. Elementary physical geography;. onsiderable rainfall, ifthe general slope be very decided, perhaps there may be nolakes and ponds, for the reason that the water flows off,meeting no obstructions which cause it to collect in the contrary, if the surface be flat, the water, findingno definite channels, spreads over the surface and forms amultitude of small ponds. In Florida and along the Gulf 170 IMPERFECT AND OBSTRUCTED DRAINAGE 171 Coast there are excellent examples, and they are commonlycalled marsh lakes. Marsh lakes are rarely more than a few feet in depth. They areseldom naviga
. Elementary physical geography;. onsiderable rainfall, ifthe general slope be very decided, perhaps there may be nolakes and ponds, for the reason that the water flows off,meeting no obstructions which cause it to collect in the contrary, if the surface be flat, the water, findingno definite channels, spreads over the surface and forms amultitude of small ponds. In Florida and along the Gulf 170 IMPERFECT AND OBSTRUCTED DRAINAGE 171 Coast there are excellent examples, and they are commonlycalled marsh lakes. Marsh lakes are rarely more than a few feet in depth. They areseldom navigable, and commercially they are of but little Europe many such lakes have been drained in order to make culti-vable land of their beds. There are several instances where such basinsare filled with water and used for fish culture for a period of severalyears, and then drained and cultivated for a like period. Marsh lakes of large size or considerable depth couldnot form in perfectly flat lands, for the reason that the. MARSH LAKES, FLORIDA water would flow off as fast as it was supplied. For asimilar reason, such lakes could not be very numerous ona surface that had a considerable slope. Glacial Lakes.—There are many thousand lake basinsthat are the result of factors with which rainfall has nodirect connection, except to fill the basins after they havebeen made. The most important are those whose basinshave been shaped largely by the action of glaciers. 172 PHYSICAL GEOGRAPHY A map of the northern part of North America showsthat the lakes of this region are its most remarkable surfacefeature. As a rule they are long and narrow, and the axes,or lines of greatest length, of each are nearly parallel. Care-ful investigations have shown that such lakes are com-paratively deeper than the marsh lakes previously described,and that, in most instances, their basins have been wroughtin the hardest rocks. In certain instances, such as thewalled lakes, their rims consist of wa
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