In Iceland, plant fossils are most commonly found in sedimentary rocks, usually as interbeds, within the basalt formation from the Miocene-Pliocene Succession (16 to 2.6 million years, the Blágrýti bedrock) and the Pliocene-Pleistocene Succession (2.6 to 0.001 million years, the Grágrýti bedrock). Interbeds may be either ancient soils in which plants grew or lake sediments that accumulated on the bottoms of ancient lakes and ponds. Plant matter carried into lakes settled to the bottom. Over time, plant and algae debris and rock fragments formed sedimentary layers. Plant remains may also be found in tephra layers and other sedimentary layers that formed in connection with volcanic eruptions. Occasionally, one can find imprints or hollows in the Blágrýti bedrock left by trees. These were created when lava flowed through forested areas during a volcanic eruption. Tree trunks became encased in the lava, destroying the trees but leaving a cavity in the solidified lava. In some cases, lava later dripped into these cavities, creating basalt casts of the tree trunks.
The oldest plant fossils that have been found in Iceland are around 15 million years old. These include leaf impressions, carbonised leaves and fruit remains, pollen grains, compressed tree trunks, and lignite. Important find sites for these fossils are located in the Westfjords: Þórishlíðarfjall in Selárdalur, Svalvogar in Dýrafjörður, Botn in Súgandafjörður, and the promontory of Breiðhilla near the village of Bolungarvík. Insect and plant fossils have also been preserved in Mókollsdalur in Strandasýsla. These are around 8–9 million years old.