The Middle Jurassic rocks of Yorkshire : A petrological and palaeogeographical study

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Abstract
I. I troduction I n Yorkshire the rocks of Middle Jurassic age consist largely of non-marine sandstones and shales usually spoken of as the Estuarine beds. Middle Jurassic times commenced and concluded with marine conditions and there were three marine invasions during the period. In these invasions the sea apparently did not extend over the whole area; hence, although the marine beds divide the Estuarine beds into as many as four divisions, in some places two or three of these divisions merge into one (Table I). In the present paper the Cornbrash is not dealt with, and the Dogger is referred to only in so far as is necessary for an understanding of the sediments above it. Although the term Middle Jurassic is usually understood to include these beds, I have ventured, for want of a better term, to use it in this paper so as to imply the exclusion of them. Where precision of statement is called for, the contractions shown in Table I will be used. The Estuarine beds with their marine divisions attain a maximum thickness of nearly 700 feet in the neighbourhood of Ravenscar. Their outcrop on the " solid " map (Geological Survey one-inch map, sheets 33, 34, 35, 42, 43, 44, 52, 53, 54, 63, 72) covers about 380 square miles; but as the outcrop is very irregular in shape, the area over which reasonably safe conclusions as to the conditions of deposition can be drawn is not less than 1500 square miles. Stratigraphy and palaeogeographical

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