Because of their dominantly red color, the shaly argillites which
comprise the bulk of this formation are the most conspicuous rocks in
the park. They lie immediately on top of the Appekunny and although
their thickness varies considerably it is greater than 3,000 feet in
several localities. Interbedded with the red argillites are thin white
layers of quartzite, a former sandstone which has been converted by
pressure into an extraordinarily hard, dense rock. Mud cracks, ripple
and current marks, raindrop impressions, and other features made while
the sediments were accumulating are common. The red color is due to
abundant iron oxide occurring mainly as a cement between the sand and
mud grains. All the rocks of Glacier Park contain some iron, or rather
contain iron-bearing minerals. These minerals have various colors unless
they have been oxidized, in which case the color is red or brown.
Oxidation of the Grinnell formation probably took place while the mud
was accumulating and during those periods when it was exposed to the
atmosphere. At such times also the mud dried and cracked, the marks of
which are so prominent on the surfaces of the layers today.
The Grinnell formation seems to be everywhere. In the Many Glacier
region it comprises the bulk of Grinnell Point, Altyn Peak, and Mount
Allen, and is no less striking in the bases of Mount Wilbur and the
Garden Wall. Ptarmigan Tunnel is drilled through it, and the trails to
Grinnell Glacier, Cracker and Iceberg Lakes cross it. Redrock Falls, on
the trail to Swiftcurrent Pass, and Ptarmigan Falls on the Iceberg Lake
trail drop over several of its highly colored layers.
From the Blackfeet Highway on top of Two Medicine Ridge one can see the
dark red rocks of this formation capping the summits of Rising Wolf and
Red Mountains. Even from the valley floor it is just as noticeable.
Sinopah Mountain standing alone and impressive across the lake from Two
Medicine Chalets carries the red banner of the Grinnell formation.
These red rocks constitute an important scenic feature for many miles
along Going-to-the-Sun Highway. If one begins his trip on this highway
at its east entrance he soon finds himself in the midst of a group of
imposing red peaks--Goat and Going-to-the-Sun on the right, Red Eagle
and Mahtotopa on the left. The road crosses the formation along a mile
and a half stretch just west of Baring Creek bridge. Innumerable loose
slabs of red rock along the side of the road contain excellent mud
cracks and ripple marks. Near Avalanche creek on the west side of Logan
Pass the highway crosses the Grinnell where it comes to the surface on
the western limb of the big syncline.
The formation is well exposed in the vicinity of Sperry Chalet and
Glacier. It forms all the mountains surrounding the basin in which the
chalet is located, and the trail from chalet to glacier lies wholly on
it. At the glacier intensely folded white quartzite layers and red
argillites are very conspicuous.
The visitor can readily trace the Grinnell from place to place
throughout the entire park area, and can thus easily visualize that it
as well as all other formations at one time filled the intervening
spaces between the mountains. (See color of cover pages.)