ROCK RIVER 



THE most picturesque portion of Rock River lies between Rockford and 

 Dixon. From Rockford north the river lies in a pre-glacial valley. 

 A few miles south of Rockford, the river abandons its old valley 

 (which runs nearly south and is largely filled with glacial debris), and takes 

 a new course in a southwest direction. This new course in places coincides 

 with the valleys of pre-glacial streams, and in places cuts across old divides. 

 In the former instances, the present valley is broad, a mile more or less, 

 with gently sloping sides; in the latter, the valley is narrow with precipitous 

 slopes. The wooded bluffs, in places picturesquely carved by rain wash, 

 are the most attractive features of the region. From their crests long vistas 

 of the river reach away to the open valley, with its cultivated slopes. 



HISTORICAL 



The rock of this region is Ordovician, for the most part Galena-Trenton 

 dolomite, with a strip of St. Peters sandstone outcropping from a point 

 near Oregon to a point south of Grand Detour, and a small area of Lower 

 Magnesian limestone south of Oregon. This whole section probably had 

 been base-leveled, the streams rejuvenated, and the valleys developed about 

 to maturity in pre-glacial times. The glacial incursions carried away the 

 mantle rock, and the upper part of the bed rock, and spread a covering of 

 glacial drift over the surface. This drift (tilt) covering smoothed off the 

 irregularities of contour of the solid rock surface, and is adequate for an 

 arable soil; at the same time that it does not completely fill the valleys, 

 leaving the drainage good. There are very few acres of waste land. The 

 latest ice-sheet which overspread this region was much older than the last 

 one which covered the region about Chicago. While the northeastern part 

 of the state was covered with the last ice-sheet, this area was exposed. 

 The earlier glacial drift was decomposing, in preparation for the rich soil of 

 the rolling prairies. Streams were cutting their valleys in the glacial drift 

 and into the underlying rock. The present valley of Rock River was 

 nearly as large as now when the Late Wisconsin ice-sheet invaded the state. 

 Thru the valley poured the floods of water from the later glacier, carrying 

 an immense load of sand and gravel, much of which was deposited, partially 

 filling the valley. This filling of gravel and sand is usually topped by a 

 few feet of clay and loam. Since the glacial period, the river has sunk its 

 valley in these fills to a depth of 25 or 30 feet, leaving the parts which 

 still remain as level, well drained terraces with a rich soil. On the terraces 

 the first settlements sprang up, and they are the sites of most of the villages 

 and cities along the river. 



