406 DESCRIPTIVE GEOLOGY. 



It is impossible to obtain here an accurate estimate of the thickness of 

 these lower limestones, but in the continuation of the range to the north, 

 beyond the limits of the map, and in the uplift to the east of Cache Valley, 

 they are found with a thickness of nearly 2,000 feet, showing a very con- 

 siderable increase in the amount of deposition over the beds of the same 

 horizon observed in the southern part of the range. 



To the north of Box Elder Peak, the beds slope gently to the north- 

 ward, until, at the northern point, the ridge is completely occupied by the 

 Wahsatch limestone, which passes under a deposit of Pliocene Tertiaries 

 which have filled Cache Valley, and extended over the low pass at the 

 "Gates" of Bear River into Salt Lake Valley. These Tertiaries consist 

 here mostly of grayish- white limestones and sandstones, partly fine-grained 

 and compact, and partly coarse and porous. Some of the beds are more or 

 less oolitic, and some again are almost completely made up of aggregations 

 of fresh-water shells of Pliocene and recent species. They are generally 

 much obscured by the Quaternary gravels of the terraces, but, where sections 

 can be obtained, are shown to have had a thickness of at least 350 feet. At 

 the northern end of the range, where the narrow-gauge railroad crosses into 

 Cache Valley, they are found to have been considerably uplifted, showing 

 angles of dip of 10° and 15°. An appearance of volcanic rock in the canon 

 of Bear River, at the "Gates", which, however, was not examined closely 

 enough to determine definitely its character, suggests the possibility that 

 this uplift maj have been determined by a late volcanic outburst. 



Eastern Uplift. — Cache Valley occupies a broad synclinal fold in the 

 Wahsatch limestone, which, on the west, forms the flanks of the main range, 

 while to the eastward, with a slight secondary roll, it occupies the broad 

 mass of hills between this valley and Bear River Plateau, in the interior of 

 which the lower formations gradually rise up from beneath it. In Logan 

 Canon, for a few miles above the entrance, the beds of the Wahsatch lime- 

 stone dip to the eastward. This dip, which is 35° at the mouth, rapidly 

 decreases as one ascends the canon, and opposite Logan Peak the beds are 

 virtually horizontal, then rising again with a dip of 20° to the westward. 

 The lower beds, as exposed at the mouth of the canon, are generally of 

 dark, rather siliceous, thinly-bedded limestones. About 1 ,500 feet above 



