"SLIDES" IN THE CONEMAUGH FORMATION 349 



on one side with the slope. This form may be so numerous as to 

 give the whole surface of such a slope a peculiar columnar structure. 

 Both kinds of hoodoos are evanescent features. 



ICE PINNACLES 



These forms are apt to coincide with the melting of snows on 

 steeply sloping fresh slide surfaces, though it does not appear 

 that slope is always necessary. They are tapering columns of 

 ice a few inches in length, coalesced toward their bases, the 

 columns extending with considerable variation at right angles to 

 the surface on which they rest. While the individual pinnacles 

 tend to be straight, many are curved and hooked in curious fashions. 

 They are of principal interest in connection with this paper in 

 that they as well as other ice masses permit the ready transmission 

 of Hght to the frozen ground beneath where it is absorbed as heat, 

 resulting in a superficial thawing of the surface and a melting of 

 the underside of the ice. No longer properly secured these ice 

 masses, with any enmeshed mud, break off and slide down. This 

 also encourages movement in the thawing muds beneath. 



TOTALITY OF EFFECTS 

 GENERAL TOPOGRAPmC EFFECT 



Taken in the large the topography is little different from a 

 t>rpical area in maturity, though broad hillsides sometimes show 

 huge re-entrants or hummocky surfaces with frequent small terraces. 

 Outcrops of resistant rock frequently indicate their position by 

 steeper slopes even when slide covered. 



A number of relatively large discontinuous terraces mark old 

 positions of the Monongahela and tributary streams. These 

 stream terraces may have some association with shde movements. 

 In fresh cuts evidences of stratification, while not infrequent, are 

 less common than till-like coverings. It is tentatively suggested 

 that since the lowering of the streams, movements within the terrace 

 deposits and the addition of slides from the hills may account for 

 the heterogeneous masses so often found. 



Where old valleys of somewhat shallow depth are practically 

 confined to shales, the slopes are more mature, as the sliding 



