Within the Siyeh there is a bed, averaging about 60 feet thick, composed almost entirely of fossil algae which apparently formed an extensive reef or biostrome on the floor of the shallow Belt Sea. The algae colonies are in the form of rounded masses up to several feet in diameter and bear a crude resemblance externally and internally to a head of lettuce or cabbage. Geologists know these algae by the genus name Collenia. Because of the rounded and smoothed surfaces on these colonies, mountain climbers frequently find the reef difficult to cross. It appears as a distinct light gray horizontal band on the east face of Mount Wilbur about midway between the base of the cliff and the peak’s summit, where it can easily be seen from Many Glacier Hotel and Swiftcurrent Camp. It is also discernible on the Pinnacle Wall above Iceberg Lake and in Mount Grinnell. The Swiftcurrent Pass trail crosses it just east of the pass, and it is also exposed along Going-to-the-Sun Highway below the big switchback on the west side of Logan Pass where attention is directed to it by a sign. Unweathered portions of the reef rock are light blue. A similar but thinner reef outcrops at Logan Pass near the start of the Hidden Lake trail. Although most of the fossil algae occur in the Siyeh they are present in the younger formations and also in the Altyn. Other than algae the only undoubted fossils of the Belt series within Glacier National Park are burrows probably made by worms. They are rare and are restricted mainly to the Siyeh formation. [Illustration: FOSSIL ALGAE IN SIYEH FORMATION, HOLE-IN-THE-WALL BASIN. (THE ROCK BOOK BY C. L. FENTON AND M. A. FENTON, DOUBLEDAY AND CO.)] At the top of the Siyeh are several hundred feet of sandy and shaly beds, mostly reddish in color, grouped by some geologists into a distinct formation known as the Spokane. At Logan Pass it is about 700 feet thick and is well exposed in the lower parts of Clements and Reynolds Mountains, and at the site of the former “Clements” Glacier. SHEPARD FORMATION. Several hundred feet of limy beds which weather yellow-brown lie on top of the Siyeh. Although named for outcrops on the cliff above Shepard Glacier (south of Stoney Indian Pass and near the site of the old Fifty-Mountain tent camp) the formation is exposed on the summit of Swiftcurrent Mountain at the head of Swiftcurrent Valley, on Reynolds and Clements Mountains near Logan Pass, and on Citadel and Almost-a-Dog, visible from Going-to-the-Sun Highway in St. Mary Valley. The formation is replete with mud cracks and ripple marks. Some rock surfaces exhibit two and three sets of the latter. KINTLA FORMATION.