Known as chalk marl, or blue chalk, it is soft enough to scratch with a fingernail but holds together until the tunnel linings are pressed into place.

On the English side a marshalling area was below the top of Shakespeare Cliff, and the New Austrian Tunnelling method (NATM) was first applied in the chalk marl here.

While on the English side the chalk marl ran along the entire length of the tunnel, on the French side a length of had variable and difficult geology.

The park was created by using 4.9 million cubic metres of chalk marl from the Channel Tunnel excavations and is found at the bottom of a section of the White Cliffs of Dover.

Above her was the metal skin of the train, the prefabricated concrete lining of the tunnel, about 45 yards of chalk marl seabed, and the English Channel.

The chalk marl showed no signs of failing or collapsing but colliery arches were installed as a precaution.

As a result the castle moats had to be cut over 10 metres deep into the layer of chalk marl.

The channel floor is formed of a relatively soft variety of limestone called chalk marl, which Mr. Kirkland said "is beautiful stuff to cut."

There, the chalk marl plunges away from the tunnel path, leaving the machines to slice through five kilometers of a highly fractured chalk laden with water under pressure.

The layer is divided into white chalk, grey chalk and chalk marl, the last being virtually impermeable and highly suitable for mechanical cutting.