These are the remains of quarry workings near the cliffs off the south west of the Island. Rock from the quarry at La Rosiere was used in the construction of the Victoria Embankment in London in the 1860's. This is what is left of the stone crusher shed where rocks were crushed, sifted and loaded onto railway trucks. This was the original terminus of the railway from St Helier, trains reached here in 1884, with the line not being extended to Corbiere until 1899.
Very little is visible from the sea as the photograph in this posting shows.