Complex sentence: Traditionally, land drains were formed in clay soils and peats by excavating a trench and forming a "tunnel" using flat stones, this was very labour intensive but could be often be done using free materials at hand.
Split into simpler sentences: Traditionally, land drains were formed in clay soils and peats by excavating a trench and forming a "tunnel" using flat stones. This was very labour intensive but could be often be done using free materials at hand.