From the parking lot adjacent to the hydro plant the lesson site is just a short walk to the south along the highway. EXERCISE EXTREME CAUTION along the highway.

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 .   

EXERCISE EXTREME CAUTION along the highway.

This panoramic view of the phonolite intrusion provides us with a vantage point that allows us to see a section of the Icebox Member of the Winnipeg Formation, a layer of dark greenish-grey shale situated between two Phonolite Intrusions. During Eocene Time magma rose from the depths to squeeze in-between the underlying Deadwood Formation (not visible) and the overlying Icebox Member. When magma is injected between older layers of rock 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. From this same vantage point a second sill is visible above the Icebox Shale. It is likely that the two parallel sills coalesce at depth into one larger magmatic body.