Echoes Of A Coastal Desert: The Minnelusa Formation

🚙  US-14A Mileage 1.2                                                    GPS Coordinates  N44.28.044 W103.15.072        3,762 ft.

Echoes of Coastal Desert Update2.pdf

Lesson Guide: PRINT

Field Exploration

Are you ready for a trip back in time? During this exploration we'll take you on a short hike down the rocky Spearfish creek-bed to the ancient shoreline of a warm, salty sea – one that occupied this region of the continent eons ago. You won't get wet but you will be inspired!

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.

The trail to the lesson site begins just to the right of the kiosk and leads to Spearfish Creek.

Once you've reached Spearfish Creek, follow the stream bed to your right (NE) for about 100 yards.

DO NOT attempt to access this site when it's raining or during the spring melt. As you can see from the size of the riverbed cobbles the stream flow can be quite high during such events.

Spearfish Stratigraphic Column

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

Minnelusa Sandstone outcrop along Spearfish Creek bank

This lesson features the Minnelusa Sandstone which is exposed along north side of stream bed. The Minnelusa Formation was deposited during Pennsylvanian and Lower Permian Time some 300 to 250 million years ago. Because this is a marine deposit we know that at the time of its deposition the Black Hill's, as an uplifted mountain range, did not exist. 

Thinly-laminated snadstone of the Minnelusa Formation

Sandy sediments, when deposited in the ocean, generally accumulates layer-by-layer close to the shoreline. In this outcrop we can clearly see those thin stratifications of sand which over the eons eventually lithified (hardened) into sandstone.

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

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