MANSFIELD: POST-PLEISTOCENE DRAINAGE. 63 
hand do not justify any quantitative statement as to the percentage 
of pebbles from any given source or direction, for it cannot be assumed 
that the collection with which comparison was made represents all 
the sources of a given type in this region. Nevertheless the divergence 
of the lines from every numbered locality toward the southwest is very 
striking and indicates that the ancient stream, which deposited the 
gravels, had its headwaters in the southwest, toward Terry Peak 
region and maintained a somewhat northeasterly course in much the 
same manner as do the present Whitewood and Bear Butte Creeks. 
While the drainage basin and direction of discharge remain practi- 
cally the same, the lines along which that discharge takes place are 
not identical with those of the ancient drainage. Bear Butte and 
Whitewood Creeks ‘are now separate streams and the latter, instead 
of flowing through the broad ancient valley, occupied by. the main 
gravel body, has been offset to a nearly parallel course about two 
miles northward. Some record of these changes is furnished by 
the gravels. At locality 16 (Plate 1), a little patch on a shoulder of 
Whitewood Creek three and a half miles below Deadwood, is found, 
in addition to the usual types of the main deposit, a peculiar spotted 
amphibolite, which has a possible source in the canyon of a small 
tributary of Whitewood Creek, about two miles south-southeast of 
Lead. At locality 9, farther down stream, all the characteristic types 
of locality 16 appear in the more recent alluvial deposits, and, in 
addition, a green porphyry similar to that described by Irving (p. 248) 
as quartz-aegirite porphyry. ‘This does not occur at any of the other 
localities visited by the writer. The records of these two localities are 
successively more recent than that of Boulder Creek valley, since 
they represent successively deeper stages of incision of the head- 
waters of Whitewood Creek and its tributaries. Moreover their posi- 
tion with reference to the main gravel deposit and the course of the 
present stream is highly significant and gives strong evidence of cap- 
ture and diversion first northward, then eastward. 
Altitudes. In the process of mapping the gravels numerous measure- 
ments of altitude were made with an aneroid barometer. The cor- 
rected readings do not agree very closely with the elevations as given 
by the contour lines on the topographic map and have little absolute 
value. Nevertheless a comparison of the readings with each other 
is instructive, in that it brings out the direction of slope of the upper 
surface of the gravels and gives a basis for estimating that slope in 
feet per mile. It shows, also, the relation of the isolated patches, 
already mentioned, to the main gravel body. From the map (Plate 
