THE RED RIVER VALLEY. 21 



and reaching- several miles from it (excepting a morainic belt of till crossing 

 the river at Goose Rapids), is fine clayey silt, horizontally stratified; but 

 the south end and large areas of each side of the plain are mainly 

 unstratified bowlder-clay, which differs from the rolling or undulating till of 

 the adjoining region chiefly in having its surface nearly flat. Both these 

 formations are almost impervious to water, which therefore in the rainy 

 season fills their shallow depressions; but these are very rarely so deep as 

 to form permanent lakes. Even sloughs that continue marshy thi-ough the 

 summer are infrequent, but, where they do occur, cover large tracts, usually 

 several miles in extent. 



In crossing the vast plain of the Red River Valley on clear days the 

 higher land at its sides and the groves along its rivers are first seen in the 

 distance as if their upper edges were raised a little above the horizon, with 

 a very narrow, strip of sky below. The first appearance of the tree tops 

 thus somewhat resembles that of dense flocks of birds flying very low 

 several miles away. By rising a few feet, as from the ground to a wagon, 

 or by nearer approach, the outlines become clearly defined as a grove, with 

 a mere line of sky beneath it. This mirage is more or less observable on 

 the valley plain nearly every sunshiny day of the spring, summer, and 

 autuixai months, especially dui'ing the forenoon, when the lowest stratum 

 of the air, touching the surface of the ground, becomes heated sooner than 

 the strata above it. 



A more complex and astonishing effect of mirage is often seen from 

 the somewhat higher land that fonns the slopes on either side of the plain. 

 There, in looking across the flat valley a half hour to two hours after 

 sunrise of a hot day following a cool night, the groves and houses, villages 

 and grain elevators, loom up to twice or thrice their true height, and places 

 ordinarily hidden from sight by the earth's curvature are brought into view. 

 Occasionally, too, these objects, as trees and houses, are seen double, being 

 repeated in an inverted position close above their real places, from which 

 they are separated by a very narrow, fog-like belt. In its most j^erfect 

 development the mirage shows the upper and topsy-turvy portion of the 

 view quite as distinctly as the lower and true portion; and the two are 

 separated, when seen from land about a hundred feet above the plain, by 



