32 GUIDEBOOK OF THE WESTERN UNITED STATES. 
former age has been determined. Unfortunately the track is so far 
below the general surface of the upland that there is little oppor- 
tunity to observe details, but if the traveler could carefully examine 
the ground, he would easily recognize the shore of this ancient lake 
just before he reaches the station at Muskoda. This old shore con- 
sists of a ridge of gravel which was heaped up by the waves that beat 
upon its western side. The large gravel pit which the railroad has 
excavated in this ridge to procure ballast can be seen from the 
Muskoda station. 
e lake was named by the geologist Warren Upham in honor of 
Louis Agassiz, who was the first to make a systematic study of 
glaciers and glacial phenomena. 
The beach at Muskoda is called” 
' Lake Agassiz was a body of fresh water 
pansion was about 110,000 square miles, 
exceeding the present aggregate area of 
the five great lakes tributary to the St. 
Lawrence. 
The shore of this ancient lake is marked 
by well-defined beaches, and it is from 
these that the existence and the extent 
of the lake are known. The beach ridges 
sare built upon typical bowlder clay or 
till that was deposited by an ice sheet, 
and hence it is known that the lake either 
followed the invasion of the ice or marked 
the closing stages of that episode. 
The next question that arises is, What 
were the conditions that led to the forma- 
tion of such a lake? It could not have 
been held in a landlocked basin, for no 
held in place by the retreating front of 
the glacier, which blocked the natural 
outlet of the water to the north and forced 
it to accumulate in this basin. The pond- 
ing of the water in the Red River valley 
as soon as the ice front retreated 
across the divide at the head of Minne- 
sota River. Lake Agassiz thus began as 
a small body of water, and expanded 
northward as the ice melted until it be- 
came of great extent. The water found 
an outlet southward (known as Warren 
River) Minnesota River through 
Browns Valley, which extends from Lake 
Traverse to Lake Big Stone along the 
boundary between South Dakota and 
Minnesota. 
The water of Lake Agassiz continued 
to flow through Warren River until the 
Keewatin (ke-wah’tin) glacier had _re- 
tired northward far enough to permit 
an outlet direct to Hudson Bay. As the 
barrier which held the lake in place was 
composed of ice, its gradual retreat af- 
forded outlsts at different levels, and at 
ess distinct. Thirty-one such beaches are 
known, all of which except the lowest ex- 
tend into the United States. Fourteen of 
them were formed by the lake when its 
outlet was to the south, and seventeen 
after it gained an outlet into Hudson 
Bay. 
In comparison with the irregularities 
of the topography of the region outside 
of the lake the shore lines are inconspic- 
uous, but on the smooth slopes of the. 
lake bed they are generally easily trace- 
able. The best-developed beach ridges 
of the lake commonly rise 10 to 20 feet 
above the adjoining land on the side that 
was next to the water and from 3 to 10 
feet on the opposite side, They vary in 
width from 10 to 30 rods and are composed 
of interstratified gravel and sand, the 
