A HYPOTHETICAL SECTION NEAR THE CLOSE OF CRETACEOUS TIME.
B PRESSURE FROM WEST CREATES LARGE FOLD AND OVERTURNS IT TOWARDS EAST.
THE NEW MOUNTAINS ARE CUT DOWN BY EROSION.
C CONTINUED PRESSURE BREAKS THE STRATA AND THE LEWIS OVERTHRUST IS
FORMED. DOTTED LINE SHOWS APPROXIMATE PRESENT OUTLINE OF
SURFACE
D SECTION ACROSS THE PARK SHOWING RELATIONSHIPS OF ROCK FORMATIONS AND
THE LEWIS OVERTHRUST
(VERTICAL SCALE GREATLY EXAGGERATED)
Erosion in the eastern part of the overthrust block, in addition to
producing its crenulated edge, has left several isolated remnants
(outliers) east of the main mass of the mountains. The best known of
these is Chief Mountain situated near the northeast corner of the park
several miles west of the Chief Mountain International Highway. It is a
mass of Altyn limestone rising vertically on its east, south and north
sides for a distance of 1,500 feet. The Lewis overthrust is well exposed
all around its base. Two smaller pinnacles immediately to the west are
similar outliers, and, like Chief Mountain, were once part of the main
mass of the Lewis Range (Figure 3D and cover sketch). Divide Peak, at
the west end of Hudson Bay Divide, is another outlier. It, too, is
composed entirely of the Altyn formation.
Although the Lewis overthrust is exposed in a great number of places
very few of these are easily accessible, and at only one does a trail
provide a close approach to the actual contact between Belt and
Cretaceous rocks. The latter site lies along Roes Creek only a few
hundred yards from East Glacier Campground. Before reaching the fault at
the base of a high cliff of Altyn limestone, the trail crosses several
outcrops of Cretaceous sandstone replete with fossil pelecypods (clams)
and gastropods (snails). The fault surface is covered by loose rock
where the trail crosses it, but on the opposite side of the stream a
zone of crushed Altyn limestone and Cretaceous shale is visible.
From U. S. Highway No. 2 just east of Marias Pass an excellent distant
view of the thrust may be obtained. About three miles to the north it
appears as a nearly horizontal line high on the side of Summit Mountain.
Above it is a vertical cliff in which white Altyn and red Grinnell are
prominent, and below is a gentler slope composed of gray-brown
Cretaceous shale.
Cretaceous rocks with relatively low resistance to Earth stresses, were
strongly crumpled and folded during the period of overthrusting. The
folded zone extends several miles eastward from the mountains (Figure 3
D), and may be seen to good advantage along Blackfeet Highway on the
north side of Two Medicine Ridge, where a series of thin shales and
sandstones has been squeezed into anticlines and synclines.
It is because of the Lewis overthrust that there are no significant
foothills on the east side of the Lewis Range. The fault has brought
into direct contact the massive and resistant Belt rocks which stand up
as mountains, and the relatively weak shales of the plains which are
carved into subdued landscape features by erosion.
[Illustration: LEWIS OVERTHRUST AT BASE OF MT. WYNN SEEN FROM HIGHWAY
EAST OF MANY GLACIER HOTEL. OVERTHRUST LIES AT BASE OF CLIFF. CRETACEOUS
ROCKS OUTCROP ON GENTLE SLOPE BELOW THRUST.
(DYSON PHOTO)]
After the Lewis overthrust had taken place, and probably following a
period of erosion, the western part of the block broke along a vertical
fault and sank several thousand feet. For a short period of time a lake,
in which clay was deposited, covered the floor of this depressed area.
The present valley of the North Fork of the Flathead River lies on this
downfaulted block (Figure 3D), and the western boundary of the
Livingstone Range marks the trace of the fault. Because the fault is of
the high-angle variety the front of this range is much straighter than
that of the Lewis Range which is formed by the notched eastern edge of
the relatively thin overthrust block. The Belton Hills and Apgar
Mountains near the park's west entrance are isolated blocks separated
from the Livingstone Range by normal faults probably dating from the
time the North Fork Valley subsided.