The Portland beds are essentially marine accumulations. They are exposed only over limited areas in this country, owing to the great Cretaceous overlaps. The thickness of strata, forming the Portland beds and the covering Purbeck layers vary much throughout the Island. In the Verne Ditch there are 20 feet of rubble and cap and 14 feet of roach. In Grove Quarry, according to the memoirs of the Geological Survey, the beds show the following thickness :

the beds are in Waycroft Quarries :-

From the Top bed here Goldsmith's Hall and The Reform Club; London, were built.

The varying thickness of the beds in the different parts of the Island are shown by a section of the two following quarries :-

Maggot Quarry:

Goslin's Quarry:

In the Memoirs of the Geological Survey the following description of the microscopic structure of Portland Base-bed and Whit-bed is given:

" They showed oolitic grains, pellets and organic fragments with some quartz grains. The Matrix was clear crystaline calcite of a granular or mixed nature. The oolitic grains were not clearly defined the concentric structure being apparently obliterated in some cases, (top-bed) and obscure in base-bed. Mr. Huddlestone found, with regard to the flint beds, shell fragments, sponge spicules, and oolitic granules. He concluded that the Portland flints were formed by the replacement of limestone by Silica,"

Dr. Hinde has remarked that from the Portland beds no entire fossil sponges are yet known, but in some chert nodule in the Island of Portland and at Upwey spicules of Pachastrilla antiqua, Moore spicules and Geoditis spicules have been found showing that in part at least the chert of these rocks is derived from sponge remains.

Portland Roach Stove is oolitic and formed of:

Trigonia Gibbosa (horse heads)
Cerithium Portlandicum (screws)
Natica elegans (casts)
Neritoma sinuosa (casts)
Pleurotomaria (casts)
Lucini Portlandica, (casts)

South of Weston village there is an upper brown and lower white roach. North East of Rufus Castle an oblique band of cherts traverses the beds. The cherty series below the Portland building stone consists of buff, shelly and sandy limestones, with cherts or flints in irregular bands and nodules

Beneath the lighthouse cliffs remarkable oblique bands traverse the rocks, appearing like ribs, and these are overlaid by approximately horizontal bands of cherts.

Whitbed is a fine grained oolitic limestone. Its durability may he due to the crystallised carbonate of lime derived from the contained shells. Where curf and flints are absent as at Weston, Whitbed lies on basebed.

The following sections will be of permanent interest.

King Barrow Quarry

Section Of Verne Fort

Rufus Castle

The Lower Portland beds are well exposed on the Western side of the Island as follows :

Southwell Shaft.

The cutting of the Southwell Shaft some twelve years ago gave the following sections :

Mr Whittaker had said that a supply of water was to be expected at the base of the Portland stone where water would naturally be held up by the clay beds that form the top of the Portland sands.

He also thought there must be joints and fissures carrying water to a lower Level.

About 120,000 gallons of water per 24 hours were found. This not being sufficient a bore hole was made with the result that salt water was obtained and the work had to be abandoned.

WE

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