DRIFT SECTION IN THE OLD ROCKY RIVER VALLEY. 335 



a glacial re-advance interrupting the general retreat. This section 

 (figure 1) displays, along its eastern half, an underlying early till, which 

 rises from below the lake level to a niaxitnum height of 25 feet, declining 

 westward and becoming hidden by the talus and finally sinking beneath 

 the shore line. Next the section has, along all its extent, a stratum from 

 5 to 20 feet thick of mostly very fine sand, with scantv fine gravel, hori- 

 zontally bedded, but often contorted in portions of small vertical range. 

 No indication of erosion of the underlying till before the deposition of 

 the sand, nor afterward of erosion of the sand previous to the deposition 

 of the overlying till was anywhere detected. Very definite and mainly 

 straight planes divide the low^er till from the sand, and the latter from 

 the till above it; but the upper till, which varies in thickness from 3 to 7 

 feet in its continuous extent through this mile, has a slightly uneven 

 upper limit, overlain by a superficial deposit of sand. This sand and 

 loam, forming the surface, is from 2 to 5 feet thick eastward, but attains 

 a maximum thickness westward of about 15 feet. 



Figure i. — Section on the Lake Shore across the preglacial Valley of Rocky River. 



A = lower tiU ; B = interglacial sand ; C = upper till ; D = delta and alluvial sand ; E = Erie 



shales. I,ength, ij/g miles ; height, 40 feet. 



No interbedding of the interglacial sand with the till below or abovj 

 was seen. The three formations — lower till, intervening sand, and upper 

 till — were evidently laid down in immediate succession,' without transi- 

 tional conditions, and with no intervals when the surface was land ex- 

 posed to stream-wearing. Excepting the superficial delta or alluvial sand 

 and loam, the whole section beneath consists of deposits made in the 

 glacial lake between 200 and 150 feet deep overhead. The height of the 

 section is quite uniformly about 40 feet above the mean surface of the 

 lake. Irregularities of thickness of the several members, and their de- 

 viation from horizontality, are shown in figure 1 ; and we need only to 

 add a few notes on the characters of the two till deposits. 



Both the lower till and the upper till have abundant shale fragments, 

 some of Paleozoic sandstones, and many of Archean crystalline schists. 

 All these varieties range in size up to 6 or 8 inches. Boulders are ver}^ 

 rare or wholly absent from nearly the entire section, except that many 

 of Archean rocks, up to three feet in diameter, were seen in the lowest 

 5 or 6 feet of the lower till where it rests on the shale at the east side of 

 the old valley. The lower till deposit has a dark bluish color through all 

 its observed extent. Surface weathering and infiltration of water and air 



XXXVIII— Bull. Geol. Soc. Am., Vol. 7, 1895, 



