Complex sentence: There was a concern that the "megadunes" characteristic of this area may shift, burying the railway; but geological research indicated that the "megadunes" are mostly formed by solid subsoil, rather than just sand.
Split into simpler sentences: There was a concern that the "megadunes" characteristic of this area may shift, burying the railway. However, geological research indicated that the "megadunes" are mostly formed by solid subsoil, rather than just sand.