34 BULLETIN 141, U. S. DEPARTMENT OF AGRICULTURE. 
dark gray, brown, or black in color. Near the margins of the smaller 
areas of this type there is not infrequently a considerable mixture 
of sandy material, and in such instances the surface soil is more 
friable and of a lighter gray color. In all of the larger areas 
where it is developed and in the central portion of even the smaller 
areas it is almost jet black and contains such large amounts of 
organic matter as to be almost muck. The depth of the surface soil 
varies to a considerable degree, ranging from 8 or 10 inches near 
the margin of the type to a depth of 18 or even 24 inches in the 
central portion of large areas or in depressed locations occurring in 
any portion of the type. The subsoil of the Clyde loam is a gray, 
drab, or blue clay, sometimes mottled with yellow or brown iron 
stains. In almost all instances this subsoil is stiff, plastic, and im- 
pervious, but in certain instances where it is underlain at no great 
depth either by layers of peat or of marl the subsoil material may be 
somewhat jointed and less impervious than the average of the type. 
The Clyde loam in the majority of areas where it has been en- 
countered is stone free, and even gravel is lacking. It is only in 
marginal areas or in locations where the surface covering of typical 
Clyde loam is somewhat thin that the stone or gravel of under- 
lying glacial formations becomes evident. In Niagara County, N. Y.,* 
a phase of the type which constitutes only a thin covering over under- 
lying glacial material is marked by the presence of stone and 
bowlders over its surface. This, however, is unusual. 
The Clyde loam invariably occupies level or depressed areas which 
at some previous time have constituted the beds of glacial lakes or 
of large swamps. Such areas occur not only within the regions 
formerly occupied by extensions of Lakes Ontario, Erie, and Huron, 
but also in the beds of many smaller extinct glacial lakes which were 
ponded between the inequalities of the rolling to ridged glacial 
drift. In all instances the mineral matter from adjoining uplands 
was washed down and deposited in the form of fine or coarse sedi- 
ments within these small or large lake beds, and as the water be- 
came shallower vegetation gained a foothold, giving rise to the in- 
corporation of large amounts of mucky or peaty organic remains 
within the zone that now constitutes the surface soil. 
The surface of the Clyde loam is almost invariably level, although 
in some areas low, rounded knolls and gentle swells within the gen- 
eral area of the ancient lake beds may also be covered by the same 
characteristic mucky swamp deposits. In all cases the area of the 
Clyde loam is distinctly depressed below the level of adjoining 
glaciated uplands and glacial moraines or below the level of the 
marginal glacial lake deposits. 
The altitude of the surface of the Clyde loam varies considerably 
in the different areas where it has been encountered. Thus in 
