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J. W. Dawson on the Glacial Theory

Sir J.W. Dawson's reasons for dissenting from the Glacial Theory
From: Acadian Geology, 4th Edition. MacMillan & Co., London, 1891.
Chapter V, pp. 66-73.

Origin of Drift

Sir J.W. Dawson If we ask what has been the origin of this great mass
of shifted sand and drifted material, which overspreads the surface
not only of the district we are now describing, but the greater part
of the land of the northern hemisphere, we raise one of the most vexed
questions of modern geology. In reasoning, however, on this subject as
regards Nova Scotia, I have the advantage of appealing to causes now
in operation within the country, and which are at present admitted by
the greater number of modern geological authorities to afford the best
explanation of the phenomena. In the first place, it may at once be
admitted that no such operations as those which formed the drift are
now in progress on the surface of the land, so that the drift is a
relic of a past state of things, in so far as regards the localities
in which it now rests. In the next place, we find, on examining the
drift, that it strongly resembles, though on a greater scale, the
effects now produced by frost and floating ice. Frost breaks up the
surface of the most solid rocks, and throws down cliffs and
precipices. Floating ice annually takes up and removes immense
quantities of loose stones from the shores, and deposits them in the
bottom of the sea or on distant parts of the coasts. Very heavy masses
are removed in this way. I have seen in the strait of Canseau large
stones, ten feet in diameter, that had been taken from below low-water
mark and pushed up upon the beach. Stones so large that they had to be
removed by blasting, have been taken from the base of the cliffs at
the Joggins and deposited off the coal-loading pier, and I have seen
resting on the mud-flats at the mouth of the Petitcodiac River a
boulder at least eight feet in length, that had been floated by the
ice down the river (Fig. 11). Another testimony to the same fact is
furnished by the rigidity with which huge piles of fallen rock are
removed by the floating ice from the base of the trap cliffs of the
Bay of Fundy. Let us suppose, then, the surface of the land, while its
projecting rocks were still uncovered by surface deposits, exposed for
many successive centuries to the action of alternate frosts and thaws,
the whole of the untravelled drift might have been accumulated on its
surface. Let it then be submerged until its hill-tops should become
islands or reefs of rocks in a sea loaded in winter and spring with
drift-ice, floated along by currents, which, like the present Arctic
current, would set from N.E. to S.W. with various modifications
produced by local causes. We have in these causes ample means for
accounting for the whole of the appearances, including the travelled
blocks and the scratched and polished rock-surfaces. This however, is
only a general explanation. Had we time to follow it into details,
many most interesting and complicated facts and processes would be
discovered. I mention merely one for an example, as it illustrates the
manner in which the land may have subsided beneath the boulder-bearing
seas. I have stated that large blocks of sandstone from the plains of
Cumberland have been carried to the summits of the Cobequid Mountains.
When these blocks were carried to their present place, the waters must
have reached to the summits of the hills; but at that time the plain
from which these blocks came must have beeen several hundred feet
below sea-level. How then could ice take them from such a depth? We
may fancy huge icebergs grounding in this deep water, but they could
not float over the hills or ground against their summits. The
explanation is that the country was gradually subsiding. While the
water was shallow, the blocks were drifted against the base of the
hills. As the land sunk, the ice-fields of successive years gradually
pushed them higher, until the summits of the hills were submerged so
deeply that the ice could no longer take up the blocks. Most of the
apparent anomalies of the drift may be explained in such ways, when
the theory of ice-bergs is once admitted.

I have retained the above explanation of the boulder-clay, which
appeared in my edition of 1855, because I have as yet seen no reason
to change my opinion on the subject, although I have since that time
had opportunities of studying the Post-pliocene of Canada and other
parts of America and Europe, and have read nearly all that has been
written by the advocates of a terrestrial origin of this deposit, in a
supposed glacial period when the whole of the northern parts of Europe
and America are imagined to have been covered with glaciers, or rather
with a universal glacier like that of Greenland, but on an enormously
larger scale. The more I have considered this hypothesis, the more
improbable it has appeared, whether in a mechanical, meteorological,
or geological point of view; and a recent visit to Mont Blanc, and the
study of the effects produced by icebergs in the straits of Belleisle,
have more fully established in my mind the belief that floating ice
and the Arctic current have been the grand agents employed. As the
glacier hypothesis of Agassiz, Ramsay, and others, has been
incorporated into the best American text-book of geology, that of
Professor Dana, and has recently been ably advocated in the case of
New Brunswick, I may here give some of my reasons for dissenting from
it, as stated in a paper published some time ago in Canada.

