PRELIMINARY MAPPING OF GLACIAL ERRATICS SOUTH OF BUFFALO, NEW YORK

It is well known that erratics can be useful in finding ice flow paths, therefore their map patterns could reveal some glacial history of a region. We have mapped erratics in the area south of Buffalo, NY, augmenting rock identification with textural/petrographic analysis in order to document patterns, if any, at the outcrop and map scales. Western NY is underlain by sedimentary bedrock, so we focused upon metamorphic and igneous rock boulders found in incised stream valleys that formed mostly after the last glaciation where post-glacial erratic concentration is by a combination of hydrologic processes and mass wasting. Size and distribution of rock types were recorded at several stations in ten streams from Lancaster in the N to Yorkshire in the S giving a field area > 700 sq. km. Textural analysis was used to distinguish between similar rock types, and at each location, a dominant rock type was determined, and rocks were collected for further fabric analysis. Results show that there is a wide variety of rock types including greenstone, quartzite, marble, matanorthosite, various migmatites, leucogneiss, banded gneiss and ferromagnesian gneiss. Relative to larger stream valleys at the lower elevations, smaller tributaries at higher elevations show high proportion of larger erratics (0.5 – 1m diameter), higher boulder frequency and wider variety of rock types. Otherwise, no discernible map pattern is found using rock size, shape or frequency. Rock type does show a pattern, however where various gneisses and quartzite are found at all localities, but analysis of dominant rock types shows a basic three-zone pattern. Banded gneiss is dominant in the north and south zones, whereas leucogneiss is dominant in the intervening middle zone. Marble is found only in the south zone and in the far west where banded gneiss dominates. A word of caution is warranted in assessment of boulder patterns because heavy rainstorms were found to have moved even the larger rocks downstream, or cause significant, almost daily changes in boulder cover. It is clear that the erratics pattern is a product of deposition and subsequent mass wasting and erosion, and may require additional data over several seasons using the same stations in order assess a pattern that will be useful in determining whether the pattern of erratics can be used for ice-flow history.