GOHMISSION GEOLOGIQVE - Arkisto.gsf.fi

INTRODUCTION.
In the Pre-Cambrian territory of Southwestern Finland, west of
the zone of the Karelides, quartzites are rare and therefore attract
special interest from geological, tectonical, and petrological points of
view. In the following we shall present a short description of the
largest area of pure quartzite within the Said territory, viz. that of
Tiirismaa in the parish of Hollola, immediately north of the marginal
moraine ridge of Salpausselkii, which here expands into a wide plateau.
The quartzite area stretches from the outskirts of the town Lahti in a
westerly direction close to a highroad from the viUage of Halviilii to
the church of Hollola. The length of the quartzite area is about seven
kilometers and its maximum breadth is a little more than two kilometers
(see map).
The scenery in question is widely known in the sporting world, as
the route of the famous Salpausselkii ski-races, annually held in
Lahti, runs over the Tiirismaa heights.
So far this interesting area has been only summarily described
in the explanation of the geological map-sheet 13 by A. F. Tigerstedt
in 1890. In the summer 1935 the junior author carried out a field
survey of the area and later a microscopic examination of the specimens
collected. The field observations were revised during a common excursion
by us in the same summer. In 1937 the junior author made
some complementary field observations. The senior author has written
the text, and he is responsible for the theoretical conclusions.
Geological investigation of Tiirismaa is greatly hampered by the
scarcity of outcrops. Even the outline of the quartzite area, as shown
on the map, could only in part be drawn on the basis of actual observations,
being drawn for the most part from the observed strikes and the
topography of the moraine-clad terrain where quartzite apparently
underlies the higher parts of the country. While there is no reasonable
doubt about the sedimentogeneous nature of the quartzite formation,
its high grade metamorphism, which has obliterated ali the finer features
of stratification and primary structures, prevents any attempt
at reconstructing with certainty the original stratigraphy. Certain
facts indicate rather complicated tectonics, as will be pointed out
below.