The facts to be accounted for are the striation and polishing of rock
surfaces, the deposit of a sheet of unstratified clay and stones, the
transport of boulders from distant sites lying to the northward, and
the deposit on the boulder clay of beds of stratified clay and sand,
containing marine shells. The rival theories in discussion are, first,
that which supposes a gradual subsidence and re-elevation, with the
action of the sea and its currents, bearing ice at certain seasons of
the year; and, secondly, that which supposes the American land to have
been covered with a sheet of glacier several thousands of feet thick.

The last of these theories, without attempting to undervalue its
application to such regions as those of the Alps or of Spitzbergen or
Greenland, has appeared to me inapplicable to the drift deposits of
eastern America, for the following reasons:

1. It requires a series of suppositions unlikely in themselves and not
warrented by facts. The most important of these is the coincidence of
a wide-spread continent and a universal covering of ice in a temperate
lattitude. In the existing state of the world, it is well known that
the ordinary conditions required by glaciers in temperate lattitudes
are elevated chains and peaks extending above the snow-line; and that
cases in which, in such latitudes, glaciers extend nearly to the
sea-level, occur only where the mean temperature is reduced by cold
ocean-currents approaching to high land, as for instance in the Tierra
del Fuego and the southern extremity of South America. But the
temperate region of North America could not be covered with a
permanent mantle of ice under the existing conditions of solar
radiation; for, even if the whole were elevated into a table-land, its
breadth would secure a sufficient summer heat to melt away the ice,
except from high mountain-peaks. Either, then, there must have been
immense mountain-chains which have disappeared, or there must have
been some unexampled astronomical cause of refrigeration, as, for
example, the earth passing into a colder portion of space, or the
amount of solar heat being diminished. But the former supposition has
no warrant from geology, and astronomy affords no evidence for the
latter view, which, besides, would imply a dimunition of evaporation
militating as much against the glacier theory as would an excess of
heat. An attempt has recently been made by Professor Frankland to
account for such a state of things by the supposition of a higher
temperature of the sea, along with a colder temperature of the land;
but this inversion of the usual state of things is unwarrented by the
doctrine of the secular cooling of the earth; it is contradicted by
the fossils of the period, which show that the seas were colder than
at present; and if it existed, it could not produce the effects
required, unless a preternatural arrest were at the same time laid on
the winds, which spread the temperature of the sea over the land. The
alleged facts observed in Norway, and stated to support this view, are
evidently nothing but the results ordinarily observed in ranges of
hills, one side of which fronts cold sea-water, and the other land
warmed in summer by the sun.

The supposed effects of the varying eccentricity of the earth's orbit,
so ably expounded by Mr. Croll, are no doubt deserving of
consideration in this connexion; but I agree with Sir Charles Lyell in
regarding them as insufficient to produce any effect so great as that
refrigeration supposed by the theory now before us, even if aided by
what Sir Charles truly regards as a more important cause of cold,
namely, a different distribution of land and water, in such a manner
as to give a great excess of land in high latitudes.

2. It seems physically impossible that a sheet of ice, such as that
supposed, could move over an uneven surface, striating it in
directions uniform over vast areas, and often different from the
present inclination of the surface. Glacier ice may move on very
slight slopes, but it must follow these; and the only result of the
immense accumulation of ice supposed, would be to prevent motion
altogether by the want of slope or the counteraction of opposing
slopes, or to induce a slight and irregular motion toward the margins
or outward from the more prominent protuberances.

It is to be observed, also, that, as Hopkins has shown, it is only the
sliding motion of glaciers that can polish or erode surfaces, and that
any internal changes resulting from the mere weight of a thick ice
sheet resting on a level surface, could have little or no influence in
this way.

