These beds have the same bright red color as those of the Grinnell. However, because they are the youngest rocks of the Belt series they outcrop only on a few mountaintops, and inasmuch as these are mainly in the northwest part of the park, comparatively few people have noticed this formation. Visitors to Cameron Lake in Waterton Lakes National Park can see it in the red north wall of Mount Custer. The mountains around colorful Boulder Pass and Hole-in-the-Wall Basin are likewise composed of it.
Within the rocks of this formation there is a great abundance of small cubes believed to be casts of salt crystals which formed when the sediments were accumulating. Their presence indicates an arid climate and intensive evaporation of the sea, similar to the condition at Great Salt Lake today.
Igneous Rocks of the Belt Series
Not all of Glacier Park’s rocks accumulated slowly and quietly as sediment in a body of water. At many places, interbedded with and cutting across the sediments, there are bodies of igneous rock which reached their present position in the form of hot molten material forced up from deep within the crust.
COLUMNAR SECTION OF BELT ROCKS
| PROTEROZOIC | ||
|---|---|---|
| KINTLA | 860′+ | Red argillite |
| SHEPARD | 600′ | Buff limestone |
| PURCELL | 250′ | Black lava |
| SIYEH | 4000′ | Dark diorite bordered by white altered limestone |
| Blue limestone. Weathers buff | ||
| GRINNELL | 1600-3000′ | Red argillite and white quartzite |
| APPEKUNNY | 2500′± | Green argillite. Some white quartzite |
| ALTYN | 2300′± | Gray limestone. Weathers buff |
PURCELL LAVA.
Soon after the youngest layers of Siyeh limestone had accumulated on the floor of the sea and while they were still under water, a mass of molten rock was squeezed up from far below and extruded in the form of a submarine lava flow over the recently accumulated sediments. Several times this lava poured out forming a total thickness varying between 50 and 275 feet. One of the best exposures is on the west side of Swiftcurrent Pass and in Granite Park just west and northwest of the chalet. In fact it is this lava flow which gives the name, albeit wrongly, to Granite Park. The material of the flow is very fine-grained and dark (basic), in contrast to the light color and coarse grain of granite. Nonetheless, many prospectors are wont to call every igneous rock, regardless of its composition, a granite. A number of ellipsoidal structures (“pillows”) up to two feet in diameter within this lava indicate that it was extruded under water. The Purcell is thickest in the vicinity of Boulder Pass, where the trail traverses its ropy and stringy surface for a distance of several hundred yards.
Later, after the Shepard and part of the Kintla formation were laid down on top of the Purcell, another similar flow spread over the sea floor.