324 J. G. Goodchild — Glacial Erosion. 



Whatever theory is proposed to account for the present form of 

 the rock surface in the Yorkshire Dale District must he based upon 

 not only the various classes of facts that are now almost on all hands 

 admitted to be the work of subaerial agencies, but also upon the 

 following points that do not so easily admit of an explanation: — 1. 

 The outcrop of eaoh limestone is weathered nearly equally all over ; 

 and as a rule swallow-holes are found only along its inner margin, 

 although there is occasionally a width of several hundred yards be- 

 tween the inner margin where the swallow-holes are found and the 

 scar that forms the outer edge of the terrace. 2. Between the tribu- 

 tary valleys the scars either extend in nearly straight lines or else 

 sweep in broad convex or concave curves, whose general regularity 

 is only occasionally interrupted by the channel of a small stream 

 from the higher ground. 3. The scars are as often found perfect at 

 elevations of several hundred feet above the bottom of the valley 

 where they occur as are those lower down. In the case of the highest 

 thick limestones of the Yoredale Rocks, known respectively as the 

 Main and Undersett Limestones, the thinness of the intervening beds 

 causes the outcropping scars of the limestones to run in pairs, which 

 often keep the same horizontal distance apart for miles, and thereby 

 render their regularity of form more than usually striking. 4. Where 

 the rocks are much disturbed, the characteristic terraces usually keep 

 to the same bed through all its variations of position and inclination ; 

 so that instances are not wanting in which the same bed forms a 

 terrace rising two hundred, or even four hundred feet within half a 

 mile. 5. Little disintegrated rock from the beds above is commonly 

 found upon the limestone terraces, even where the absence of such 

 debris cannot be accounted for by stream action. 6. And lastly, the 

 terraces and scars developed along the outcrop of each limestone are 

 usually even more perfect than those of the less-easily-weathered 

 sandstones that they are associated with. 



Bearing these points in mind, let us see how far the commonly 

 received theories will apply in the present case. 



As the terraces here referred to are seldom horizontal for any great 

 distance, and sometimes have a slope of even several degrees, it is 

 obvious that their marine origin is quite out of the question. This 

 theory therefore will be passed without further mention. 



The other great class of agents at work developing the surface 

 characteristics of each rock is usually designated Subaerial Denuda- 

 tion. Under this term most writers include also the abrading -work 

 of ice ; but in the present communication Subaerial Denudation will 

 be taken to mean only that kind of alteration of the form of the 

 ground that is effected by the separate or the combined action of the 

 weather and running water ; while the term Glacial Erosion will 

 be used for the abrading work of moving ice. For the present we 

 have to deal only with the effects of Subaerial Denudation upon the 

 particidar kinds of rock that make up the hills of the Dale District. 

 Of these by far the most important rock is limestone, which is found 

 interstratified with the other rocks in bands whose thicknesses range 

 from a few inches to a hundred feet or more. In character most of 



