The picture above belongs to a Silurian outcrop near Checa, a locality in the province of Guadalajara (Spain). In the image can be distinguished two blocks of rock (quarcite) included in slates, these two blocks are dropstones, clasts released by melting from the base of a glacier falling in the muddy sediment (the slates we can observe) of the sea.
In the slates can be observed fossils, the ones highlighted in the circles. These fossils are graptolites, colonial animals that extends from the Middle Cambian (520 m.y. aprox.) to the Lower Devonian where they are extinct (390 m.y aprox.). They lived in marine environment in a wide range of conditions: shallow waters, open shelf and ramp. These fossils are relevant in stratigraphy since the evolution and characteristics lets to determine the age of the rocks in which they appears, specially in the Ordovician (500-435 m.y). The group on the left circle is Monograptus, a group found from the Silurian to the Devonian, although the presence of Gliptogratidae (the picture on the right circle) lets to determine more exactly the age for these deposits, since this group extends from the Ordovician (Llandeilo) to the Lower Silurian, that is, Monogratus and Gliptogratidae coexists during the Lower Silurian, in this way the serie can be included under the the Lower Silurian (435-430 m.y.).
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