Movement of the last ice sheet in the Mount Tom quadrangle was due south during time of major advance as indicated by striations, drumlins, and indicator stones. Erratics of the Belchertown Tonalite, derived from the northeast portion of the Easthampton quadrangle, have been carried southward a distance of at least 16 miles. The presence of tonalite boulders on the summit of Mount Tom, elevation 1,205 feet, testifies to uplift of erratics through a vertical distance of 1,000 feet.
Three tills are recognized on the basis of color, grainsize parameters, and location: (1) a reddish-brown sandy till occurs west of the Holyoke Basalt ridge, (2) a brown silty till lies east of the basalt ridge, and (3) a grayish-brown till-with intermediate grain size occurs in the Easthampton quadrangle and the north-central portion of the Mount Tom quadrangle. The three tills are the same age and are equivalent to the upper till of southern New England. During deglaciation, readvance occurred from the northeast over a minimum distance of 3.5 miles in the southeast portion of the quadrangle. Evidence for readvance consists of (1) till over stratified drift, (2) southwest-oriented till fabrics on south-trending drumlins, (3) west-southwest-trending striations cutting south-trending striations, (4) southwest-oriented glaciotectonic structures, and (5) data from borings.
West of the basalt ridge, northward retreat of an active ice margin was punctuated by four stillstands, during which outwash deltas were deposited in proglacial lakes. From oldest to youngest the deposits associated with the four stillstands are named: (1) Paper Mills delta, (2) Barnes delta, (3) Pomeroy Street delta, and (4) White Brook delta. East of the basalt ridge a series of small ice-contact deltas were deposited in high proglacial Lake Hitchcock, which expanded northward with the retreating ice margin. Deposition culminated along the upper east margin of the quadrangle with the formation of a large ice-contact delta named the Holyoke delta.
Continued northward retreat of the ice margin both east and west of the basalt ridge permitted water of Lake Hitchcock to extend through the Holyoke Narrows and into the valleys of the Manhan River and Broad Brook in the north-central portion of the Mount Tom quadrangle. Drainage of Lake Hitchcock 10,700 years ago (Flint, 1956) initiated rapid downcutting by streams, as shown by numerous stream terraces along the Connecticut, Westfield, and Manhan Rivers, and along lesser tributaries.
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Surficial geology of the Mount Tom quadrangle, Massachusetts