The present account is based on the geological survey, of 110 oquare miles of dissected, upland country developed in the, p2, E, H, and RI faunal divisions of the Carboniferous system and located in the S. E. of the Aekrigg Block in the Pennine Chain of Yorkshire (refer to Plate 2). The succession embraces approximately 1750' of strata both of Yoredale (limestone, shale, sandstone) -and 'of Millstone Grit facies (sandstones and shales). The sediments of Yoredale facies comprise eight rhythmic units, or cyclothems, five of which are of lower Carboniferous age. All remaining beds are of upper Carboniferous age. They ý include the three highest Yoredale cyclothems and rocks of Millstone Grit faciese The S,im onstone cyclothem varies greatly. in thickness'from north to south, this beine due to n, thinning of the shäles and sandstones. The-- Middle, cyclothem. is characterised by great local thickening of the Middle Limestone. This latter-horizon shows a tripartite division throughout the -area, with a central part carrying abundant Gia ntorroductus. A'less persistent band with, 0rionastraea, occurs at several localities near the base of the limestone. The-measures above the. Middle Limestone are everywhere thin and in the south of the area yield an extensive normal shale fauna, but are replaced by sandstones in the north which may however belong to -the Five-Yard or possibly the Three Yard cyclothem. Owing to the absence of the Five Yard Limestone in the north of the area no full proof, of the age of, these latter beds is forthcoming.The Five' Yard L1mest"one is restricted to the döü. th° of the area. Its lower Leaf is characteri'sed' by a Lithostrot ion- Gipontopi ductus fauna in upper Coverdole and upper ialdendale. The 3h231es above the 1imestorie are 'cönfined to the söütli of the area and carry a large fauna which includes several new species and s 'new genus of ostracod. ' Sudýeticerns ordinetüm, Moore, a P2 goniatite, is also recorded. The Three Yard Limestone consists of two leaves, an upper persistent leaf and a lower impersistent one. The measures above the Three Yard Limestone are chiefly : andstönes in they north of the area, but shales dominate in the extreme south. Revised correlations of the beds at thekThree Yard-Five Yard Limestone level are put forward for the - outcrops of these beds in upper Coverdale 'and Nidderdale on the bäs'is of the- present work, 'using -the findings of Moore (1955) as essential evidence. The Underset Lim°stoneý-varies considerably ' in' thickness and carries within it a coral bed in' the extreme P1. W. of the area. Local 1d1e velopments of chert above`nnd below the limestone and of calcareous shales bel6%v the main leaf of the limestone are detailed. The sediments above the limestone are everywhere thick, except in the N. E. "of the area, where they thin abruptly. ' ? Cravenoceras sp. has been recorded about 25' above the top of the Ünderset Limestone, providing confirmatory evidence for placing the baue of the Pendleicn eboüt 10` Above the top of `this limestone. The Main Limestone varies jqr`eatly in`thickness, possibly owing to overstep at the base of the Little Limestone which is overlain by the Richmond Chert Series. These latter beds are extremely, thick in the'N. of the area. Axtreassossment of the chert problem is presented in the light of the field evidence from the present-ground. A cane is put for local chertification below the plane-of the intra--Eli unconformity in upper Nidderdale. The -Crow cyclothem- is, recognised in the north of the present area. ' It is overlapped by-the sub-Grassington Grit (intra-E1) unconformity, which transgresses southwards across the sediments of 'the upper four cyclothems , in the ý area. A synopsis of present knowledge relating to the. antra-Ei unconformity, is given and the natureiof the overstep in the present area is elucidated by means of-a map and sections. The, Grassington Grit Group is shown to°alter laterally into a serien- of shales with impersistent sandstones ` in,,, the north of the: area. Cravenoceras cowlinaense is first recorded"inloitu from the area and is described from several localities together with a-large supporting-fauna. A widespread distribution of the Cockhill Marine Band is demonstrated and C. cowlin aense is recorded from--the extreme north of the area. By means of these occurrences a'nörthwards thinning of the Grassington Grits and shales is "shown to occur, a process of thinning which is-shown to be continued farther to the N. W. in the 'work of Scanlon-}(1955). 0 Sediments of lovier Arnsbergian age are the Nidderdsle shales with local impersistent sandstones and the, -Red Scar Grit with an imperaistent coal, the variations of which are treated in detail. The Red Scar Grit develops ,a fossilif erous phase in the upper leaf in the north of-the ground tind a fauna is also recorded from the shales with the, Woogill Coal. The Colaterdale Marine Series consist of shales with a thin limestone characterised by Cravenoceratoides nitidus (Phillips), overlain and underlain by shales with Anthracoceras au cilobum (Phillips) and Dimor-phoceras sp. together with a large additional fauna. A local thickening of the beds beneath the Colsterdale Limestone accompanied by the development of a nuculid-gastropod phase occurs in the north, in Colsterdale. Diagrams and text with full details of the faunal divisions and changes at this level are given. The Nar Hill Beds and the Lower Follifoot Grit, also of Arnsbergian age, consist of variable sandstones with shales prominently developed in the former unit. Beds at these levels have been re-named-following the discovery of Homoceras bevrichianum (Haug) and Homoceras aff. subplobosun (Bisat) in the Ganister Beds. The Ganister Beds are the only sediments of Sabdenian age and include the above mentioned fossils which constitute the first record of H zone goniatites on the Askrigg Block. The Cayton Gill Shell Bed yields an extensive suite of fossils and is overlain by the Agill Sandstone which has yielded the only extant Upper Carboniferous ophiuroid. The Libishaw Sandstone and Capelshaw Shales are locally fossiliferous. New correlations of sediments at the level of these R1 shell beds are put forward. The massive Brimham Grits are separated by shales which yield a band with Lim sp. The beds dip in an easterly direction at about ,2 degrees and though no continuous mesh of faults occurs, they are locally abundant. Aa-analysis "ofstructural trends is given together with a structure contour map. The margins of the drift and periglacial phenomena associated with the Main Dales Glaciation are described and include new records of overflow channels. A synthesis of work on fossils js iprovided, Nvith , comments , on the zonal, use of several forms, palaeo-ecology and techniques of separation of specimens. The chief rock types are examined from. the genetic and descriptive standpoints. Comparison , is made with the . conclusions of other authors relating to ,. the genesis of . sediments of the Yoredale and Millstone Grit facies. _ Accounts of previous workings for coal, lead, etc.., are given together with a statement-of the problems which have beset dam builders in the past when constructing reservoirs, in the area.