Acta naturalia Islandica - 01.02.1946, Blaðsíða 36

30
TRAUSTI EINARSSON
chapter we shall be able to point out a simple ancl natural explana-
tion of the different modes of consolidation.
Here we shall enter further into the question of the grey con-
glomerates, which hitherto have generally been assumed to be mo-
raines.
We have emphasized their largely glassy composition, a fact which
was originally noted by Keilhack. This holds also in cases where the
conglomerates rest on crystalline lavas. It is further important to
notice that blocks or pebbles of brown tuff or breccia are extremely
rare in the conglomerates. At any rate I have not noticed such blocks.
Hence it is rather unnatural to assume that the material of the con-
glomerates derives from the breaking down of tuffs or breccias.
Now as to striated stones contained in the conglomerates two facts
must be kept in mind. The first is the phenomenon of secondary stri-
ation. These hard conglomerates are just in such a condition that
boulders protruding out of them may be striated by glaciers without
much danger of being loosened and carried away. On the other
hand, post-glacial weathering would suffice to wipe out eventual
striae on the fine matrix of the conglomerates and yet in most cases
permit the striated basaltic boulders to remain in place. This pheno-
menon, it seems, has aroused little notice and it might be suggested
that the striae detected on boulders in the conglomerates are partly
of this nature.
The other fact is the notorious scarcity of striated boulders in the
assumed old moraines. Observers often remark that by a thorough
search they managed to find one or a few striated stones. And this
scarcity should be further accentuated if secondary striae are taken
into account.
The significance of this scarcity of striated stones struck me especi-
ally as I walked across one of Iceland’s well-known sandar, the
Breiðamerkursandur. From the eastern part of this sandur the
glacier Breiðamerkurjökull has retreated since the beginning of this
century, and here nearly every, if not every stone of the size of a
human head or larger which I looked at, was very clearly glacially
striated.
This may be an exceptional case, but nevertheless, I think that the
occurrence of striated blocks in Pjeturss’ “moraines” is so rare as to
call for a special explanation. I think the question is legimitate, wheth
er the reported striation of such a small percentage of boulders in