284 Tlie American Geologist. November, i899 
found. But before proceeding to their description, it will be 
well to mention, in somewhat more detail, the general features 
of the later geologic history of the district. 
The rapidly undulating drift plains above the valley proper 
were formed at a time when the site of Morrison county was 
occupied by a portion of a great continental glacier. This 
was in process of general recession, and when its margin had 
retre:ited northward to a sufftcient distance to uncover the dis- 
trict, the great streams flowing away from the melting ice 
deposited the broad plains of stratified gravel and sand before 
mentioned. As soon as the border of the ice-sheet had moved 
northward as far, say, as the northern portion of the state, the 
Mississippi river ceased to deposit gravel and sand in Morrison 
county, but on the contrary it then began to trench its present 
trough into the sand-plain. The major portion of the excava- 
tion of this trench was accomplished during the earlier portion 
of the post-Glacial period of geologic time. Later, when the 
land had become somewhat depressed below its late Glacial 
altitude, the Mississippi river deposited during flood times a 
flood-plain of gravelly alluvium. This occurs at various places 
in the present valley, but is not continuous, existing rather as 
isolated fragments. It varies in hight from three to eight or 
ten feet above the low-water level. This modern flood-plain, 
as we shall see, is of considerable importance to the archaeolo- 
gist. 
In several places in Morrison county the post-Glacial Mis- 
sissippi in trenching its present channel encountered ledges of 
schistose and slaty rocks, which have formed rapids in its bed. 
In these metamorphic rocks there are occasional veins of 
white quartz, which may reach a thickness of several feet, and 
v.^ere especially suited to the needs of primitive peoples. Being 
the only outcrops of good implement material in a very ex- 
tensive area, they were much resorted to by the aborigines, 
who constituted them important "quarry sites." The principal 
of these was at Little Falls, but several others were established 
farther down the river, notably at Pike rapids, and at several 
outcrops of schist and quartz several miles above the mouth 
of the Two rivers. Many flakes or quartz fragments are 
found scattered on the surface or in the soil in the vicinity of 
these quartz outcrops. The question of their age is narrowed 
