LANDSLIDES AND ROCK AVALANCHES 



281 



substance that has slowly flowed down 

 from the cliffs at the back of the cirque 

 and gradually extended to the outer edge 

 of the basin. The singular billowy sur- 

 face and the curved, often concentric 

 lines near the front or foot of the mass, 

 and which closely resemble those caused 

 by the cooling of lava streams, strongly 

 add to the appearance of slow move- 

 ment." 



Scores of such rock streams can be 

 seen in the San Juan Alountains. An 

 interesting variation is what is termed 

 by Doctor Cross "Slumgullion mud 

 flow," which dammed a fork of the 

 Gunnison River and formed Lake San 

 Cristobal. At the head of a tributary 

 of Slumgullion Gulch, 11,500 feet alti- 

 tude, certain rocks had been decomposed 

 into a soft, crumbling sand, underlying, 

 however, other rock masses. In time, 

 perhaps during some abnormally wet 

 season, this incoherent decomposed ma- 

 terial became so extensively softened as 

 to be unable to bear the load of rock 

 above, and gave way ; the overlying 

 rocks broke into fragments, and the 

 whole mass of mud and rock fragments 

 rushed as a flow down the lateral gulch 

 to the main vSlumgullion Gulch, and 

 down that to the Lake Fork, six miles 

 from the place of starting. On reaching 

 the Lake Fork, whose course is here at 

 right angles to Slumgullion, the flow 

 turned north and ended about three- 

 fourths of a mile below the mouth of the 

 Slumgullion. The volume was suffi- 

 cient to dam the main stream and to 

 cause the formation of Lake San Cristo- 

 bal, which now extends for nearly two 

 miles up the Lake Fork Valley. The 

 end of the flow is at about 8.900 feet 

 altitude, 2.600 feet lower than its start- 

 ing point. A sparse forest growth on 

 the surface of the flow shows that the 

 flow occurred manv rears ago. On the 

 upper part of the flow the trees are in 

 many places overturned or tilted at 

 various angles, testifying to recent move- 

 ment. 



RICCENT LANDSLIDES IN THE SAN JUAN 



While the great landslides of the San 

 Juan region are doubtless a thing of the 



past, the recent disaster at Frank shows- 

 that xA.merican mountains are not en- 

 tirely trustworthy, and in the case of the 

 San Juan there has been an actual 

 though not very tremendous landslip- 

 within the jiresent generation. How- 

 ever, had a city been located upon the 

 three square miles of disturbed area, the 

 movement was sufficient to have ruined 

 it as effectually as did earthquake and 

 fire the cities of San Francisco and Mes- 

 sina. 



Late in July, 1886, there appeared in 

 the Denver newspapers a report that an 

 earthquake had occurred in the Cimar- 

 ron Creek Valley. A few days later 

 Dr Whitman Cros's, of the U. S. Geolog- 

 ical Survey, accompanied by a photogra- 

 pher, visited the area. The scene of the 

 so-called earthquake was a well-timbered 

 basin, and evidences of such disturbances 

 as had been described were everywhere 

 visible. In some parts nearly all the 

 trees were overturned ; in others they 

 stood at various angles, presenting a 

 weird picture. In places were bare 

 slopes, presenting a fissured and step- 

 like structure as if from the dropping 

 down of successive sections of the earth. 

 The movement as described by Cross 

 was a downward sliding of the whole 

 surface, unequal in different places. The 

 impression produced was that a sliding 

 or almost a flowing movement had taken 

 place, involving the whole area of some 

 three square miles. The mischievous 

 agent which had produced the result was 

 unquestionably water. Mud streams 

 were here and there found in which tree 

 trunks and rocks were embedded, while 

 columns and mounds of moist earth were 

 pressed up through cracks by movements 

 of some part of the mass. This appears 

 to have been more in the nature of a sur- 

 face soilslip than a rock or landslide. 



In another localit}- in the San Juan, 

 namely, the C. H. C. 1 lill, near the town 

 of Rico, progressive slipping is actually 

 in eff'ect at this time. At one jioint the 

 stump of a tree has been split open since 

 the tree was felled, and the two portions, 

 have separated about five feet in a period 

 of four years. The crack was traced for 

 some hundreds of feet. It is suggested 



