Jökull - 01.12.1988, Page 22

Fig. 3. A: Historical recession curves for Svínafells-
jökull (North and South) and Skaftafellsjökull,
based on annual measurements by the Icelandic
Meteorological Office and later by the Icelandic
Glaciological Society (since 1932), and
lichenometric dating of earlier moraines (Skafta-
fellsjökull only). Dates of individual readvances
(and of corresponding push-moraines) are indicated.
B: Mean annual temperature data for Fagurhólsmýri
(15 km southeast of Svínafell), 1903-1985.
Mynd 3. A: Línurit yfir hörfun Svínafellsjökuls
(nyrðri og syðri) og Skaftafellsjökuls. Línuritið er
byggt á árlegum mœlingum Veðurstofu og síðar
Jöklarannsóknafélags lslands frá 1932 og á aldurs-
greiningum skv. mœlingum á skófum á eldri
jökulurðum (á aðeins við um Skaftafellsj ökul).
Framrásir og tilsvarandi jökulgarðar eru auðkennd
með ártölum á myndinni.
B: Meðalárshiti á Fagurhólsmýri 1903-1985.
underlying trend upon which shorter-term fluctua-
tions are superimposed, and in accounting for the
contrasts in behaviour of the two glaciers.
The overall rate of retreat of Svínafellsjökull has
been much slower than that of Skaftafellsjökull, and
has been punctuated by more frequent episodes of
readvance (Fig. 3A). This differential response of
the two glaciers is thought to be largely a function
of the differences in elevation of their respective
accumulation areas (King and Ives, 1955), and is
reflected in the resulting morphology of the
moraines (Figs. 4 and 5).
MORPHOLOGY AND ORIGIN
OF THE MORAINES
The Skaftafellsjökull moraines (Fig. 4A) are for
the most part clearly defined and widely separated
asymmetric structures with a modest relief of 2 to 3
metres on their steeper distal margins. The Svína-
fellsjökull moraines by contrast (Fig. 4B) are much
higher features, especially at the southem end where
they rise up to 60 metres above the adjacent sandur,
and show evidence in the complexity of superim-
posed and overlapping ridges, of repeated oscilla-
tions of the glacier snout. The crests of individual
ridges, both at Svínafellsjökull and Skaftafellsjökull,
reflect the intricate scalloped outlines of former
positions of the glacier snout, testifying to their ori-
gin as ice-marginal rather than subglacial features.
Eyles (1978) has suggested that the Skaftafells-
jökull ridges represent local thickening of ground
moraine (lodgement till) by ice-frontal pushing (as
found by Boulton (1986) at Breiðamerkurjökull),
whereas Price (1970, 1977), in his detailed study of
the fabric and intemal stmcture of morphologically
similar moraines at Fjallsjökull, concludes that
squeezing of water-soaked till from beneath a
rapidly retreating ice-front may be an altemative
mechanism. Conditions favourable for the latter pro-
cess (rapid recession, abundant meltwater and poor
drainage due to low relief) are found at Skaftafells-
jökull, but there is also evidence for minor push-
moraine formation along the present margin of the
glacier. It seems likely therefore that most ridges
here will have been formed by a combination of
20 JÖKULL, No. 38, 1988