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Portland Base Bed

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Local Name(s): Portland Best Bed,
Portland Bottom Bed,
Portland White Bed,
Portland Base Bed. 		Example of Portland Base Bed
Stone Group: Sedimentary
Stone Type: Limestone
Geology:
Era:   Mesozoic
Period:   Jurassic
Epoch:   Upper
Geological Sub-Divisons:

Portland Formation, Portland Stone Member (Portland Stone). Three main beds are important:- Roach (normally top of the series), Whit Bed (middle) and Base Bed (lower).
General Colour Description:

A whitish brown, creamy/grey colour when first quarried it becomes paler in colour (white) when exposed to the atmosphere. It is slightly lighter in colour than Portland Whit Bed.
Hand Specimen Description:

Fine grained oolitic limestone with an even, close texture and little shell content. It is not so markedly oolitic in appearance as Portland Whit Bed and contains less shell.
Petrographic Description:

A pure, fine grained oolite (95% calcium carbonate, 1% quartz), it is formed from micrite ooids with a small quantity of micrite occuring as the matrix. The shell fragments are elongated to rounded in shape and can be up to 5mm across. The small shell fragments comprise oysters, bivalves and alga.
General Comments:

In use, both Portland Whit Bed and Portland Base Bed can be indistinguishable.

Stone Identifiers: Fossiliferous,  Oolitic.
Reacts dilute to HCl? Yes