520 O. H. HERSHEY TERTIARY AKD QUATERNARY GEOLOGY"^ 



tance of 1 mile. These streams have built their alluvial fans of coarse 

 gravel on it, but between the fans it remains in a finely preserved condi- 

 tion. The north border is a steep bank with lobate outline produced by 

 erosion by the Modern river. The height varies from 27 to 29 feet above 

 unburied remnants of the Modern floodplain, and the width of the ter- 

 race varies from 80 to 150 yards. It descends westward down the valley 

 at a rate only a little less than the Modern flood plain. 



No bedrock appears in the bank, and this terrace is apparently en- 

 tirely built of unconsolidated materials, chiefly brown stratified gravel 

 like that of the Modern alluvium except that it has few cobbles 6 inches 

 in length and practically no boulders. The unique feature is the upper 

 deposit, a structureless bed of non-pebbly light brown silt 2 to 6 feet 

 thick. The line between the gravel and silt is ratlier sharp and appar- 

 ently undulating. The silt is composed of angular and subangular 

 grains of quartz and other minerals occurring in the rocks of the region 

 and was probably derived from the soil of the district and deposited by 

 the river at flood stage. It is relatively impervious to water and gives 

 rise to a thin and rather sterile soil, contrasting strongly with the fertile 

 dark brown soil at the surface of the Modern alluvium. 



There is probably a small remnant of the terrace at the mouth of Elk 

 Creek, 1% miles above Kellogg, and another at the mouth of Govern- 

 ment Gulch, 2 miles west of Kellogg. In both the gravel is overlaid by 

 2 to 6 feet thickness of pebbleless silt. 



The age of the deposits in this terrace is indicated by the subsequent 

 erosion. Opposite the terrace at Kellogg the river has eroded a valley 

 900 yards wide and at least 35 feet in average depth, and except for the 

 few small remnants described above it has practically destroyed the de- 

 posits for many miles above and below the terrace at Kellogg. They 

 belong to a relatively late epoch of the Quaternary Era, but are much 

 older than the glacial material in the cirques of the high mountains. It 

 is my impression that this 30-foot terrace is of the same age and due to 

 the same conditions as a low terrace at the mouth of Deer Creek Valley 

 in Montana, belonging to the earlier glacial stage there recognized. 



SIXTY-FOOT TERRACE 



At a small remnant of this terrace at a railway cut between Deadwood 

 and Government gulches the bank to a height of 27 feet is Prichard 

 slate overlaid by 8 feet thickness of coarse river gravel abounding in 

 cobbles and small boulders. The original terrace surface is present far- 

 ther back along the wagon road. A better remnant occurs at the Sweeny 

 mill on the east side of the mouth of Government Gulch. The Prichard 



