These stratigraphical difficulties stem from the variable genesis of the
sediments involved. The stratigraphical column of the Middle Palaeozoic of
Gotland shows an alternation of sediments, typical of deposition in a shallow
sea, bordered by a base-levelled continent. Small-sized epeirogenetic movements
of the sea floor resulted in very marked differences in the facies of
the sediments (marlstone, limestone, oolite, sandstone). Moreover, in several
localities in Gotland, the succession of sediments which is normal for an
epicontinental basin, is overshadowed by the occurrence of reefs. These
reefs formed protruding elevations on the s e a bottom and influenced the process
of sedimentation. This caused a number of local differences in the
facies of the deposits, even within a certain bathymetrical zone. Besides,
each of the several facies may contain a fauna largely typical of the special
environment, more s o than of the time of deposition, making stratigraphical
work even more difficult.