SEDIMENTATION. 17 



and bowlderets found in the valley drift to ice floes. No doubt ice floes 

 often deposited such stones, as well as large bowlders, but I have lately 

 made some observations in Colorado which show that large stones, and even 

 bowlders, may be deposited by water upon and within sand. I have 

 examined the track of several so-called cloudbursts soon after they occurred. 

 Near the centers of these violent thunderstorms a fall of 6 or 8 inches of 

 rain and hail is not unusual. This great precipitation takes place within a 

 few hours, sometimes within a few minutes. The rain water soon collects 

 on the lower slopes, fills the beds of the streams, and then covers their 

 flood plains to a depth of several feet, sometimes overwhelming a broad 

 prairie. As the waters flow down the hillsides the hail is rolled along in 

 front as a sort of moving dam several feet high. Here and there the waters 

 break through this dam and shoot with great velocity down ithe slopes of 

 the prairie, soon to be stopped again by the hail. In this way the waters 

 are soon concentrated and confined within channels varying from 10 feet to 

 several hundi'ed feet in breadth, bordered by walls or dams of hail from 1 

 to 4 feet high. 



Dm'ing one of these floods in El Paso County the flow was so rapid as 

 to transport slabs of sandstone 4 feet square and 2 feet thick. These 

 bowlders were iron-cemented and heavier than ordinary sandstone. The 

 velocity of the current must have been 10 miles or more per hour. In 

 narrow ravines of erosion (washes or arroyos) the erosion was verj'' great. 

 Blocks of clay were undermined and rolled along in the boiling torrent 

 until they were nearly round. A stream 200 to 300 feet wide, and about 

 20 feet in depth at the deepest place, issued from the mouth of a narrow 

 valley at Templetons Gap, near Colorado Springs. It became somewhat 

 wider as it entered the broad open plain, yet for one-third of a mile it was 

 swift enough to transport the bowlders above mentioned. Previous to the 

 flood the plain at this point was composed of sand loosely grassed over. 

 The bowlders were dropped upon the sand plain, which was but little 

 eroded by the swift currents. Then as the flow slackened, sand was depos- 

 ited upon and around the bowlders to the depth of fi-om 1 to 3 feet. The 

 geologist of the future will find the bowlders surrounded on all sides by 

 stratified sand. Before I saw and studied these cases it would have seemed 

 to me impossible that water could have deposited fine sand and large 

 bowlders in juxtaposition in this way. Two or three miles farther down on 



MOJNf XXXIV 2 



