THE MOFFATDALE REGION 96?> 



This one fact of occurrence alone is of the utmost significance, but may 

 not here be dwelt on. In this Moifat series six distinct graptolite zones 

 have been recognized, and they constitute the well known Birkhill divi- 

 sions of the Llandovery. In making a series of detailed sections northwest- 

 southeast across southern Scotland, it was soon found that there was a 

 very regular transition of each black shale band northwestward into 

 barren sandstones and grits, and it was especially interesting to note that 

 as we traced any particular graptolite zone away from the Moifatdale 

 region — east, north, or west — the black shale facies changed to a sand- 

 stone and grit and finally to a conglomerate, and that a concomitant of 

 tliis change was the gradual disappearance of the graptolite fauna and 

 the appearance of worm tracks, of various trails, and "finally of eurypterid 

 fragments, all clear indications of continental deposition by rivers. We 

 have made a large numljer of sections across the southern belt of Scotland 

 at right angles to the strike, and this is generally also approximately at 

 right angles to the old shoreline. The same lithological and faunal re- 

 placement is seen in every section, the sequence holding so true to type 

 indeed that one can predict with only small error or with none at all Just 

 what facies and what graptolites will occur at any given point, provided 

 one already lias two points located at tlie extremities of the section. The 

 acccmipanying map lias l)een made to show the actual migration of the 

 strand-line soutliward ; it is plotted to scale, and the different contour 

 lines represent the last northward appearance of the graptolites in each 

 zone before the shale passes north westAvard into the barren grit and con- 

 glomerate facies. Allowance has been made for the folding which took 

 place at the end of the Siluric and which involved all of these rocks, so 

 tliat they now strike northeast-southwest. A measurement of the sections 

 showed that they were foresliortened at least twice, so that in attempting 

 to represent tlie conditions as they were during the time of deposition of 

 the sediments it has been necessary to project all points twice their present 

 geographical distance apart in a northwest-southeast direction. Thus the 

 scale of the map is true for Lower Siluric time, but it is evident that the 

 position of the various cities located on the map will show a distortion at 

 right angles to the strike, all points being plotted twice as far apart in 

 this direction as they are at present ; the scale on the strike remains the 

 same for the past as for the present. This careful plotting at once brings 

 out the fact that Moft'atdale, far from being in what was once a locus of 

 deep-sea sedimentation, was as a matter of fact in the very center of a 

 small bay or of a lagoon. We must indeed interpret the evidence as point- 

 ing to a deltaic origin of all of the sediments, for the black shales could 

 not have been deposited under even normal near-shore marine conditions, 



LXXI — Bull. Geol. Soc, Am., Vol. 28, 1916 



