214 THE EVOLUTION OF DERBYSHIRE SCENERY. 



cohesion is very slight, the shales being remarkably friable, and the 

 flaky fragments slipping over each other with readiness. It was 

 explained in last year's Journal^' how the pressure of overlying 

 gritstones had produced flexures in the Yoredales of the Amber 

 Valley. In the present cases a somewhat different effect has 

 been produced. The shales have resembled in their behaviour 

 a viscous substance. The pressure of the overlying mass has 

 caused them to be squeezed out towards the valley, where 

 there is but sUght resistance laterally, and no overlying 

 mass. If it is difficult to conceive how this could be brought 

 about ; let it be borne in mind that much of the water falling on 

 to the plateau would find its way down to the Yoredale shale, 

 permeating and lubricating the upper laminte. Now the over- 

 lying rocks are of a broken character, being thinly bedded and 

 divided vertically by joints. Accordingly, as the shales are 

 squeezed out from below, they will tend to carry with them — 

 float out, as it were — portions of the overlying strata. But the 

 separation from the parent rock depends also upon another pro- 

 cess. It must not be overlooked that the slight dip would tend 

 to direct the water sinking through the plateau south of the valley, 

 northwards into the latter, in preference to the lower valleys to the 

 south. Hence the shale, as it approaches the valley, would be 

 more mobile, and hence more susceptible to being squeezed out. 

 But, where this squeezing out takes place, the shale will become 

 correspondingly thinner, and, as a consequence, the overlying 

 rocks will unequally sink those portions next the valley at a 

 greater rate than those behind them. This unequal subsidence 

 will obviously aid the fracturing. The process would be accelerated 

 Avhen once begun by the lateral thrust exerted by the " creep " 

 produced on the shale between the detached mass and the parent 

 rock, and by the talus falling into the crevasse from the sides ; also 

 by the increased action of water finding its way in large quantities 

 behind the detached mass 'and thus soaking the semi-solid mass 



* " On Some Contorted Strata in the Yoredale Rocks, near Ashover." By 

 John Ward. 



