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VALLEY BELOW THE SEA-LEVEL—TERRACES OR PLAINS. 233 
It is believed that it gradually breaks down into a slope and thus becomes merged in the general 
slope of the Desert. 
In crossing the Desert, several banks or terraces are ascended and descended. One is found 
at the Alamo Well, and another at Cook's Well. It is impossible to decide whether these banks 
are continuous between the two points, or whether they mark the limits of two plains or terraces 
at different elevations. It is believed that they do, and that other and intermediate banks or 
terraces are found between the Colorado and the lowest part of the Desert. At the Colorado, 
only two terraces are found—the bottom-land and the upper plain covered with pebbles. But 
further inland the banks or terraces are very different, being formed of clay, and it is probable 
that an intermediate terrace is formed between the bottom-land and the upper plain. This 
indicates an increased difference of level between the bottom-land and the upper plain, a condi- 
tion which must of necessity exist if the upper plain does not conform in its descent to the 
channel of New River. We may conclude, that on the Desert there are at least three terraces, 
or plains, marked by abrupt descents. 
The first or lowest, borders the Colorado and forms its bank; the second, forms a bluff at 
Cook's Well and beyond, and the third is the high pebbly plain of the Desert. The first terrace 
is properly the river-bottom, and is subject to partial overflow. It is formed of a fine alluvial 
clay from the river, and is well wooded near the river with a growth of willows and cotton- 
wood trees. Near the head of the Gulf, this terrace, or possibly one at a lower level, is over- 
flowed during the spring tides. It is traversed in many places by dry arroyos and is watered 
by New River, which holds a relation to it like that of the Colorado. 
The second terrace is composed of an older alluvion, being entirely of clay, without sand or 
gravel. The extent and boundaries of this terrace are not accurately known. It forms a steep 
bank above Cook's Well, and may extend to the Alamo, where there isa similar bank. Its 
elevation probably diminishes towards the Colorado until it becomes merged in the first terrace 
near the Indian village, or Algodones. The trail follows this terrace, or plain, for a long 
distance, and the surface is wooded in some places by mezquite bushes. It is probable that 
the surface of this plain gradually descends towards the interior, conforming to the inclination 
of the first terrace. 
The third terrace forms the highest plain of the Desert. Its margin is seen in the vertical 
banks which overhang the bottom-land of the Colorado and Gila. It also extends from Pilot 
Knob westward, and borders the north side of the road as far as a point about half way between 
Cook’s Well and the Alamo. Its extent beyond this point is not known, but it is supposed to 
break down into a gradual slope to the level of the second terrace. The road is believed to 
ascend this slope between the Alamo and Cook's Well, and thus to be for a part of the way 
"pon the upper plain. This denuded or broken part of the upper plain is not, however, like 
the surface around Pilot Knob and the Sand-Hills; it is without many pebbles, and is more 
sandy. : 
The probable extent of the third or upper plain is readily seen upon the map, it being colored 
asP ost-Tertiary. I have also endeavored to represent the general character of these terraces, and 
their relation to the Sand-Hills, by a section from north to south between Cook's Well and the 
Alamo—Section 14, Sheet VIII. ; 
All these terraces become merged into a continuous slope between the Alamo and the Big 
Lagoon. Thus, in passing from Carrizo Creek towards the Gila, the traveller, after descending 
to the level of the Lagoon, ascends by almost imperceptible degrees to the surface of the upper 
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