3. The transport of boulders to great distances, and the lodgement of
them on hill-tops, could not have been occasioned by glaciers. These
carry downward the blocks that fall on them from wasting cliffs. But
the universal glacier supposed could have no such cliffs from which to
collect; and it must have carried boulders for hundreds of miles, and
left them on points as high as those they were taken from. On the
Montreal Mountain, at a height of 600 feet above the sea, are huge
boulders of feldspar from the Laurentian Hills, which must have been
carried 50 to 100 miles from points of scarcely greater elevation, and
over a valley in which the striae are in a direction nearly at right
angles with that of the probable driftage of the boulders. Quite as
striking examples occur in many parts of this country. It is also to
be observed that boulders, often of large size, occur scattered
through the marine stratified clays and sands containing sea-shells;
and whatever views may be entertained as to other boulders, it cannot
be denied that these have been borne by floating ice. Nor is it true,
as has been often affirmed, that the boulder clay is destitute of
marine fossils. At Isle Verte, Riviere du Loup, Murray Bay, and St
Nicholas on the St Lawrence, and also at Cape Elizabeth, near
Portland, there are tough stony clays of the nature of true "till,"
and in the lower part of the drift, which contain numerous marine
shells of the usual Post-pliocene species.

4. The Post-pliocene deposits of Canada, in their fossil remains and
general character, indicate a gradual elevation from a state of
depression, which on the evidence of fossils must have extended to at
least 500 feet, and on that of far-travelled boulders to several times
that amount; while there is nothing but the boulder clay to represent
the supposed previous ice-clad state of the land, except the scratches
on the rock surfaces, which must have been caused by the same agency
which deposited the boulder clay.

5. The peat deposits, with fir-roots, found below the boulder clay in
Cape Breton, the remains pf plants and land-snails in the marine clays
of the Ottawa, and the shells of the St Lawrence clays and sands, show
that the sea at the period in question had nearly the temperature of
the present Arctic currents of our coasts, and that the land was not
covered with ice, but supported a vegetation similar to that of
Labrador and the north shore of the St Lawrence at present. This
evidence refers not to the later period of the Mammoth and Mastodon,
when the re-elevation was perhaps nearly complete, but to the earlier
period contemporaneous with or immediately following the supposed
glacier period. In my former papers on the Post-pliocene of the St
Lawrence, I have shown that the change of climate involved is not
greater than that which may have been due to the subsidance of land,
and to the change of course of the Arctic current, actually proved by
the deposits themselves.

These objections might be pursued to much greater length; but enough
has been said to show that there are, in the case of north-eastern
America, strong reasons against the existence of any such period of
extreme glaciation as supposed by many geologists; and that if we can
otherwise explain the rock striation and polishing, and the formation
of fiords and lake basins, the strong points with these theorists, we
can dispense altogether with the portentious changes in physical
geography involved in their views, and which are not necessary to
explain any of the other phenomena.

On these points, the Report of the Geological Survey of Canada throws
new light; though Sir William Logan, with his usual caution, has not
committed himself to theoretical conclusions; and in one or two local
cases he seems to favour the glacial theory. It has long been known to
geologists, that in north-eastern America two main directions of
striation of rock surfaces occur, from north-east to south-west, and
from north-west to south-east; and that locally the directions vary
from these to north and south and east to west. Various attempts have
been made, but without much success, to account for these directions
of striation by the motion of glaciers; and while it is quite easy for
any one prepossessed with this view to account in this way for the
striation in a particular valley or part of a valley, yet so many
exceptional facts occur as to throw doubt on the explanation, except
in the case of a few of the smaller and steeper mountain gorges.

In the Report of the Survey of Canada a valuable table of these
striations is given, from which it appears that they are locally
distributed in such a way as to throw a decided gleam of light on
their origin.

