TRANSPORT OF DIABASE BLOCKS AND BOULDERS BY THE SUSQUEHANNA RIVER AT CONEWAGO FALLS, NORTHWESTERNMOST LANCASTER COUNTY, PA

Conewago Falls occurs on a north-dipping (<10°), Jurassic-age, diabase sheet eroded to its present level by the Susquehanna River. The diabase is bounded, above and below, by hornfels baked from sedimentary rocks of the Triassic-age Gettysburg Fm. (above) and New Oxford Fm. (below). The diabase separates at 1-2 m intervals along parting planes that parallel bedding-plane orientations of adjacent sedimentary rocks. Widely and irregularly spaced vertical joints cut the diabase generally in NE-SW and NW-SE orientations. The relatively flat diabase surfaces with 1-2 m topographic relief are exposed only during very low river levels.

Strewn on the exposed surfaces are many diabase blocks, very large rocks with angular to subangular edges, that were eroded from outcrops within this diabase exposure and then transported downstream for distances of a few centimeters to many meters. More than 1,200 definitely moved blocks have been identified on an enlarged aerial photograph. Dimensions of 151 blocks were measured in situ. The mean size is 5.2 x 3.2 x 1.3 m. The largest measured block is 11 x 4.5 x 1.5 m. Potholes eroded into some blocks allow positive identification of upside-down blocks.

In addition, there are many 1-2 m, nonequidimensional, diabase boulders that are moderately well rounded. There are also many very well rounded diabase boulders less than 1 m in diameter. Local derivation of all these boulders is doubtful because of their excellent rounding. They were probably eroded from another river-eroded diabase exposure 5 km upstream and rounded during fluvial transport to their present positions.

Movement of the large blocks could have been by several mechanisms: (1) traction transport by extreme floodwaters, (2) rolling during extreme floodwaters, and (3) flotation while encased in ice. Movements by (1) and (2) are real. Movement by (3) is plausible. Extreme floodwaters occurred during multiple Pleistocene deglaciations and also during multiple catastrophic drainages of Glacial Lake Lesley on the West Branch Susquehanna River.