Review and Comment 
355 
On the other hand the content of fine sediment in the ordi¬ 
nary flood stage of mountain streams averages about 1-100th 
part. 
The coarse material, disregarding the fine, was found to be 
1-2,500th in the case of the Reuss and 1-1,500th in the case of 
the Linth. Contrasting the two Heim says : 
“Her G-escheibe und Schlammtransport der Gletscherbache ist 
also sehr viel geringer, geradezu verschwindend gegenuber 
demjenigen der gowohnlichen Bache and Flusse. ” 31 
Regarding the maKing<of lake basins by ice-action he says (p. 
379): “ The sweeping out of an old river bottom (Alluvionsboden) 
to a small lake basin, has never been observed in existing 
glaciers, much less the digging out of such a basin in hard rock. ” 
He further says that the contrast between ice action and the 
ordinary forces of erosion is so great that valley making comes 
practically to a standstill when ice takes possession. This he 
says (p. 389) is not a theoretical deduction but a matter of 
observation. 
He considers the ground moraine to come from the following 
sources (pp, 400, 401): 
1. In the case of many glaciers it comes in great part from the 
surface moraines (Obermoranen). 
2. A second source is found in the accumulated products of 
weathering. 
3. The wear of the ice on the firm rock produces in most 
cases nothing but fine sand and rock flour (Schleifschlamm). 
The cases are very rare in which rock fragments are broken 
off by the ice from its bed. 
Heim’s position on the main question is expressed as fol¬ 
lows (p. 400): 
,! The slow sweeping out of a mass of sediment from a 
valley by a glacier [De Mortillet] I hold as not impossible from 
the facts, which we gather from existing glaciers although 
an actual occurrence has not yet been proven; likewise, some 
widening of deep valleys in soft sandy rocks, and the* grind¬ 
ing out of shallow troughs also in hard rock I hold as conceiva¬ 
ble, although direct proof is lacking, and the indirect has never 
been given without sharp qualifications. ” 
“The grinding out of large lake basins in rock by glaciers, 
31 Handbuch der Gletseherkunde, p. 365. 
24 
