114 



M.K. HOWARTH 



lower horizons down as far as bed 500 and those of bed 525 are 

 especially conspicuous, pyrites is very common at most horizons up 

 to bed 526.7 but does not occur higher. So the strong sideritic 

 mudstone of bed 527 is a good base for this member. Such sideritic 

 mudstone nodules and occasional continuous beds are a feature of 

 the whole thickness of the member up to the base of the Staithes 

 Sandstone Formation. Several of them form prominent features on 

 the scars, eg. beds 527, 531, 543, 545, 559 and 560.2 (a distinctive 

 pair of nodule beds), 569, 578.4 and 589. 



Pyritous Shale Member (beds 497-526; 26. 1 8 m thick). Consists 

 of dark grey, soft and micaceous shales, with many calcareous and/ 

 or sideritic mudstone nodules; there are many nodules or irregular 

 masses of iron pyrites, especially in the lower part. This member is 

 similar to the Ironstone Shale Member in containing both calcareous 

 and sideritic mudstone nodules, but it also contains much iron pyrites 

 as irregular nodules or masses of crystals. 



The base is defined here as the bottom of bed 497 in Robin Hood*s 

 Bay. This is the horizon at which the sand and silt content almost 

 disappears and calcification is much diminished, leaving the beds 

 above as softer shales. Iron pyrites is common at many horizons, 

 appearing variously as irregular masses or strings of pyrites, or pyrite- 

 rich concretions, and many of the fossils are partly pyritized in these 

 beds. The softness of the shale leads to this part of the succession 

 forming the wettest and lowest part of the bay relative to sea level. 



Siliceous Shale Member (beds 447-496; 38.74 m thick). Dark- 

 grey shales, interbedded with much harder and lighter-coloured beds 

 of calcified mudstones, silts and fine sandstones; a few nodules and 

 doggers of calcareous or sideritic mudstone occur. This member 

 forms the series of hard calcified beds alternating with soft shales 

 that is typical of the lower part of the succession in Robin Hood's 

 Bay. Red-weathering sideritic nodules are now scarce, and most of 

 the harder beds are calcareous cemented muds and silts, in which the 

 arenaceous content is higher than in the underlying Calcareous Shale 

 Member. Large, circular 'cheese' doggers of hard argillaceous lime- 

 stone, containing vertically orientated cystals of calcite, occur at 

 several horizons in both this member and the Calcareous Shale 

 Member below. Such doggers are generally up to only about 10 cm 

 thick, but they can reach 2.5 m in diameter; they occur at 7 levels 

 between beds 450 and 47 1 . 



The base is defined here at the bottom of bed 447 (Low Scar) in 

 Robin Hood's Bay. This is the first prominent bed of very hard 

 calcified shale or argillaceous limestone that has a significant sand 

 content. Sand and silt occur in many of the hard calcified shales or 

 sandstones at horizons up to the base of the Pyritous Shale Member. 

 Most of the prominent 'scars' in the bay are formed of beds in this 

 member - ie. Low Scar, Middle Scar (Gryphaea Scar), High Scar 

 (Lower Triplet), Double Band (Cowling Scar and Billet Scar). Upper 

 Triplet, East Scar and Landing Scar. 



Calcareous Shale Member (beds 41 8-446; 23.35 m thick). Dark- 

 grey shales, interbedded with hard, calcified, silty mudstones; doggers 

 of calcareous mudstone and some beds of limestone also occur; 

 cone-in-cone enveloped calcareous mudstone nodules also occur at 

 several horizons. This member is less arenaceous than the Siliceous 

 Shale Member, the hard beds now having no sand and less silt, and 

 the hardness being due mainly to calcification. The very prominent 

 Low Balk and the less prominent Pseudo Low Balk are formed by 

 limestones or highly calcified shales. Large 'cheese' doggers, simi- 

 lar to those in the Siliceous Shale Member, are found at three levels 

 in beds 425^129. 



In Robin Hood's Bay the base has to be placed at the lowest 



horizon exposed, ie. at the base of bed 418. If it is thought that the 

 base should coincide with the base of the Redcar Mudstone Forma- 

 tion, then it must be defined at the same level (ie. 288.87 m depth) in 

 the BGS Felixkirk Borehole (Cox etal. 1998: 35). 



EXPOSURES IN ROBIN HOODS BAY NOW 



The extent of the foreshore exposures of the solid geology on the 

 north Yorkshire coast has always depended on the vagaries of shift- 

 ing sand and boulder cover, algal growth, barnacle growth, and major 

 cliff falls, all caused or cleared away by the actions of tides and 

 storms. But in the last 25 years much more extensive, and possibly 

 more permanent, sand, boulder, algal and barnacle cover, and mussel 

 beds have made major inroads into the amount of rock exposed in 

 some areas. Especially serious on some of the scars are mussel beds 

 that trap mud and silt to form a thick, impenetrable cover that 

 completely obscures the rock underneath. At the end of the 1 990s the 

 foreshore exposures were largely obscured from Way Foot at the 

 bottom of Robin Hood's Bay Town northwards to just south of 

 Dungeon Hole. In fact there are few or no exposures of beds 497-525 

 owing to the sand and seaweed cover, which has possibly been 

 exacerbated by the high concrete seawall built to protect Robin 

 Hood's Bay in 1975. That seawall covers the cliff face of the same 

 beds, so that they are not now exposed in either the cliff face or on the 

 foreshore. Exposures improve upwards from bed 526, especially 

 north of the Dungeon Hole fault, though there is still much algal 

 growth and large areas are covered by loose boulders. Around the 

 north side of the bay in the top half of Map 1 (Fig. 5) the foreshore 

 continues to be washed clean by tides and storms and exposures are 

 still good. Exposures seaward and south of Boggle Hole (Map 3; Fig. 

 8) are also better, and they are good in front of Peter White Cliff (Map 

 4; Fig. 1 1 ). The lowest beds on the latter map, especially from bed 

 430 down to below Low Balk have always suffered from algal cover, 

 of which Laminaria is a significant factor at those low sea-levels, but 

 barnacle growth is also very pronounced and makes observations 

 difficult on some beds. 



For most of the past two centuries the foreshore of Robin Hood's 

 Bay has been largely clear of such cover, and collectors from Young 

 & Bird in the 1 820s, Phillips, Simpson, Tate & Blake, the Geological 

 Survey in 1880-1910, up to Bairstow in the period from 1928 to the 

 1950s (see Fig. 7) were able to make significant fossil collections 

 from all the beds. In particular, most of them obtained large speci- 

 mens of Apoderoceras from the Taylori Subzone of beds 501 to 526. 

 No such specimens can be collected today. The foreshore was largely 

 clean in 1969 when Bairstow conducted a field party from the 

 William Smith Jurassic Symposium to the Bay, but deterioration 

 proceeded rapidly from the early 1970s. Bairstow's work could not 

 be repeated today, at least for the beds on Map 2 (Fig. 6) from Robin 

 Hood's Bay town northwards to the top of that map. 



CORRELATION WITH PREVIOUS 

 DESCRIPTIONS 



The Lower Lias of Robin Hood's Bay was mentioned by Young & 

 Bird (1822, 1828) and Phillips (1829, 1875), and a few ammonites 

 from the bay were figured by them, but their descriptions were not in 

 sufficient detail to be correlated with the work in this paper. Prior to 

 Bairstow's work, detailed descriptions were published by Simpson 

 (1868, 1884), Tate & Blake (1876) and Buckman (1915). all of 

 whom numbered their beds from the top downwards. After Bairstow 

 prepared his maps and stratigraphical descriptions, detailed accounts 



