SKETCH OF THE GEOLOGY OF THE ST. ELIAS REGION.
GENERAL FEATURES.
In the preceding narrative, many references have been made to the
character of the rocks and to the geological structure of the region
explored. It was not practicable during the journey to carry on
detailed geological studies, but such facts as were noted are of
interest, for this reason, if for no other: they relate to a country
previously unknown.
My reconnoissance enabled me to determine that there are three
well-defined formations in the St. Elias region. These are--
1. The sandstones and shales about Yakutat bay and westward along the
foot of the mountain to Icy bay, named the _Yakutat system_.
2. A system of probably later date, composed of shale, conglomerate,
limestone, sandstone, etc., best exposed in the cliffs of Pinnacle
pass and along the northern and western borders of the Samovar hills,
and named the _Pinnacle system_.
3. The metamorphic rocks of the main St. Elias range, called the _St.
Elias schist_.
YAKUTAT SYSTEM.
The rocks of this system are of gray and brown sandstones and nearly
black shales. They are uniform in lithological character over a large
area, and are usually greatly crushed and seamed. So great has been
the crushing to which they have been subjected that it is difficult to
work out a hand specimen with fresh surfaces. Fragments broken out
with a hammer are almost invariably bounded by plains of previous
crushing, and are usually somewhat weathered.
These rocks form the bold shores of Yakutat and Disenchantment bays,
and were the only rocks seen along our route from Yakutat bay to
Pinnacle pass. The whole of the Hitchcock range is composed of rocks
of this series, as are also the Chaix {168} hills and the hills west
of Icy bay and the southern portion of the Samovar hills. North of
Pinnacle pass there are rocks undistinguishable lithogically from
those about Yakutat bay. These are exposed in Mount Owen and on each
side of Dome pass; they also form the bold spurs about the immediate
bases of Mount Augusta, Mount Malaspina, and Mount St. Elias. In the
three instances last named these rocks dip beneath the schist forming
the crest of the St. Elias range, and it is probable that a great
overthrust there took place before the formation of the faults to
which the present relief of the mountains is due.
All the mountain spurs of Mount Cook, so far as is known, are composed
of sandstones and shales of the Yakutat series, with the exception of
the Pinnacle pass cliffs. Nearly all the débris on the glaciers from
Disenchantment bay to the Seward glacier, and probably beyond, is
derived from the rocks of this system. The distribution of the rocks
from which the débris was derived may be ascertained in a general way
by tracing out the sources of the glaciers. Medial moraines on the
Hayden and Marvine glaciers, however, have their sources on the
northern slope of Mount Cook, and are composed of gabbro and
serpentine. These rocks were not seen in place, and their relation to
the Yakutat series can only be conjectured.
Although the rocks of this system are stratified, it is impossible to
determine their thickness, for the reason that they have been greatly
crushed and overthrust. This is well illustrated in the Hitchcock
range, which, as already explained, trends about northeast and
southwest, and is composed of strata of shale and sandstone, having a
nearly east-and-west strike and a uniform dip toward the northeast.
Were the rocks in normal position their thickness would be incredible.
In addition to this negative evidence, there is the crushed condition
of the strata to show that movement has taken place all through their
mass; and in a few instances thrust faults were distinguished, dipping
northeastward at about the same angle as the lines of bedding. In the
crushing to which the rocks have been subjected the shales have
suffered more than the sandstones, and have been drawn out into
wedge-shaped masses, the sharp edges of which usually point toward the
northeast, which is presumably the direction from which the crushing
force acted.
The hypothesis that the rocks in the St. Elias region have been
crushed and overthrust explains many otherwise {169} inharmonious
facts, and accounts for the superposition of the St. Elias schist upon
rocks of the Yakutat system.
Coal has been discovered in the rocks of the Yakutat system about two
miles west of the southern end of Disenchantment bay, and is reported
to be of workable thickness. I saw thin lignite seams at the surface
at this locality, but as the shafts were filled with water I was
unable to examine the coal in the openings, and cannot vouch for its
thickness. Samples obtained from the mine show it to be a black
lignite which would apparently be of value for fuel. Fossil leaves are
reported to occur in connection with the lignite, but these have never
been seen by any one who could identify them.
