526 THE GEOLOGY OF MINNESOTA. 



[Grasses. Drift. 



provision, it seems quite certain that ten acres will be sufficient for the needs of an ordinary 

 household, thus leaving each acre of willows ten years to grow before cutting, in which time they 

 attain a diameter of six to eight inches and a bight of twenty to thirty feet. 



The surface of these counties, having for the greater part a smooth, gently undulating or 

 rolling contour, with few or no boulders, presents a vast, fertile expanse, waiting only to be 

 ploughed and sown to yield fifteen to thirty bushels of wheat per acre. Till thus changed into 

 cultivated farms, it annually produces its thin growth of prairie grasses, one to two feet high, 

 which are excellent for pasturage till the first severe frosts, about the middle of September; by 

 which they are whitened and killed to the roots, not continuing green after frosts like the culti- 

 vated grasses. Then, after a few days of drying, it is ready to be swept by prairie-fires at any 

 time when they come, until it is covered by the snow of winter; and, should it escape through 

 the autumn, it is again in danger of fires during a month or more in spring, from the departure 

 of the snow until the green grass shoots up anew. 



The most abundant species of grass found upon the prairies of this part 

 of Minnesota are as follows: beard-grass (Andropogon furcatus, Muhl.), com- 

 monly here called "blue-joint," Indian grass (Cnrysopogon nutans, Benth.), 

 muskit grass (Bouteloua racemosa, Lagasca), and porcupine grass (Stipa spartea, 

 Trin.), common on land neither very dry nor very moist ; another species 

 of beard-grass (Andropogon scoparius, Michx.), and a second muskit-grass 

 (Bouteloua hirsuta, Lagasca), common on dry swells; the fresh-water cord- 

 grass (Spartina cynosuroides, Willd.), in sloughs, making the principal mass 

 of their hay ; and rice cut-grass (Leersia oryzoides, Swartz), with the last. 

 The prairies also bear a great variety of flowers, including numerous spe- 

 cies of aster, golden-rod, sunflower, and blazing-star or button snakeroot, 

 and the rose, lily, harebell, phlox, fringed gentian, and many others. 



GEOLOGICAL STRUCTURE. 



Glacial and modified drift. 



The bed-rocks of Murray and Nobles county have no outcrops, nor are 

 they reached by any wells, so far as learned of in this survey. Drift forms 

 the surface, consisting almost wholly of the unmodified deposit of the ice- 

 sheet, which is called till, boulder-clay, or hardpan. Clay is the principal 

 ingredient, containing always more or less of grit, gravel, and large stones, 

 but boulders exceeding a foot in diameter are usually very rare, so that 

 perhaps in some cases none would be found in ploughing a quarter-section. 

 Though the soil to the depth of a foot or more appears to contain less 

 gravel than the earth excavated in cellars and wells, some intermixture 

 of gravel may nearly everywhere be noticed upon ploughed land; and the 

 true loess, which thinly covers much of Rock county, does not extend east 

 into the counties here described. Under the black soil, the till has a yel- 



