ABSTRACT The Colorado River extensional corridor, which stretched by a factor of 2 in the Miocene, left a series of lowland basins and intervening bedrock ranges that, at the dawn of the Pliocene, were flooded by Colorado River water newly diverted from the Colorado Plateau through Grand Canyon. This water and subsequent sediment gave birth, through a series of overflowing lakes, to an integrated Colorado River flowing to the newly opened Gulf of California. Topock Gorge, which the river now follows between the Chemehuevi and Mohave Mountains, is a major focus of this field guide, as it very nicely exposes structural, stratigraphic, and magmatic aspects of the Miocene extensional corridor, a core complex, and detachment faults as well as a pre-Cenozoic batholith. Topock Gorge also is the inferred site of a paleodivide between early Pliocene basins of newly arrived Colorado River water. Overspilling of its upstream lake breached the divide and led the river southward. The Bouse Formation in this and other basins records the pre–river integration water bodies. Younger riverlaid deposits including the Bullhead Alluvium (Pliocene) and the Chemehuevi Formation (Pleistocene) record subsequent evolution of the Colorado River through a succession of aggradational and re-incision stages. Their stratigraphic record provides evidence of local basin deepening after river inception, but little deformation on a regional scale of the river valley in the last 4 m.y. except in the Lake Mead area. There, faults interrupt both the paleoriver grade and incision rates, and are interpreted to record 100’s of m of true uplift of the Colorado Plateau. Warren Hamilton’s insightful work beginning in the 1950s helped set the stage for interpretation of Mesozoic orogeny and Cenozoic extension in this region, as well as the record of the Bouse Formation.

ABSTRACT This field trip guide presents a summary of the Paleozoic bedrock geology of the Twin Cities (Minneapolis and St. Paul) region as well as an introduction to the Quaternary depositional and erosional history of the Twin Cities area, focusing on the carving of the Mississippi River gorge by glacial River Warren. Within this geological context, the philosophy behind a variety of pedagogical techniques is discussed, together with student learning goals, opportunities, and objectives for each locality.

Abstarct The glacial geology of the area north of the Great Bend of the Wabash River in west-central Indiana (Fig.1), 1) illustrates important early developed principles regarding multiple continentalglaciation, the movements of adjoining ice lobes, and the interrelated deposits. Perhaps the first documentation of a lithologically definable till stratigraphy that is basic to interpretation of glacial movement and sedimentation throughout much of the Midwest was made in the Great Bend area. A half century after the first documentation was made, the till stratigraphy here would emerge as the regionally mappable sequence. Further, it would illustrate cross-cutting relationships of morainal ridges andthe interbedding of multiple-source deposits. Here, therefore, is a clear demonstration of how deposits first classified on the basis of surface morphology relate to those that are classifiable on a mappable, till-stratigraphy basis. Four vantage points in the Great Bend area are particularly focal for illustration ofthe basic concepts that are addressed here.Foremost is the Lovers Leap section (also known historically as the Stone Creek section), which is exposed on the northwest side of Big Pine Creek, a Wabash River tributary, 4 mi (6.4 km) north of Williamsport, Warren County, Indiana, NW¼NW¼ NW¼Sec.23, T.22N., R.8W., Williamsport 7½-minute Quadrangle (Fig. 2d), 2D). Here well exposed are three superimposed tills and other drift materials that a pioneer of continental-glaciation theories, Thomas Chrowder Chamberlain, used to demonstrate his beliefs. In fact, our Figure 3 is taken from Chamberlin’s contribution to James Geike’s ( 1894) The Great Ice Age and was

Abstract The New York bluffs of Lake Erie, which stretch northeastward 100 km from the Pennsylvania border, and those of Lake Ontario, which extend eastward 212 km from the Niagara River along the south shore of the lake, expose one of the most continuous sets of glacial drift in the northeast. The Erie bluffs are predominantly Devonian shale. However, the shale is mantled by sandy-silt till remarkable for its lateral uniformity in texture, but with distinct “down-ice” trends in lithologic properties relating to the regional bedrock or glacial reworking. Proglacial lacustrine silt, clay, and sand, deposited in 90 to 50 m of water in glacial Lakes Whittlesey and Warren between 13.5 and 12 ka, overlie the till. The bluffs of Lake Ontario are cut only sparingly in Ordovician red sandstone and shale and expose three major lithostratigraphic units. A lower red or pink sandy-till sheet is overlain by widespread gray and red glaciolacusrrine silt and clay. These fines provide major input to a younger, sillier, purplish-gray or gray till marked by both massive basal facies and upper subaqueous-flow or basal melt-out diamicton. A blanketing glaciolacustrine sequence occurring in the lower, nondrumlin areas is largely related to glacial Lake Iroquois. Locally, proglacial sands, lenses of flow till, or stone concentrations indicate a short readvance during lake formation. The two major tills of the Lake Ontario bluffs correlate with a similar sequence in adjacent Ontario, Canada. These, in turn, are tied to the Halton Till of the Port Huron advance. The lower till sheet of the United States Erie shore is related to a preceding advance, which is tentatively identified as the Lake Escarpment glaciation, and the Wentworth Till of nearby Canada.