It would seem that the dominant direction in the valley of the St
Lawrence, along the high lands to the north of it, and across western
New York, is north-east and south-west; and that there is another
series of scratches running nearly at right angles to the former,
across the neck of land between Georgian Bay and Lake Ontario, down
the valley of the Ottawa, and across parts of the Eastern Townships,
connecting with the prevalent south and south-east striation which
occurs in the valleys of the Connecticut and Lake Champlain, and
eslewhere in New England, as well as in Nova Scotia and New Brunswick.
What were the determining conditions of these two courses, and were
they contemporaneous or distinct in time? The first point to be
settled in answering these questions is the direction of the force
which caused the striae. Now, I have no hesitation in asserting, from
my own observations as well as from those of others, that for the
south-west striation the direction was from the ocean toward the
interior, against the slope of the St Lawrence valley. The
crag-and-tail forms of all our isolated hills, and the direction of
transport of boulders carried from them, show that throughout Canada
the movement was from north-east to southwest. This at once disposes
of the glacier-theory for the prevailing set of striae; for we cannot
suppose a glacier moving from the Atlantic up into the interior. On
the other hand, it is eminently favourable to the idea of ocean drift.
A subsidence of America, such as would at present convert all the
plains of Canada and New York and New England into sea, would
determine the course if the Arctic current over this submerged land
from north-east to south-west; and as the current would move up a
slope, the ice which it bore would tend to ground, and to grind the
bottom as it passed into shallower water; for it must be observed that
the character of slope which enables a glacier to grind the surface
may prevent ice borne by a current from doing so, and vice versa.

Now we know that in the Post-pliocene period eastern America was
submerged, and consequently the striation at once comes into harmony
with other geological facts. We have, of course, to suppose that the
striation took place during submergence, and that the process was slow
and gradual, beginning near the sea and at the lower levels, and
carried upwards to the higher grounds in successive centuries, while
the portions previously striated were covered with deposits swept down
from the sinking land or dropped from melting ice. It would be easy to
show that this view corresponds with many of the minor facts.

Further, the theory thus stated accounts for the excavation of the
deep and land-locked basins of our great American lakes. Ocean
currents, if cold, and clinging to the bottom.must cut out pot-holes,
just as rivers do, though geologists are too apt to limit their
function to the throwing up of banks. The course of the present Arctic
current along the American coast has its deep hollows as well as its
sand- banks. Our American lake-basins are cut out deeply into the
softer strata. Running water on the land would not have done this, for
it could have no outlet; nor could this result be effected by
breakers. Glaciers could not have effected it; for even if the
climatal conditions for these were admitted, there is no height of
land to give them momentum. But if we suppose the land submerged so
that the Arctic current, flowing from the north-east, should pour over
the Laurentian rocks on the north side of Lake Superior and Lake
Huron, it would necessarily cut out of the softer Silurian strata just
such basins, drifting their materials to the south-west. At the same
time, the lower strata of the current would be powerfully determined
through the strait between the Adirondac and Laurentide Hills, and,
flowing over the ridge of hard rock which connects them at the
Thousand Islands, would cut out the long basin of Lake Ontario,
heaping up at the same time, in the lee of the Lawrentian ridge, the
great mass of boulder clay which intervenes between Lake Ontario and
Georgian Bay. Lake Erie may have been cut by the flow of the upper
layers of water over the Middle Silurian escarpment; and Lake
Michigan, though less closely connected with the direction of the
current, is, like the others, due to the action of a continuous
eroding force on rocks of unequal hardness.

The predominant south-west striations, and the cutting of the upper
lakes, demand an outlet to the west for the Arctic current. But both
during depression and elevation of the land, there must have been a
time when this outlet was obstructed, and when the lower levels of New
York, New England, and Canada were still under water. Then the valley
of the Ottawa, that of the Mohawk, and the low country between Lakes
Ontario and Huron, and the valleys of Lake Champlain and the
Connecticut, would be straits or arms of the sea, and the current,
obstructed in its direct flow, would set principally along these, and
act on the rocks in the north and south and north-west and south-east
directions. To this portion of the process I would attribute the
north-west and south-east striation. It is true that this view does
not account for the south-east striae observed on some high peaks in
New England; but it must be observed that even at the time of greatest
depression, the Arctic current would cling to the northern land, or be
thrown so rapidly to the west that its direct action might not reach
such summits.

