That “Mountain” Has Lost It’s Top!
Posted by: 
Volcanoguy
N 43° 46.582 W 121° 31.867
10T E 618208 N 4848083
Sign at bulletin board for McGregor Memorial Viewpoint.
Waymark Code: WM1RND
Location: Oregon, United States
Date Posted: 07/02/2007
Views: 41
This sign is located on the bulletin board for McGregor Memorial Viewpoint.
Marker Name: That “Mountain” Has Lost It’s Top!
Marker Text: The mountain in the distance is Newberry Volcano. Stretching 500 square miles, Newberry has left it’s mark on Central Oregon. Beneath your feet lies pumice ash from the ancient eruption that left the mountain topless and created a giant lake once comparable in depth to Crater Lake. Newberry’s flank eruption at Lava Butte created a flow of lava which dammed up this quiet river, the Deschutes, and left this area under the waters of Lake Benham for 3000 years. Top that off with 3 to 8 feet of ash from Mt. Mazama, and you’ve got one eclectic volcanic parfait beneath your feet!
There are several errors in the content of this sign. At the location of the viewpoint if there is pumice from the caldera forming eruption it is probably at a level below the current river level. That eruption occurred about 80,000 years ago. About 75,000 years ago an eruption from Newberry’s northwest flank block the river in the Benham Falls area and the river deposited river and lake sediments that the viewpoint is located on. By 8,000 years ago the river had carved a river channel below the elevation of the river today. The eruption of Mt. Mazama (Crater Lake) then deposited the surface pumice. Finally about 7,000 years ago the eruption of Lava Butte again blocked the Deschutes River near Benham Falls forming Lake Benham which backed up into the river channel at this location and refilled the channel to a level about 20 feet above the current river level. Since 7,000 years ago the river has downcut to its present level.
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