Complex sentence: There are plenty of prehistoric monuments (especially in its borders) and grazing has been long-practised (as shown by the "seles" presence), but the most significant one is the usefulness given to the river by the blast furnaces.
Split into simpler sentences: There are plenty of prehistoric monuments (especially in its borders) and grazing has been long-practised (as shown by the "seles" presence). The most significant one is the usefulness given to the river by the blast furnaces.