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 🚙 US-14A Mileage 8.2      GPS Coordinates    N 44.23.795  W 103.54.011     4,786ft.
Molten The Monster Update.pdfLesson Guide: PRINT
Lesson Guide: PRINT
Its life began as molten mushÂ
Which rose up from the deep
To squeeze into the upper crustÂ
As rock it now doth sleep
Field Exploration
Field Exploration
Magma . . . the stuff of volcanoes, high temperature and the "underworld." During this geologic exploration you'll be taken on a short hike to an igneous intrusion where you'll see solidified magma in the flesh, touch its crumbly surface and investigate its role in the geologic history of Spearfish Canyon. It's a rare opportunity to experience a body of rock that originated deep within the Earth.
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.
Igneous Intrusion
Located along the right side of the highway is a body of solidified magma that squeezed upward and in-between two older rock layers eons ago. Such a body is called an Igneous Intrusion. EXERCISE EXTREME CAUTION along the highway.
Homestake Hydro Electric Plant NO.2Â Â Â
From the parking lot adjacent to the hydro plant the lesson site is just a short walk to the south along the highway. The lesson site is close to a blind corner so be sure to stay clear of the highway at all times. It is not recommended that you bring children but if you choose to do so be sure to hold their hand.
 EXERCISE EXTREME CAUTION along the highway.
EXERCISE EXTREME CAUTION along the highway.
A short walk down the shoulder of the highway will bring you to this vantage point where two intrusions are readily visible. The intrusions are composed of an igneous rock called phonolite. The Icebox Member of the Winnipeg Formation, is the dark greenish-grey shale that's sandwiched between these two units of solidified magma. During Eocene Time, magma rose from the depths to squeeze in-between the underlying Deadwood Formation (not visible) and the overlying Icebox Member (dark grey) with a second intrusion positioned above the Icebox Shale as well. When magma is injected between and parallel to pre-existing rock layers, the resulting intrusion is called a sill. The Icebox and Deadwood Formations, then, must be older than the sill itself for they had to be present prior to the injection of the magma. It is likely that the two parallel sills coalesce at depth into one larger magmatic body. Â Note the position of the Icebox and the underlying Deadwood Formations in the Stratigraphic column below to reference the location of our roadside intrusions.Â
A short walk down the shoulder of the highway will bring you to this vantage point where two intrusions are readily visible. The intrusions are composed of an igneous rock called phonolite. The Icebox Member of the Winnipeg Formation, is the dark greenish-grey shale that's sandwiched between these two units of solidified magma. During Eocene Time, magma rose from the depths to squeeze in-between the underlying Deadwood Formation (not visible) and the overlying Icebox Member (dark grey) with a second intrusion positioned above the Icebox Shale as well. When magma is injected between and parallel to pre-existing rock layers, the resulting intrusion is called a sill. The Icebox and Deadwood Formations, then, must be older than the sill itself for they had to be present prior to the injection of the magma. It is likely that the two parallel sills coalesce at depth into one larger magmatic body. Â Note the position of the Icebox and the underlying Deadwood Formations in the Stratigraphic column below to reference the location of our roadside intrusions.Â
Igneous Intrusion (Sill) Outcrop.
Igneous Intrusion (Sill) Outcrop.
The ribbed appearance of this exposure is due to a process called flow-sheeting. Simply stated, when magma is injected into an existing body of rock it does so in as a series of pulses instead of as one single event. Each influx adds to the intrusion "sheet-by-sheet" until, eventually, the intrusive activity comes to an end . Â
In this close-up view of the intrusion, note the blocky and fractured nature of the outcrop and the general absence of horizontal layers which are prevalent in sedimentary rocks. Sets of vertical, horizontal or diagonal cracks are called  joints and are common in igneous rocks which helps to identify them.
The northern Black Hills is host to numerous igneous intrusions most of which are roughly the same age. The graphic depiction of these intrusion are generalized above in orange but do not include all possibilities. Â Age and thickness are approximates.
Be sure to download our Onsite Geology Lesson PDF posted above for a detailed geologic investigation of this site.Â
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