The Geologic Story of Glacier National Park

by James L. Dyson

DIORITE SILL.

Few persons visit the park without noticing the pronounced black layer, within the Siyeh formation, present on many of the high peaks. It is most in evidence on the face of the Garden Wall viewed from the vicinity of Many Glacier Hotel, although it is plainly visible also in Mount Wilbur and the wall above Iceberg Lake. Passengers on the Waterton Lake launch can see it cutting across the stupendous north face of Mount Cleveland. From Going-to-the-Sun Highway it can be seen on Mahtotopa, Little Chief, Citadel, Piegan, and Going-to-the-Sun Mountains, and on the west side of the Garden Wall, where it also forms the cap of Haystack Butte. It is everywhere about 100 feet thick, and thus can be used as a very accurate scale for determining the height of mountains on which it is discernible.

This imposing layer of rock, unlike the lava, never reached the surface in a molten state, but was intruded between beds of sedimentary rock and thus became a sill instead of a flow. We need only a glance to determine its intrusive nature. Wherever it occurs it is bordered at top and bottom by thinner gray layers. These are Siyeh limestone which was changed to marble by the tremendous heat of the diorite during its intrusion. This effect is termed contact metamorphism by geologists. Because this contact-metamorphosed zone is at both top and bottom of the sill we know the latter was intruded into the adjacent rocks. Lava flows, even though covered later by sediments, of course alter only the underlying rocks.

THE GARDEN WALL AND GRINNELL GLACIER. THE WALL IS COMPOSED OF SIYEH LIMESTONE ABOVE THE LEVEL OF THE GLACIER AND THE GRINNELL FORMATION BELOW IT. (DYSON PHOTO)

The sill can readily be examined in a number of places where trails cross it, notably at Swiftcurrent and Piegan Passes, and north of Granite Park near Ahern Pass. But nowhere is it as accessible as on Logan Pass. It lies beneath the parking lot at a depth of only a few feet, and is exposed on both sides of the pass. To inspect it, one need walk only about 200 yards along the trail leading to Granite Park. In a distance of less than 100 feet the trail traverses from fresh Siyeh limestone across the entire altered (contact-metamorphosed) zone, here 12 to 20 feet wide, into the center of the sill. All parts of the sill and adjacent rocks can readily be examined and studied in detail at this site.

TOP OF THE DIORITE SILL OF BLACKFOOT GLACIER. THE MAN IS STANDING ON THE SILL. LIGHT ROCK OVERLYING SILL IS CONTACT-METAMORPHOSED SIYEH LIMESTONE. (DYSON PHOTO)

A number of dikes[3] of Belt age, some of which undoubtedly were feeders to the sill and flows, cut vertically up through the sedimentary formations. Some of the dikes are less resistant to weathering and erosion than the rocks surrounding them; consequently their more rapid removal results in the formation of narrow vertical chimneys or recesses which appear as snow-filled chutes on the mountainsides in spring and early summer. Such a feature almost invariably indicates the presence of a dike. From Many Glacier Hotel one of these can be seen on the red mountain in front of Mount Wilbur. Another, 1,500 feet high, transects the Pinnacle Wall at the outlet of Iceberg Lake. The dike which forms this impressive chute is less than thirty feet wide. Though not so conspicuous as the sills some of these dikes are of interest because they contain various ore minerals, principally copper, which today form small deposits along their borders. About the beginning of the century these were responsible for a short-lived mining boom, the best known vestige of which is the remains of the mill at Cracker Lake. The old Cracker Mine, with entrance now caved in, was driven along a dike which has a width of over 100 feet.

From the boat landing at the head of Josephine Lake the dump of another mine appears as a tiny gray-green mound on a narrow shelf high on the precipitous wall of Grinnell Point. Like the Cracker Mine this one was dug along the edge of a similar but smaller dike. All these deposits are insignificant in size and of no commercial value. Had they been important this great area might never have been set aside as a national park.