Jökull - 01.12.1982, Síða 55

Fig. 2. Topographical map of the Trjávidarlækur basin. Roman numbers indicate the location of the
profiles (Fig. 4). BH 16 is a drillhole.
Mynd 2. Landslag við Trjáviðarliek. Rómverskar tölur sýna staðsetningu sniða (Mynd 4). BH 16 er borhola.
in of the area shown in Fig. 1. They are far smaller in
volume and areal extent than the Tungnárhraun
lavas and maximum length is about 10 km.
2.3 Tephra. Characteristic for the area are
thick beds of tephra which originated in big Plinian
eruptions of Hekla (Thorarinsson 1958, Larsen and
Thorarinsson 1977). The uneven surface of the lava
plains is smoothed by the tephra beds, and depress-
ions in the bedrock and the mountain slopes are
covered with thick sheets of tephra. Tephra of a
known age is an excellent chronological tool. The
tephra is found either in a primary condition or
reworked, as will be discussed in the next chapter.
Thin tephra layers from more distant volcanoes like
Katla and Vatnaöldur are also found.
2.4 Organic soil. The term is used here for org-
anic silt, and especially peat which is found in
abundance in the Trjáviðarlækur basin.
description of profiles
The soil profiles measured are found in the banks
of the Trjáviðarlækur channel (Fig. 2). The under-
lying rock seen upstream in the channel is tillite
(Fig. 5). A schematic section along the channel is
shown in Fig. 3, and the four measured profiles are
shown in Fig. 4. Profiles I and II show the upper
part of the series, and profiles III and IV show the
lower part.
Layer oj sand with gravel. Lowest in the sediment
sequence is a layer ofsand with gravel, which can be
seen in profiles III and IV in Fig. 4 and in Figs. 6
and 7. Cross bedding is common. The sand grains
are mostly fragments of glass, but plagioclase and
even olivine crystals are also conspicuous. The
gravel pebbles are most often subrounded basalt
rock fragments. The sand is unconsolidated except
l'or the uppermost few meters, which seem to have
been cemented to some degree by bog iron.
Organic sandy silt. Overlying the sand is a layer of
organic silt and sand, generally 0.5-1 m thick. It
has a rather indistinct horizontal bedding with al-
ternating silty/sandy layers (Profiles III and IV in
Fig. 4). In the silty parts, fresh-water diatoms are
found in abundance. In the upstream end of the
channel, a 3 cm thick peat layeris imbedded (Pro-
file III in Fig. 4), and a sample taken for radio-
carbon dating gave the age of8,950 years BP (Table
1). The datings were carried out by Dr. Ingrid
JÖKULL 32. ÁR 51