262 ARTHUR C. VEATCH 



The obstructed valleys- — All the valleys which come from the 

 hill region into the plain along the line of hills on the western 

 side of the triangular upland have a very abnormal character. 

 At the points where the streams pass from the upland to the 

 plain, two long ridges of loess and sand 20 to 40 feet high jut 

 out from each side like the arms of a great dam. These two 

 parts almost meet, and the stream passes through the narrow 

 V-shaped space between them. These dams are continuous with 

 the loess-capping of the hills, which is so regular here that it 

 looks much like a great artificial embankment. Although these 

 dams are best developed along this line of hills, a similar tend- 

 ency to dam the mouths of valleys on the east and south sides 

 of the triangular hill land is shown. 



These peculiar loess dams must be taken into consideration 

 in any theory accounting for the manner of deposition of the 

 loess of this region. The fact that valleys have been found 

 facing in all directions, seems opposed to a wind origin. A pre- 

 vailing southwest wind blowing over dried mud flats in the 

 River and Pigeon plains could have formed all the dams on the 

 western side of the triangular upland, but could not have formed 

 some of the others. For this reason it seems probable that the 

 loess of this region was deposited on the bordering hills as a 

 natural levee by the swollen waters of the river, and that the 

 dams across the mouths of these valleys represent continuations, 

 of this levee. 



Drive?i-well area. — In all the plain region bounded by these 

 loess-capped hills, that is, all the River Plain, Lake Plain, and 

 that portion of Pigeon Plain south of the terrace, excepting a 

 very narrow strip in a few places along the base of the hills, 

 wells reveal a great trench filled with an irregular series of clays 

 and water-bearing sands and gravels. This is the region of the 

 driven wells. In the hill region and most of the region in Pigeon 

 Plain north of the terrace all wells strike rock at comparatively 

 shallow depths. 



At Rockport wells have been driven 70 feet in the river 

 alluvium without reaching rock. The normal depth of wells in 



