Geology of a stream terrace deposit in Massachusetts and its utilization as aggregate

The geology of a stream terrace deposit was studied at Pochassic, near Westfield, Massachusetts. This is roughly circular with an area of about five km 2 and a thickness of 30 m. It was formed in the Late Pleistocene as an alluvial fan. The fan extends southward from a south-flowing stream at the edge of the mountains surrounding the former glacial Lake Westfield. The upper surface lies about 40 m above the present river system, which is the second highest level of five stream terraces recognized in the local fluvioglacial sequence. The outer margin of the deposit has been truncated by a post-glacial river, and other parts have been incised by small streams. The main exposures examined are extensive sand and gravel workings. The alluvial fan has three main environments, the distal parts being composed mainly of bedded sand with a pebble content of less than five percent. The mid fan is built of sand layers interbedded with gravel layers, both the thickness and the coarseness of the gravel layers increasing toward the proximal area on the rim of the original lake basin. The materials consist of stream-flood deposits and channel deposits, constituting two more or less distinct facies. Higher up at the head of the stream is the upland source area of glacial till where the ice melted. The petrography was also studied. The sand and gravel are derived from metamorphic and igneous rocks. The particles are divided into three groups: dark rocks, including certain schists; light rocks, mainly granite and quartzite; and quartz. Pebbles in the gravel layers are mostly smooth and ellipsoidal. Those composed of banded rocks tend to be discoidal in shape, and those of rocks with uniform texture are more nearly spherical. Pebble sizes in the mid fan are mixed and vary from layer to layer, the proportion over 12.5 cm exceeding ten percent toward the proximal area. In the sand layers of the distal part about nine percent is finer than 300 µm, consisting of fine sand and silt. There is no material of clay size in the deposit. About ten percent of the dark rocks are weathered, only about two percent of the light rocks, and none of the quartz. The processing equipment being used on the site consists of gyratory and cone crushers, dry and wet screens, and washers. The chief products are concrete sand and several sizes of crushed stone aggregate. There is normally enough sand from the matrix in gravel units to meet the demand, and sand units are used to only a small extent. The main workings are in the mid fan, but are heading toward the proximal area. The treatment plant is described, and tables are included showing the sieve fractions, composition, and condition of the processed and washed concrete sand. Largely because of the similar geologic origin of all of the materials and the fact that both flood and channel deposits were transported only by stream flow, uniform aggregates and processed sand can be produced in large quantities by conventional treatment procedures. Environmental concerns are diminished because the deposit is in a remote, undeveloped area.