The rocks of the Yakutat system, wherever seen, dip northeastward,
except when greatly disturbed near fault-lines. East of Disenchantment
bay the inclination of the beds is from 15° to 20°; farther westward
the dip increases gradually all the way to the Hitchcock range, where
the prevailing inclination is from 30° to 40°, and frequently still
greater. Beneath Mount Malaspina and Mount St. Elias the Yakutat
sandstones dip northeastward at an angle of about 15°, and in the
hills west of Icy bay the dip is about the same. Exceptions to the
prevailing dips occur along the immediate shore of Yakutat bay,
northwest of Knight island, and at the southern extremity of each of
the mountain spurs between Yakutat bay and Blossom island. At these
localities the rocks are frequently vertical or nearly so, owing their
high dip to the proximity of lines of displacement. The faults
indicated by these unusual dips also mark the boundary between the
mountains and the seaward-stretching plateau of alluvium and ice.
The crushing, overthrusting and faulting that has affected the rocks
of this system render it doubtful whether the coal seams which occur
in it, even if of requisite thickness, can be worked to advantage.
Some of the samples of coal obtained at the openings made near Yakutat
bay were slickensided, showing that movements in the coal seam had
there taken place.
As already stated, the rocks of the Yakutat series are remarkably
uniform in character throughout the extent now known, and offer but
little variety. The sandstones are intersected in every direction by
thin quartz seams, which stand in relief on the weathered surfaces,
giving the rocks a peculiar and {170} characteristic appearance. The
first important change in the geology along the route traversed by us
was met on reaching Pinnacle pass.
PINNACLE SYSTEM.
The rocks of this system, as already stated, are best exposed in the
great fault-scarp forming the northern wall of Pinnacle pass. They are
more varied in composition and have preserved a better record of the
conditions under which they were deposited than the sandstones and
shales of the Yakutat system.
Only an approximate section of the rocks exposed in the Pinnacle-pass
cliff was obtained.
Sandstone and conglomerate weathering into spires . . . 500 feet.
Evenly bedded, sandy shale in thin layers . . . . . . . 600 "
Coarse conglomerate; bowlders of crystalline rock . . . 50 "
Thinly bedded, dark-colored sandstone and shale . . . . 500 "
Reddish conglomerate . . . . . . . . . . . . . . . . . 10 "
Light-gray sandstone, with thin, irregular coal seams . 40 "
-----
Total . . . . . . . . . . . . . . . . . . . . . . . . 1,800 "
There is also a compact, crystalline, gray limestone near the upper
portion of the series, which escaped notice in the cliffs. At the end
of the Pinnacle-pass cliffs, however, where the rocks are turned
northward by the great fault which decides the course of the Seward
glacier, and dip eastward at a high angle, the limestone is well
exposed, and has a thickness of about 50 feet. In many places the
surfaces of the layers are covered with fragments of large _Pecten_
shells. Associated with the limestone there are reddish shales, much
crushed and broken, and a peculiar conglomerate. The pebbles in the
conglomerate are of many varieties, and were observed at places along
the Pinnacle pass cliffs. Their most marked peculiarity lies in the
fact that they have been sheared by a movement in the rocks and
sometimes broken into several fragments which have been reunited,
probably by pressure. These faulted pebbles are characteristic of the
strata from which they were derived. Similar pebbles were afterward
obtained in the Marvine glacier near its junction with the Malaspina
glacier, thus indicating that there are other outcrops of the
conglomerate about Mount Cook, near where the Marvine glacier {171}
has its source. Two quartz pebbles from the conglomerate of Pinnacle
pass are shown in the accompanying illustrations. The larger pebble
(shown in figure 7) is of bluish-gray quartz, and the smaller one
(depicted in figure 8) is of white quartz. The fragments into which
they have been broken are now firmly united. The engravings are
photo-mechanical (Moss process) reproductions from the objects.
[Illustration: FIGURE 7--_Faulted Pebble from Pinnacle Pass_.]
