Complex sentence: Investigation of the Meare Pool indicates that it was formed by the encroachment of raised peat bogs around it, particularly during the Subatlantic climatic period (1st millennium BC), and core sampling demonstrates that it is filled with at least of detritus mud.
Split into simpler sentences: Investigation of the Meare Pool has indicated that it was formed by the encroachment of raised peat bogs around it, particularly in the Subatlantic climatic period (1st millennium BC). Core sampling has shown that it is filled with at least of detritus mud.