Nor would I exclude altogether the action of glaciers in eastern
America, though I must dissent from any view which would assign to
them the principal agency in our glacial phenomena. Under a condition
of the continent in which only its higher peaks were above the water,
the air would be so moist, and the temperature so low, that permament
ice may have clung about mountains in the temperate lattitudes. The
striation itself shows that there must have been extensive glaciers,
as now, in the extreme Arctic regions. Yet I think that most of the
alleged instances must be founded on error, and that old sea-beaches
have been mistaken for moraines. I have observed in Canada and Nova
Scotia many old sea-beaches, gravel-ridges, and lake-margins, I have
seen nothing that could fairly be regarded as the work of glaciers.
The so-called moraines, in so far as my observation extends, are more
probably shingle beaches and bars, old coast-lines loaded with
boulders, trains of boulders or "ozars." Most of them convey to my
mind the impression of ice-action along a slowly subsiding coast,
forming successive deposits of stones in the shallow water, and
burying them in clay and smaller stones as the depth increased. These
deposits were again modified during emergence, when the old ridges
were sometimes bared by denudation, and new ones heaped up.

I shall close these remarks, perhaps already too tedious, by a mere
reference to the alleged prevalence of lake basins and fiords in high
northern latitudes, as connected with glacial action. In reasoning on
this, it seems to be overlooked that the prevalence of hard
metamorphic rocks over wide areas in the north is one element in the
matter. Again, cold Arctic currents are the cutters of basins not the
warm surface-currents. Further, the fiords on coasts like the deep
lateral valleys of mountains are evidences of the action of the waves
and currents rather than that of ice, I am sure that this is the case
with the numerous indentations of the coast of Nova Scotia, which are
cut into the softer and more shattered bands of rock; and show, in
raised beaches and gravel ridges like those of the present coast, the
levels of the sea at the time of their formation.

In Nova Scotia we have the means of applying another crucial test to
this theory of lake basins. The whole surface of the peninsula has
been striated and polished; and it has been estimated that one-third
of its area is occupied by lakes, most of them of small dimension. Now
these lakes are almost entirely confined to those metamorphic regions
where unequal hardness and imperfect facilities for drainage tend to
promote their occurrence. It is evident, therefore, that we are to
seek for the origin of the lake basins in these local causes, and not
in any universal covering of glacier. Further, as I have above shown,
the manner in which the great Canadian lakes have been cut out of the
softer materials, and their relations to the neighboring harder
portions of the country, prove that these great basins may be due to
the action of ocean-currents, a cause to which I would attribute also
the greater part of the smaller lakes of Nova Scotia. To these reasons
I may add the following comparative statements of the effects of
glaciers and icebergs, deduced from examinations of the glaciers of
Mont Blanc and the icebergs of Belleisle:

1. Glaciers heap up their debris in abrupt ridges. Floating ice
sometimes does this, but more usually spreads its load in a more or
less uniform sheet.

2. The material of moraines is all local. Icebergs carry their
deposits often to great distances from their sources.

3. The stones carried by glaciers are mostly angular, except where
they have been acted on by torrents. Those moved by floating ice are
more often rounded, being acted on by the waves and by the abrading
action of sand drifted by currents.

4. In the marine glacial deposits mud is mixed with stones and
boulders. In the case of land glaciers most of this mud is carried off
by streams and deposited elsewhere.

5. The deposits of floating ice may contain marine shells. Those of
glaciers cannot, except where, as in Greenland and Spitzbergen,
glaciers push their moraines out into the sea.

6. It is of the nature of glaciers to flow in the deepest ravines they
can find, and such ravines drain the ice of extensive areas of
mountain land. Icebergs, on the contrary, act with greatest ease on
flat surfaces or slight elevations in the sea bottom.

7. Glaciers must descend slopes and must be backed by large supplies
of perennial snow. Icebergs act independently, and, being water-borne,
may work up slopes and on level surfaces.

8. Glaciers striate the sides and bottoms of their ravives very
unequally, acting with great force and effect only on those places
where their weight impinges most heavily. Icebergs, on the contrary,
being carried by constant currents and over comparatively flat
surfaces, must striate and grind more regularly over large areas, and
with less reference to local inequalities of surface.

9. The direction of the striae and grooves produced by glaciers
depends on the direction of valleys. That of icebergs, on the
contrary, depends upon the direction ofmarine currents, which is not
determined by the outline of the surface, but is influenced by the
large and wide depressions of the sea-bottom.

10. When subsidence of the land is in progress, floating ice may carry
boulders from lower to higher levels. Glaciers cannot do this under
any circumstances, though in their progress they may leave blocks
perched on the tops of peaks and ridges.

Related Links

Sir J. William Dawson - Nova Scotia Museum site
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