18 THE ARTESIAN WELLS OF COLORADO, 
ment surveys, sometimes for long distances, as in the case 
of the Prairie Ditch, which runs on a section line for 
twenty-six miles. Though at a mean elevation of over 
7,500 feet, agriculture is a success, and attention lias been 
attracted to its large crops of the cereals and potatoes. 
The second premium crop of the American Agriculturist 
of 1890, over 800 bushels of potatoes to the acre, was 
raised near Del Norte. 
The valley is surrounded on all sides by the highest 
mountains of the State, so that the rainfall in the valley is 
scanty and irrigation is more than ordinarily necessary. 
The streams, which come principally from the West, soon 
sink, with few exceptions, and in consequence have built 
their beds higher than the surrounding plain with the 
sediment and debris which has been left as the waters 
have sunk. This is true of the Rio Grande as well, as 
is evident by the canals shown on the map, which run 
nearly at right angles to the river. 
The uniform appearance of the valley, as well as the 
conditions which have made it an artesian basin, is due 
to the fact that in former geological times it was an 
immense lake, formed by the damming of the Rio 
Grande by the large mass of basalt in the lower end of the 
valley, and which is probably also the cause of the abrupt 
bending of the Conejos and other rivers to the north. In 
consequence of the lake formation, the characteristics are 
fairly uniform over the whole area, though there is much 
variability, as is to be expected, in the thickness and num¬ 
ber of the strata. Near the ancient bed of the Rio Grande 
there is especially great variation; elsewhere there is great 
uniformity over considerable distances. The water is 
found everywhere, so far as learned, above the rock, which, 
in the western part of the basin, is comparatively near 
the surface, but at Alamosa is not found in the well 
which is 1,000 feet deep. The wells are sunk so easily 
