THE GREAT MARLBOROUGH CONGLOMERATE 



353 



Fig. 3. — Part of McKay's section 

 in the Mead River, showing the junc- 

 tion between the Grey Marl (2), and 

 the Great Marlborough Conglom- 

 erate (i). 



weaker rock. The junction between the Grey Marl and the con- 

 glomerate passes as a conspicuous line up the sides of this basin, 

 and, since the face on which the section is exposed runs for some 

 distance more nearly parallel to the strike than to the dip, the 

 effect is to make the dip of the ^ 

 junction, when viewed from down- 

 stream, or from the center of the 

 hollow, appear much more nearly 

 horizontal than it really is. The 

 apparent dip is, owing to the 

 absence of a hard line of division 

 between the Grey Marl and Amuri 

 Limestone, compared by the eye 

 with the true dip of the limestone outcropping in the monoclinal 

 ridge (see Fig. 2), and there is thus produced the effect which was 



sketched by McKay 

 as shown in Fig. 3.^ 

 Fig. 4, however, which 

 is a photograph looking 

 along the strike of the 

 junction, shows that 

 there is no appreciable 

 discordance of dip. 



2. Although the 

 surface of junction be- 

 tween the Grey Marl 

 and the conglomerate 

 was originally plane, it 

 is now broken by a 

 number of small faults, 

 each with a downthrow of only a few feet, which appear to be 

 tension faults formed while the conglomerate and the underlying 

 strata were in their original horizontal position. These, collectively, 

 have let down, by trough-faulting, a wedge of conglomerate into 

 the Grey Marl, giving the surface of junction a somewhat undu- 

 lating form shown in Fig. 4, in which the faults can be seen. 



' After McKay, op. cit. (1886), section p. 94; see also Hector, op. cii., p. xxxvi. 



Fig. 4. — -View of the junction between the Grey 

 Marl and Great Marlborough Conglomerate, looking 

 south-southwest across the Mead River. 



