The Old Man Of The Sea: The Deadwood Formation

🚙 US-14A Mileage 6.0 GPS Coordinates N 44.24.971 W 103.52.867 4,281 ft.

Old Man of the Sea Update2.pdf

Lesson Guide: PRINT

The oldest layer in the Hills

It truly looks the part

But filled with curiosities

That make it stand apart

Field Exploration

The Deadwood Formation . . . one of the most intriguing layers of sedimentary rock in the world! In this roadside exploration you'll learn about the Cambrian/Ordovician Sea, search for unusual sedimentary structures, and ponder a time so ancient that animals had not yet inhabited the land.

This pdf Lesson Guide contains the specific directions for reaching our featured site as well as a detailed explanation of its geologic story. Mileage and coordinates are provided in each document and roadmaps and other learning aids can be found at the Lesson Resources link in the sidebar menu. The document is most useful if taken with you to the lesson site. Exercise extreme caution at the lesson site and along roadways.

Are you ready. . . . Let's Go! S. V. Fogarty & W. R. Stevens

*Mileage starts at the intersection Colorado Bvd. and US 14A near Spearfish.

Deadwood Formation

The Deadwood Formation is Cambrian in age and is composed of sandstone, shale, limestone and some basal conglomerate. This exposure is directly across from the Bridal Veil viewing platform. Many of the weathered Deadwood fragments that have accumulated at the base of the outcrop in the photo above contain a wide variety of unique and interesting sedimentary features. Use extreme caution along the highway.

The Deadwood Formation is one of the thicker rock units in the canyon and also one of the most variable in terms of rock type. Age and thickness are approximates.

Deadwood Bedding

The Deadwood consists of an assorted mix of sediments. Note the variability in the thickness of each layer. An outcrop displaying this character is often described as "flaggy." See the Stratigraphic Column below for the position of the Deadwood Formation within the stack of rock layers exposed here in Spearfish Canyon.

Deadwood Outcrop

A magnificent roadside exposure of the Deadwood Formation. The Deadwood is primarily Cambrian in age so it represents the oldest sedimentary rock in the Black Hills. Like the weathered and wrinkled skin of an old man, it even looks like it's been around for a long time! Maximum thickness for the Deadwood in the canyon is roughly 400 ft. See the Stratigraphic Column above for the position of the Deadwood Formation relative to the other sedimentary formations exposed in the canyon.

Load Casts

These irregular and unusual rocky surfaces form when layers of sand or silt are deposited on top of beds of mud. As the overlying sediments settle downward into the mud an irregular surface forms on the sole or underside of the sandy, overlying layers.

Bioturbation Structures

Organisms living on the shallow seafloor often left tracks, trials, burrows and other marking behind from their activities and movements in the bottom sediments. These markings are often preserved in the sediemnt when it later hardens into stone. Such examples can be found here in the Deadwood Formation.

Glauconite Mineralization

Glauconite is an iron-rich, blue-green mineral that encrusts many of the Deadwood rock fragments. It commonly forms in shallow marine environments thus providing a clue as to its origin.

Be sure to download our Onsite Geology Lesson PDF posted above for a detailed geologic investigation of this site.

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