St. Catherine's Point

Question 1

Where is St. Catherine’s Point situated?

Answer 1

St. Catherine’s lighthouse is located at the most southerly point of the island at the Western end of the Undercliff.

Question 2

Describe the geological make up of the ground at St. Catherine’s point and the formation of strata.

Answer 2

The various layers of rock are formed in layers of strata which are predominantly horizontal and only have a slight dip of between 1 and 1.5 degrees to seaward. The geology is built up of the Ferruginous sands of the lower greensand, layers of sandrock, gault clay, the upper greensand and the lower chalk at the top of the formation.

Question 3

Describe the types of geological failures occurring at St. Catherine’s Point.

Answer 3

The main visible failures are the various mudslies in the area. There are 9 mudslides within the St. Catherine’s Point area which rest upon a deep-seated compound slide. This compound slide consists of three main failures:

1. A block at the rear of the slide which has subsided and rotated
2. A block slide controlled by bedding
3. A frontal apron of debris that meets and is eroded by the sea. The front of this apron is also subject to rotational slip mechanisms.

Question 4

What are the main signs that there is ground movement when inspecting the lighthouse?

Answer 4

Damage to the boundary walls of the lighthouse is very apparent with cracking and various misalignments being the main features.

Question 5

What kind and magnitude of movements is the lighthouse experiencing at St. Catherine’s Point?

Answer 5

The lighthouse is moving translationally at a rate of 10-20mm per year caused by the main slide and apron.
Additional translational movement occurrs due to the movement from the rotational fialures at the toe of the debris apon.
Tilts of approximately 15” of arc
The movements are also affected by rainfall and coastal erosion.