The Helmet
September 11, 2018
Photographer: Rod Benson
Summary Author: Rod Benson
The photo above shows an unusual peak called The Helmet, found in the Madison Range of southern Montana. It was taken as friends and I descended from a neighboring peak called Sphinx Mountain. The Helmet (9,449 ft or 2,880 m) is so-named because it resembles the comb on a Spartan’s helmet -- Sphinx Mountain looks like Egypt’s famous Sphinx when viewed from the north. Besides their unusual shapes, the two peaks share another distinctive feature; both are made of conglomerate, a sedimentary rock (the sediment is gravel).
It's odd to find a whole mountain made of layer upon layer of conglomerate, but that's the case with Sphinx Mountain and The Helmet. Both mountains are composed entirely of thick layers of limestone conglomerate – pebbles, cobbles, and boulders of limestone embedded in a reddish sandstone matrix. Altogether, the beds of conglomerate are over 2,000 ft thick (607 m).
It's believed that the gravel here was deposited here during the Eocene period (56 – 34 mya), when the area was a basin. This basin, which was probably much more extensive during the Eocene, presently occupies an area of only 2 sq mi (5.2 sq km) -- and it’s not a basin anymore. Over millions of years the layers of gravel became stone, and then were pushed up as the Rockies formed. Now Sphinx Mountain (10,876 ft or 3,315 m), one of Montana’s most iconic peaks, stands as a remnant of this gravelly basin. The Sphinx conglomerate is found only on Sphinx Mountain and The Helmet. It was first identified by geologist A. C. Peale during geological surveys of the region in the 1880s. To see more photos of Sphinx Mountain, The Helmet, and the Sphinx conglomerate, click here. Photo taken on July 23, 2018.
Photo Details: Camera: Apple iPhone 8 Plus; Exposure Time: 0.0005s (1/1845); Aperture: ƒ/1.8; ISO equivalent: 20; Focal Length (35mm): 28.