546 PRINCIPLES OF STRATIGRAPHY 



cave scars in the mountainside and accumulate in convex masses 

 in the valley bottoms. 



Such a rock fall in one of the upper Ganges branches in the 

 Himalayas brought down 800,000.000 tons of rock in three days, 

 tilling the narrow valley to a depth of a thousand feet with debris. 

 (See ante, p. 127.) The rock fall here was facilitated by the loos- 

 ening and undermining of the sloping strata of rock. 



A remarkable case of rock fall occurred in the little town of 

 Frank, Alberta, Canada, in 1903. where the Cretacic sandstones and 

 shale of Turtle Mountain suddenly gave way and fell into the val- 

 ley below, forming a huge heap of debris, which spread out in the 

 valley bottom. The cause of the slide was the loosening of the mass 

 through coal-mining operations. 



In 1 88 1 a great rock fall occurred in the village of Elm in 

 Switzerland. Loosened by undermining in a slate quarry, a huge 

 mass of rock broke away on the mountainside and fell onto the 

 plateau in which the quarry was situated. Striking this surface, 

 the mass broke, and numerous large fragments were hurled by the 

 impact through the air. and fell into the valley below, where they 

 formed a rock stream which in all essentials resembles that of the 

 Alaskan region above described. (Heim-20.) 



In both of the above-mentioned cases of rock falls or slides the 

 fragments showed percussion marks, while chips were sometimes 

 split off as with a hammer. The recent extensive rock slides of the 

 Panama Canal are other examples of such phenomena. 



Rock and Soil Slides Started by Earthquakes. Slides are not in- 

 frequently started by earthquakes, especially where slopes are steep 

 and the material more or less water-soaked. As a result of the 

 earthquake of November 16, 191 1, a portion of the shores of the 

 Lake of Constance (Bodensee) sank into the sea. The lake border 

 (Seehalde) was deformed for a total length of about 16 km.; on 

 the left for a distance of 10 km. to the extent of 3^ km., and on 

 the right shore for a distance of 6 km. length to the amount of 

 about lyz km. The edge of the sublacustrine shelf was locally 

 translated 2.4 to 18 m. lakeward, the point of abrupt descent 

 ( Absturztiefe) was moved between 1.4 to 8 m., the elevations on the 

 border of the break rose from 0.2 to 2 m. In the expansion about 

 9,000 cubic meters of rock and soil material were moved. 



Ancient Examples of Rock Streams and Slides. 



Many brecciated rock masses occurring in older geological for- 

 mations, of which the explanation is in doubt, may here find an 