[Illustration: FIGURE 8--_Faulted Pebble from Pinnacle Pass_.]
In the northern and western part of the Samovar hills the rocks of the
Pinnacle system again appear, forming a bold angular ridge, curving
southward and reaching the border of the Agassiz glacier. The southern
face of this range is precipitous and, like the Pinnacle pass cliffs,
exhibits the edges of northward-dipping strata. Its northern and
western slopes are heavily snow-bound. It is in reality a continuation
of the Pinnacle pass fault, but thrown out of line by the cross-fault
which marked out the course of the Seward glacier.
The Yakutat and Pinnacle systems are so easily recognized that their
distribution can be distinguished at a glance, when the outcrops are
not concealed beneath the nearly universal covering of snow. The rocks
of the Yakutat series are heavily bedded sandstones and shales, and
have in general a light-brown tint; while the rocks of the Pinnacle
series are thinly bedded and dark in color, appearing black at a
distance.
The presence of a _Pecten_ (_P. caurinus_ (?) Gld.) in the limestone
of the Pinnacle series has already been mentioned. Other fossils were
obtained from sandstones and shales at the crest of the cliffs above
Pinnacle pass at an elevation of 5,000 feet. These {172} were
submitted to Dr. W. H. Dall, who kindly identified them as follows:
_Mya arenaria_, L.;
_Mytilus edulis_, L.;
_Leda fossa_, Baird, or _L. minuta_, Fabr.;
_Macoma inconspicua_, B. and S.;
_Cardium islandicum_, L.;
_Litorina atkana_, Dall.
All of these species are stated by Dall to be still living in the
oceanic waters of Alaska. The very recent age of the rocks in which
they occur is thus established.
In strata closely connected with the layers in which these shells were
found there occur many fine leaf impressions, a few of which were
brought away. These have been examined by Professor L. F. Ward, who
has identified them with four species of _Salix_, closely resembling
living species. The report on these interesting fossils forms Appendix
D.
The age indicated by both invertebrates and plants is late Tertiary
(Pliocene) or early Pleistocene. This determination is of great
significance when taken in connection with the structure of the
region, and shows that the mountains in the St. Elias region are
young.
Not only was a part, at least, of the Pinnacle system deposited during
the life of living species of mollusks, but also the whole of the
Yakutat series, the stratigraphic position of which is, if my
determination is correct, above the Pinnacle system. After the
sediments composing the rocks of these two series were {173} deposited
in the sea as strata of sand, mud, etc., they were consolidated,
overthrust, faulted, and upheaved into one of the grandest mountain
ridges on the continent. Then, after the mountains had reached a
considerable height, if not their full growth, the snows of winter
fell upon them, and glaciers were born; the glaciers increased to a
maximum, and their surfaces reached from a thousand to two thousand
feet higher than now on the more southern mountain spurs, and
afterward slowly wasted away to their present dimensions. All of this
interesting and varied history has been enacted during the life of
existing species of plants and animals.
The relative age of the Yakutat and Pinnacle series is the weakest
point in the history sketched above. The facts on which it rests are
as follows: At Pinnacle pass the sandstones and shales forming the
southern wall belong to the Yakutat system and are much disturbed,
while the northern wall, or the heaved side of the fault, is composed
of the rocks of the Pinnacle system, inclined northward at an angle of
30° or 40°. North of this fault-scarp, in the foothills of Mount Owen,
sandstones and shales, seemingly identical with those of the Yakutat
system, again occur, although their direct connection with the rocks
south of Pinnacle pass was not observed, owing to the snow that
obscured the outcrops. Again at Dome pass a similar relation seems
evident, but cannot be directly established. The immediate foothills
of Mounts Augusta, Malaspina, and St. Elias are also of sandstone,
lithologically the same as the Yakutat series. The conclusion that the
Yakutat system is younger than the Pinnacle-pass rocks was reached in
the field after many other hypotheses had been tried and found
wanting, and to my mind it explains all the observations made. Even
should the supposed relations of the two series under discussion be
reversed, it would still be true that a very large part of the rocks
of the St. Elias region were deposited since the appearance of living
species of mollusks and plants, and that the prevailing structure of
the region was imposed at a still later date. This will appear more
clearly after examining the structure of the region.
