History of lake Agassis. — Up ham. 229 
one foot per mile through an extent of 400 miles along the west 
side of this lake in North Dakota and 3Ianitoba. 
As lake Agassiz gradually extended to the north, following the 
receding ice-barrier, it received successively by three outlets the 
drainage of the glacial lakes of the Saskatchewan and Souris 
basins. These streams took the course of the She}'enne, Pembina, 
and Assiniboine rivers, each bringing an extensive delta deposit. 
With the first retreat of the ice from the Missouri Coteau a glacial 
lake began to exist in the valley of the South Saskatchewan in the 
vicinit} - of the Elbow, probably outflowing at an earl}' time by the 
way of Moose Jaw creek, and through a glacial lake in the upper 
Souris basin, to the Missouri near Fort Stevenson. Later the out- 
flow from the lake Saskatchewan may have passed to the lake 
Souris by way of the Wascana river, after passing through a gla- 
cial lake which probably extended from Regina sixty miles to the 
west in the upper Qu'Appelle basin. When the Dakota ice-lobe 
was melted back to the vicinity of Devil's lake, the drainage of 
lake Souris passed southeast by the Big Coulee, one of the head 
streams of the Sheyenne, flowing thence for some time southward 
by the James river to lake Dakota, but later eastward and south- 
ward by the Sheyenne into lake Agassiz. A manuscript report 
of a reconnoissance in North Dakota by major W. J. Twining, 
in 1869, describes the valley of Big Coulee as 125 feet deep and a 
third of a mile wide, enclosing several shallow lakes along its 
course. " This great valley, " he writes "preserves its character 
to within twelve miles of the Mouse [Souris] river and connects 
through the clay and sand ridge with the open valley of that 
stream." 
The Sheyenne delta, reaching from the Lightning's Nest fifty 
miles northwest to the south bend of the Maple river, and having 
a maximum width of nearly thirty miles to the northeast from the 
south bend of the Sheyenne. probably covers an area of SO0 square 
miles to an average depth of 40 feet. A large portion of this 
delta is doubtless modilied drift, which was brought down by 
glacial streams from the melting surface of the ice-sheet, their 
coarser gravel with much sand being deposited in the high plains, 
that slope southward along the outer side of the great moraines 
that pass south of Devil's hike their finer gravel and sand being 
carried by the Sheyenne to this delta, and their finest silt and clay 
being spread in the quiet water of the lake over a much Larger ad- 
