| Summary: | In the Last Glacial Maximum, approximately 25,000 years ago, a 1500 ± 500 m thick ice sheet covered most of Iceland. A glacier tongue is believed to have occupied the Þingvellir rift valley where Lake Þingvallavatn now resides. Lake Þingvallavatn is believed to have formed as an ice-dammed lake between the receding Iceland ice sheet and the Ölfusvatnsfjöll in Grafningur in south Iceland. Terraces and other landforms in the Grafningur area are believed to mark the former position and height of the early stage of Lake Þingvallavatn. The aim of this research was to identify these terraces and look into their depositional environment. After they were identified on aerial orthophotographs, a map and digital elevation model of the area was constructed. Three outcrops of sediments were observed in the area of Krókur to identify their depositional environment. Cross-bedding, ripples and horizontal planar lamination indicated that they were likely deposited through fluvial processes. Different tilts of layers suggested different origins of sediment transportation. The results were the following depositional environments; glaciofluvial outwash, a prograding delta and an alluvial fan. The terraces were then connected to the geological history of the area, particularly the early stage of Lake Þingvallavatn. Key words: Geology, deglaciation, Iceland ice sheet, cross-bedding, Lake Þingvallavatn, Grafningur. Fyrir um 25.000 árum er talið að um 1500 ± 500 m þykkur jökull hafi þakið nánast allt Ísland. Jökultunga lá í Þingvallasigdældinni sem nú geymir stærsta náttúrulega stöðuvatn Íslands, Þingvallavatn, 82 km2 að stærð. Þingvallavatn er talið hafa myndast sem jökulstíflað lón milli hörfandi jökuls og Ölfusvatnsfjalla. Hjallar í Grafningnum eru til marks um þær breytingar sem vatnið hefur tekið frá myndun þess. Hjallarnir voru auðkenndir á loftmyndun og útbúið var kort af þeim ásamt þrívíddarlíkani af svæðinu. Þrjár opnur með setlögum voru skoðaðir í grennd við bæinn Krók til að kanna betur setmyndunarumhverfi þeirra. Við ...
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