ST. ELIAS SCHIST.
The rock forming several thousand feet of the upper portion of the St.
Elias range is a schist in which the planes of bedding {174} are
preserved. The dip of the strata is northeastward, and has exerted a
decided influence on the weathering of the mountain crests. As the
opportunities for examining this formation were unsatisfactory, a
detailed account of it will not now be attempted.
GEOLOGICAL STRUCTURE.
The abnormal thickness of the Yakutat series, due to crushing and
overthrust, has been referred to, as has also the superposition of the
St. Elias schist upon rock supposed to belong to the Yakutat system.
The plane of contact between the sandstone and the overlying schist of
the St. Elias range dips northeastward at an angle of about 15°,
corresponding, as nearly as can be determined, with the dip of the
strata in the sandstone itself. All of the observations made in this
connection indicate that the schist has been overthrust upon the
sandstones. After this took place the great faults to which the range
owes its present relief were formed.
About Mount Cook, however, and in the elevated plateau east of Yakutat
bay, the conditions are different from those observed along the base
of the St. Elias range. The only displacements known in the Yakutat
system south and east of Pinnacle pass is the great fault which
presumably exists where the rocks of the foothills disappear beneath
the gravel and glaciers of the Piedmont region, the faults referred to
belonging to the same series as those which determine the southern and
southwestern borders of the St. Elias range and many of the foothills
south of the main escarpment. Besides the great faults which trend
from St. Elias toward the northeast and northwest, there are several
cross-faults, one of which determines the position of the Seward
glacier through a portion of its course, while another marks out the
path of the Agassiz glacier; and two others may be recognized just
east of the summit of St. Elias, which have dropped portions of the
eastern end of the orographic block forming the crowning peak of the
range.
The southern face of Mount St. Elias is a fault-scarp. The mountain
itself is formed by the upturned edge of a faulted block in which the
stratification is inclined northeastward. As has just been mentioned,
the mountain stands at the intersection of two lines of displacement,
one trending in a northeasterly and the other in a northwesterly
direction. The one trending {175} northwestward extends beyond the end
of the northeast fault. The point of union is at the pass between
Mount St. Elias and Mount Newton. The upturned block, bounded on the
southwest by a great fault, projects beyond the junction with the
northeasterly fault. It is this projecting end of a roof-like block
that forms Mount St. Elias. That this is the case may be clearly seen
when viewing the mountain from the glacier near the base of Mount
Owen. Such a view is shown on plate 20. The crest-line of St. Elias
extends with a decreasing grade northwestward from the culminating
peak, and the northern slope of the ridge is the surface of the tilted
block.
[Illustration: PLATE 20. MT. ST. ELIAS, FROM THE SEWARD GLACIER.]
From what has been stated already, it will be seen that the St. Elias
range is young. Its upheaval, as indicated by our present knowledge,
was since the close of the Tertiary. The breaking of the rocks and
their upheaval is an event of such recent date that erosion has
scarcely modified the forms which the mountains had at their birth.
The formation of glaciers followed the elevation of the region so
quickly, that there was no opportunity for streams to act. The ice
drainage is consequent upon the geological structure, and has made but
slight changes in the topography due to that structure.
About Mount Cook, and in the elevated plateau east of Yakutat bay,
there has been deeper erosion than about Mount St. Elias. The glaciers
in this region occupy deep valleys radiating from the higher peaks;
but whether these are really valleys of erosion is not definitely
known. In some instances, changes of dip on opposite sides of the
valleys indicate that they may in part be due to faulting; but, owing
principally to the fact that every basin has its glacier, it has not
been practicable, up to the present time, to determine how they were
formed.
The crests of the mountains are always sharp and angular, by reason of
the rapid weathering of their exposed summits, but while
disintegration is rapid, no evidences of pronounced decay are
noticeable. The peaks on the summits of the St. Elias range are either
pyramids or roof-like crests with triangular gables. These forms have
resulted from the weathering of schist in which the planes of bedding
are crossed by lines of jointing.
{176}