MODERN SUPERFICIAL DEPOSITS. 
343 
There are numerous points where the accumulation of superficial detritus is local in its 
character, and very circumscribed in its limits. These deposits often consist of an inter¬ 
mixture of the older drift, with boulders which have been removed by the change in the 
course of a river, the bursting of a lake, or some other phenomenon of the kind. They are 
recognized by numerous characters; but a general distinctive feature is that of containing 
materials of both northern and southern origin, confusedly intermingled. They often rest 
upon previously formed superficial detritus, and are distinguished by position, as well as by 
the character of the materials. Several cases of this kind have been enumerated in the 
Annual Reports; and many others might be added to the list, were it necessary to increase 
the number for the sake of illustration. 
In passing along the margins of the broad valleys, such as the Genesee and others, we 
find, opposite the mouth of every stream or deep ravine entering the valley, a mound of 
greater or less extent, and undistinguishable, upon the surface, from the surrounding deposits 
of similar nature. Occasionally, however, the bursting of a cloud, or a powerful spring flood, 
reveals to us the mode of these formations, in the production of mounds of earth, pebbles and 
fragments of rock which are swept down by the resistless torrent, and heaped up as the 
stream emerges into the broader valley. The more ancient deposits of this kind often cover 
many acres, and, from all the analogies, we must suppose that they were formed in the same 
manner as recent ones of the kind. Sometimes, however, it appears as if a lake may have 
burst its barriers upon the high ground, and the waters suddenly discharging, have carried 
forward this immense mass of materials. These deposits often occur opposite the inlet of 
ravines in which no water flows at the present time. 
In many of the river valleys the accumulation of materials indicates the whole to have been 
occupied by a deep lake, and that the fine sand and loam has been brought down by streams, 
and spread over the bottom. Terraces are often left at successive points, indicating the stages 
of recession in the waters occupying these lakes, as the outlet has been lowered. 
In the valley of the Conhocton, opposite the junction of the valley of the Canandaigua lake, 
there are several successive terraces, as represented in the woodcut at the head of the chapter. 
It would appear that at the same time in which the current was flowing down the valley of 
the Conhocton, a still more powerful current, probably loaded with detritus, came in by the 
valley from the north, and was carried against the south side, when these materials were 
deposited in the eddy current, while the stream turned its course to the east. 
Along the courses of nearly all the streams and rivers, we find evidences of their having 
once stood at a higher elevation than at present. These evidences consist in long terraces of 
pebbles and sand, often successively repeated ; and from being continuous, and at the same 
elevation for long distances, no farther facts are required to indicate their nature. It would 
require too much space to describe all the examples of this kind which have been examined. 
A few of the more extensive and important cases will be noticed. 
The deep depression known as the Genesee valley, extends from Rochester, southward, as 
far as Dansville. Following the same direction we find, after rising several hundred feet, 
