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The exposed geology of Hiiumaa is composed of [[Paleozoic]] [[limestone]] which dips towards the South, covered by [[glacial sediment]]s. In the North of the island the limestones are [[Ordovician]] and they young upwards to the [[Silurian]] in the South.<ref name=":4">{{Cite journal|last1=Schwartz|first1=Maurice|last2=Bird|first2=Eric|last3=Orviku|first3=Kaarel|date=1995-01-09|title=The Provenance of Beaches on the Estonian Islands of Hiiumaa, Saaremaa and Muhu|url=http://journals.fcla.edu/jcr/article/view/79510|journal=Journal of Coastal Research|language=en|volume=11|issue=1|issn=0749-0208}}</ref> These limestones formed at 30 degrees South and have since been moving North with the rest of the Estonian block.<ref name="Suuroja2002">{{cite book|last1=Suuroja|first1=Kalle|title=Impacts in Precambrian Shields|year=2002|chapter=Natural Resources of the Kärdla Impact Structure, Hiiumaa Island, Estonia|pages=295–306|doi=10.1007/978-3-662-05010-1_12|issn=1612-8338|series=Impact Studies|isbn=978-3-642-07803-3}}</ref> Bore holes have found [[Cambrian]] [[sedimentary rock]]s and a [[crystalline]] [[Basement (geology)|basement]].<ref name="PuuraSuuroja1992">{{cite journal|last1=Puura|first1=Väino|last2=Suuroja|first2=Kalle|year=1992|title=Ordovician impact crater at Kärdla, Hiiumaa Island, Estonia|journal=Tectonophysics|volume=216|issue=1–2|pages=143–156|doi=10.1016/0040-1951(92)90161-X|issn=0040-1951|bibcode=1992Tectp.216..143P}}</ref>
In the [[Ordovician]] (c. 455 million years ago) the sea floor was hit by a [[meteorite]] forming the 4 km wide Kärdla [[
The limestone is overlain by [[Pleistocene]] glacial deposits that were deposited as the ice cap retreated 11 to 12 thousand years ago. These include [[terminal moraine]]s, the two most prominent being one in the South of Island running towards the North-East and another forming the [[Kõpu, Hiiu County|Kõpu]] Peninsula.<ref name=":4" />
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