Qualla: Castilletes Formation

Castilletes Formation: Introduction

Fri, 06 Mar 2026 00:00:00 GMT

Photo credit Grundkarte Milenioscuro, Relief Alexrk2, CC BY-SA 3.0. Fourteen million years ago, the northernmost tip of what is now Colombia looked nothing like the desert that bakes there today. Shallow seas lapped into estuaries and lagoons. Rivers pushed sediment into deltas where crocodilians the size of buses lurked in turbid water. Turtles with shells broader than dinner tables navigated warm shallows alongside hundreds of species of snails, clams, and crabs. All of this vanished, of course -- but not without leaving a record. The Castilletes Formation, a 440-meter-thick stack of mudstones, siltstones, and sandstones in the Cocinetas Basin of La Guajira, Colombia, preserves one of the most detailed snapshots of Middle Miocene life in northern South America. Defined by the geologist Rollins in 1965 and named after the village of Castilletes, this formation dates to roughly 16.7 to 14.2 million years ago, spanning the Burdigalian and Langhian stages of the Neogene period.

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Image: Grundkarte Milenioscuro, Relief Alexrk2 | CC BY-SA 3.0

Castilletes Formation: Layers of a Vanished Coastline

Fri, 06 Mar 2026 00:00:00 GMT

Photo credit Llez (H. Zell)., CC BY-SA 3.0. The rocks themselves tell a story of shifting environments. The Castilletes Formation consists of fossiliferous mudstones, siltstones, and medium-grained to conglomeratic sandstones -- the kind of mixed sedimentary sequence that forms where rivers meet the sea. Geologists interpr...

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Image: Llez (H. Zell). | CC BY-SA 3.0

Castilletes Formation: When Giants Patrolled the Estuaries

Fri, 06 Mar 2026 00:00:00 GMT

Photo credit Llez (H. Zell)., CC BY-SA 3.0. The vertebrate fossils are what seize the imagination. Among the crocodilians recovered from the Castilletes Formation are specimens comparable to Purussaurus, a caiman genus whose largest members may have exceeded ten meters in length -- apex predators capable of taking almost a...

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Image: Llez (H. Zell). | CC BY-SA 3.0

Castilletes Formation: A Seabed Catalog

Fri, 06 Mar 2026 00:00:00 GMT

Photo credit Llez (H. Zell)., CC BY-SA 3.0. If the vertebrates are dramatic, the invertebrates are staggering in their sheer abundance. Hundreds of species of gastropods, bivalves, crustaceans, and other marine organisms have been identified from the formation, creating a census of Middle Miocene Caribbean life that few ot...

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Image: Llez (H. Zell). | CC BY-SA 3.0

Castilletes Formation: Reading the Deep Past in Desert Rock

Fri, 06 Mar 2026 00:00:00 GMT

Photo credit Llez (H. Zell)., CC BY-SA 3.0. Today the Cocinetas Basin sits in the arid northernmost department of La Guajira, where the Guajira Desert stretches to the Caribbean under a punishing sun. It takes an act of imagination to stand in this desiccated landscape and picture the warm, productive coastal waters that o...

Read more: Castilletes Formation on Qualla

Image: Llez (H. Zell). | CC BY-SA 3.0