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.
[Illustration: 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.
[Illustration: 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.
[Illustration: MOUNT WILBUR AND THE PINNACLE WALL VIEWED FROM MANY
GLACIER HOTEL. THE UPPER PART OF APPEKUNNY AND ALL OF THE GRINNELL AND
SIYEH FORMATIONS ARE VISIBLE. THE SNOW-FILLED CHUTE LEFT OF THE WORD
"GRINNELL" IS FORMED BY THE SAME DIKE WHICH PASSES THROUGH THE PINNACLE
WALL.
(HILEMAN PHOTO, COURTESY GLACIER PARK CO.)]