. Elementary physical geography . posits(Chapter XVII.). The stream valleys which were carvedbefore the ice covered the country, were dammed, or in otherways interfered with by glacial deposits, or by glacial action,so that when the ice retreated, the rivers found it impos-sible to flow overthe land withoutbecoming locallytransformed tolakes (Fig. 167).The scores ofthousands of lakesand ponds thatexist in northernUnited States andEurope, are mostlydue to glacial ac-tion (Figs. 168 and190). Other acci-dents to riversmay produce lakes in a similar way. Thus a lava flowmay dam a stream and form a
. Elementary physical geography . posits(Chapter XVII.). The stream valleys which were carvedbefore the ice covered the country, were dammed, or in otherways interfered with by glacial deposits, or by glacial action,so that when the ice retreated, the rivers found it impos-sible to flow overthe land withoutbecoming locallytransformed tolakes (Fig. 167).The scores ofthousands of lakesand ponds thatexist in northernUnited States andEurope, are mostlydue to glacial ac-tion (Figs. 168 and190). Other acci-dents to riversmay produce lakes in a similar way. Thus a lava flowmay dam a stream and form a lake; or an avalanche maydo the same; or the growth of a mountain across a streamvalley may transform it to a body of quiet water. A largemajority of the lakes in the world are a result of accidents,either of these or other kinds. In many cases the origin iscomplex, several causes uniting to produce the lake basin. Not a few lakes in the world are the result of other depressions on the surface of a land which has. Pig. 167. Avalanche Lake, Adirondack^, Part of a rivervalley transformed to a lake. (Copyrighted, 1889,by S. R. Stoddard, Glens Falls, ) 300 PHYSICAL GEOGRAPHY. been newly added to the continent, when filled with waterare formed into lakes. This is the origin of the large num-ber of lakes in Florida, and of Lake Drummond in the Dis-mal Swamp. Others may be produced during and as aresult of the natural development of streams. Such lakesas the oxbow cut-offs described above (Figs. 135 and 160),or those formed by the irregular growth of deltas (Fig. 153),are dependent upon the development of streams.
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