REPORT OF THE DIRECTOR I914 I45 



bottom of the chasm, where one would expect to get some clew to 

 the character of the underlying strata, the heavy covering of rock 

 debris conceals everything. 



CAUSE OF THE RIFT 



Examination of the accompanying figures will make clear the 

 history of the development of the great rift. The lapping of the 

 Grenville strata against the side of the hard, homogeneous syenite 

 of the mountain mass, with the weaker beds practically in contact 

 with the igneous rock, was decidedly favorable for undermining 

 (figure i) due to removal of the weaker material by erosion or 

 solution, or both combined, by waters coming down from the higher 

 portion of the mountain during heavy rains or melting snows. 

 Instead of the type of undermining indicated in figure i there may 

 have been removal only of certain calcareous layers or possibly a 

 thorough honey-combing of the underlying weaker mass thus greatly 

 weakening the support of the overlying mass. 



Finally the undermining proceeded far enough so that a great 

 block of gneiss, already practically separated by a prominent joint- 

 plane from the ledge of the mountain side, was suddenly pulled over 

 by the force of gravity (probably aided by the wedge-work of ice) 

 as shown in figure 2. This great block, from 600 to 700 feet long 

 and from 100 to 250 feet high, swung through an angle of 60 to 70 

 degrees with greatest subsidence toward the north, thus readily 

 accounting for the marked differences of strike and dip of strata 

 on opposite sides of the rift. It is difThcult to conceive how such 

 an undermining process could have been carried far enough unless 

 we postulate relatively weaker or more soluble rock underlying the 

 Grenville gneisses. 



That the great block fell no less than 75 to 100 years ago is 

 proved by the fact that trees of considerable size have grown within 

 the chasm, while the large accumulation of talus materials suggests 

 a much longer time than 100 years. On the other hand, the rift is 

 certainly Postglacial as indicated by the utter absence of any evi- 

 dence of glaciation within it. 



As a result of a moderate amount of weathering and the sliding 

 of joint-blocks down the steep slopes, the present-day conditions 

 were reached as shown in figure 3. 



The geological principles of undermining etc. here set forth are 

 by no means uncommon, but the development of such a great rift 

 in this manner is somewhat unusual, and it appears to be quite 

 unique in the Adirondack region. 



