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.