Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 168

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practically barren of vegetation and glacial drift is generally
thin or absent. Numerous tectonic fractures and dykes are
easily spotted in the aerial photographs as long, narrow de-
pressions on the glacially scoured plateau and, in case of
thickcr dykes, as ridges and walls on mountain slopes and
beaches. In some instances slight relative movement on frac-
tures could be ascertained in aerial photographs. The ease
with which a particular lineation may be spotted depends
to a large extent on its angle to the drainage direction. It
goes without saying that a fracture system running parallel
with the drainage will be accentuated by erosion, whereas
a fracture running perpendicular to the run-off direction
may not be revealed at all. Variations in topography are,
however, sufficient to reveal all the main fracture trends.
Data on dykes and fractures are of necessity lumped together
in this report, as the classification of a lineation is dependent
on field work in most cases, except where a dyke is visible
in the aerial photographs. It must be kept in mind during
the following discussion that the observed lineations can be-
long to any of three types: faults, joints and other related
tectonic phenomena, or dykes.
The orientation of each tectonic lineation in a group of
aerial photographs was determined. Data were grouped at 10
degree intervals and a rose-diagram constructed for each
area as indicated on Fig. 1. Two rose-diagrams on western
Snæfellsnes are, however, based only on data of dyke and
fracture orientation collected in the field and grouped at
22^4 degree intervals, or as N, NNE, NE etc. Data on dykes
and fractures at Brjánslækur in Bardaströnd are from Fried-
rich (1966). A total of 2035 observations of dyke and frac-
ture trends are plotted in Fig. 1. Examination of the rose-
diagrams reveals a complementary set of fractures over the
entire area, making an acute angle of 70 degrees on the ave-
rage. In the northem and north-eastem part of the region
a northerly trend is dominant, but gradually decreasing in
magnitude southwards, being replaced by a set of north-east