APRIL 26, 1912] 
grading of stream-beds. They are close com- 
panions and they are among the chief causes 
of degradation in any region, being more 
noticeable in mountainous semi-arid regions 
because inequalities of stream flow are greater 
in such regions. 
Every water course (or glacier course) is 
subject to cross-cutting, on a greater or 
smaller scale according to conditions. In 
semi-arid lands it is often violent. 
Take a stream flowing in a semi-arid re- 
gion over the surface of a sedimentary deposit 
of say 20 feet in depth. This stream bed ter- 
minates in some other stream bed which may 
be dry most of the time, the water of the first 
evaporating or seeping away before the junc- 
tion is reached. For some reason this master- 
stream corrodes its bed more rapidly than the 
tributary—by sudden flood, perhaps, in which 
the tributary does not share. The tributary 
stream bed is cut away at the junction, leav- 
ing it high above the bed of the master-stream 
—the water, if there is any at this point, cas- 
ceades. The tributary enters by what has been 
termed a hanging-valley. The process of 
retrograding then begins in the tributary 
stream-bed. It is a process of corrosion by 
undercutting. It may be slow or fast, de- 
pending on many conditions. In arid and 
semi-arid regions, banks of dry earth often 
remain vertical for years. This is partly be- 
cause the surface behind the wall (as pointed 
out by Stanton in his investigation of the 
landslides on the Canadian Pacific Railway) 
becomes impervious to water and, like a roof, 
prevents the earth mass from becoming satu- 
rated. Eyen when chunks of this earth fall 
beside a stream they do not readily disinte- 
grate unless completely submerged, because 
the clay forms on their surface an impervious 
coating. I have seen earth-cliffs 30 to 40 feet 
high with all the characteristics of a rock-cliff 
erosion. The run-off is quick. Steady rains 
are infrequent. The earth mass can not be- 
come saturated. But given an abnormally 
steady and long-continued rainfall, in com- 
bination with a “cloud-burst,” at the head- 
waters, and all is changed with startling 
rapidity. It is dramatic. The tributary 
SCIENCE 
657 
flood undercuts its precipice steadily and mass 
upon mass of the earth drops into the flood to 
dissolve. The precipice travels up stream. 
The tributary stream-bed is immediately 
placed on a par with its master. When the 
storm is over the precipice of earth if found 
at all is found miles towards headwaters. 
The former stream instead of flowing on 
the surface amidst verdure and willows and 
cottonwoods, now glides at the bottom of a 
desolate and barren earth canyon. 
In the retrograding it has cross-cut the 
mouths of other stream-beds, where in time, 
the operation described is repeated over and 
over, as the country is gradually lowered by 
these forces. 
There have been many excellent examples 
of cross-cutting and of violent retrograding, 
two of which I may mention. In the 60’s of 
the last century one occurred at the little 
Mormon settlement of Santa Clara in south 
Utah. In a single night the stream was meta- 
morphosed. At Kanab, south Utah, a similar 
example took place about 1900. In an aston- 
ishingly brief time the Kanab creek meander- 
ing amongst willows and vegetation was trans- 
formed into a waste for miles of its course. 
The channels through which the Colorado 
River broke into the Imperial Valley exhibited 
retrograding in its violent form and excellent 
photographs were made at the time. 
In the Mukuntoweap Valley in southern 
Utah cross cutting and retrograding in rock 
may be studied. Some small valleys that were 
cross-cut ages ago remain almost unaltered. 
They hang 1,000 feet above the master-stream. 
The explanation seems to be that during the 
Glacial Epoch the high plateaus at the head 
waters of the Virgin river were heavily piled 
with snow and ice which, melting in the sum- 
mers, carried on rapidly the work of cor- 
rosion in the main streams which had sources 
in the very high lands, while the shorter 
stream-beds with lower sources were even then 
arid and were left behind, having no power of 
retrograding to keep level with master 
streams. In other words the corrosion of the 
master-streams ran away from the erosion of 
the drier valleys, leaving the latter hanging 
