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

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 Sea, search for unusual sedimentary structures, and ponder a time so ancient that animals had not yet inhabited the land. 

This pdf document contains the specific directions for reaching our featured Lesson Site as well as a detailed explanation of its geologic story. Mileage to the site as well as its specific coordinates are listed on the header above and also in the lesson document. Roadmaps and other learning aids can be found at the Lesson Resources link in the menu and are helpful to have handy in the field. Please print this document and take it with you or read it thoroughly before departing. 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 contain a wide variety of unique and interesting sedimentary features. Use extreme caution along the highway.

Deadwood Outcrop

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.

Spearfish Stratigraphic Column

Note the position of the Deadwood Formation. Age and thickness of geologic formations are approximate.

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.

Glauconite

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.

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. 

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

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