NEWLY MAPPED, PARTIALLY THRUSTED RECESSIONAL MORAINES, TILL BENCHES, AND OTHER ICE-MARGINAL FEATURES SUPPORT ACTIVE ICE SHEET RECESSION IN THE MOUNT OSCEOLA QUADRANGLE, WHITE MOUNTAINS, NEW HAMPSHIRE

Mapping in the Mount Osceola quadrangle in the White Mountains of New Hampshire reveals several recessional moraines along the Hancock Branch of the Pemigewasset River. The moraine segments are defined by linear to slightly arcuate boulder-crested ridges that diverge from the adjacent till hillsides and are bisected by the Hancock Branch. In most cases, the moraines are nested within larger slope breaks of mountain shoulders or hollows, which are interpreted as zones of increased glacial erosion. On a finer scale, till benches step down in elevation from drainage divides toward the valley, suggesting periodic ice-marginal deposition during the nunatak stage of deglaciation. Along trend of the benches are two moraines about 1500 m long, 300 m wide, and 50 m high, which had once dammed glaciolacustrine sediments in proglacial settings.

The North Fork moraine is composed of stratified till overlying laminated to cross-bedded fine sand and can be broken down into three parts: distal, middle, and proximal. A package of fine sand dipping NE in the proglacial direction defines the distal part, which contains two till interbeds of angular cobbles and boulders in a matrix of indurated silt. Above the till interbeds, trough cross-bedded sand and silt drapes show multiple cross-cutting bedsets, where sandy inclusions and soft-sediment deformation suggest reworking of frozen sediment or dewatering, perhaps due to glacial thrusting. The middle part is composed of massive boulder-rich till with interbedded silt stringers. The contact between the distal sand and middle till packages variably shows dropstones, but in other areas, sand infills formerly hollow spaces between till boulders without dropstone deformation. The proximal part of the moraine is composed of SE-dipping stratified till-gravel couplets of diamict topped by thin gravel beds. Long axes of till clasts form an organized cluster trending 158° and plunging 31°. Within the stratified till is a gravel package that hosts a thrust plane and related splays that strike 056° and dip 45° SE, which forms an imbricate anticlinal stack dipping in the up-ice direction, parallel to the till stratification. The moraines here support active glacial recession and slight readvance of the thinning Laurentide ice sheet, which continued to occupy the valley as lobes after the nunatak stage.