MANSFIELD: POST-PLEISTOCENE DRAINAGE. 61 
Over two hundred, were examined and classified according to type, 
as, for example, Algonkian quartzite, Cambrian quartzite, and quartz 
porphyry. Representatives of each type were compared with trimmed 
and labelled specimens collected by Professor Jaggar and Mr. Bout- 
well in 1898-9. Possible sources of each type were thus determined. 
In a number of cases the identification of a pebble was difficult or 
Uncertain because of weathering and discoloration. In a majority of 
instances, however, the pebbles were identified with a considerable 
degree of certainty. The sources thus found were entered upon the 
Map (Plate 1) as green rectangles and were connected by green lines 
with the localities at which the respective pebbles were collected.’ 
In this way several important facts were brought to light which will 
be discussed in a later section. A separate map (Plate 3) was also 
made on which were platted the altitudes of the different portions of 
the gravels. 
No such maps were constructed for the Bighorn region, since, for 
reasons that will appear later, no attempt was made to follow out the 
courses of the streams that deposited the gravels. 
Tse BLack Hırıs DISTRICT. 
Tue Gravers. Character and Distribution. At many places in 
the Black Hills, both within and without the region under discussion 
(Figure 1), occur patches of gravel and boulders, sometimes a hundred 
feet or more in thickness. The pebbles consist of well rounded frag- 
ments of quartz, schist, sedimentary rocks, and porphyries, which have 
an average size of one to three inches, while boulders six inches and 
even a foot in diameter are not uncommon. They occupy broad val- 
leys, saddles on the divides, and patches on the shoulders of the valley 
sides far above the present streams. Their distribution in this region 
is indicated on the map (Plate 1). The main gravel body lies in 
Boulder Creek valley, where the gravels range in thickness from fifty 
to one hundred feet. In Boulder Park proper they are very thin and 
the underlying strata (Minnekahta limestone or Spearfish Red Beds) 
frequently appear at the surface. Indeed it may be questioned 
whether the deposits there found are of the same age as the main 
gravel body or only later, resorted gravel. From the comparatively 
uniform size and character of pebbles found in different parts of the 
1 The colors were not reproduced in the plate and for simplicity some of the lines were 
Omitted. 
