Complex sentence: Close to the source the underlying geology is a thin layer of Fuller's earth clay over Yeovil sands; the resulting light soil made the area important for the production of flax and for market gardening in the past.
Split into simpler sentences: Close to the source ot the river the underlying geology is a thin layer of Fuller's earth clay over Yeovil sands. The resulting light soil made the area important for the production of flax and for market gardening in the past.