [CRMO npshistory.com]

• Death Valley National Park (DEVA), California and Nevada—[DEVA Geodiversity Atlas] [DEVA Park Home] [DEVA npshistory.com]

• Delaware Water Gap National Recreation Area (DEWA), New Jersey and Pennsylvania—[DEWA Geodiversity Atlas] [DEWA Park Home] [DEWA npshistory.com]

• Devils Tower National Monument (DETO), Wyoming—[DETO Geodiversity Atlas] [DETO Park Home] [DETO npshistory.com]

• El Malpais National Monument (ELMA), New Mexico—[ELMA Geodiversity Atlas] [ELMA Park Home] [ELMA npshistory.com]

• Glen Canyon National Recreation Area (GLCA), Arizona and Utah—[GLCA Geodiversity Atlas] [GLCA Park Home] [GLCA npshistory.com]

• Grand Canyon-Parashant National Monument (PARA), Arizona—[PARA Geodiversity Atlas] [PARA Park Home] [PARA npshistory.com]

• Guadalupe Mountains National Park (GUMO), Texas—[GUMO Geodiversity Atlas] [GUMO Park Home] [GUMO npshistory.com]

• Hawai’i Volcanoes National Park (HAVO), Hawai’i—[HAVO Geodiversity Atlas] [HAVO Park Home] [HAVO npshistory.com]

• Katahdin Woods and Waters National Monument (KAWW), Maine—[KAWW Geodiversity Atlas] [KAWW Park Home] [KAWW npshistory.com]

• Lava Beds National Monument (LABE), California—[LABE Geodiversity Atlas] [LABE Park Home] [LABE npshistory.com]

• Mississippi National River and Recreation Area (MISS), Minnesota—[MISS Geodiversity Atlas] [MISS Park Home] [MISS npshistory.com]

• Puʻuhonua O Hōnaunau National Historical Park (PUHO), Hawaii—[PUHO Geodiversity Atlas] [PUHO Park Home] [PUHO npshistory.com]

• Salinas Pueblo Mission National Monument (SAPU), New Mexico—[SAPU Geodiversity Atlas] [SAPU Park Home] [SAPU npshistory.com]

• White Sands National Monument (WHSA), New Mexico—[WHSA Geodiversity Atlas] [WHSA Park Home] [WHSA npshistory.com]

Fossils and Paleontology