erratics and perched blocks
Roche moutonnée shaped by ice moving left to right (NE-SW) but cross-cut by grooves trending NW-SE, enlarged below.
A large whaleback at The Rambles
Roches moutonnées in Scotland
The smooth up-ice or stoss side of a roche moutonnée
The jagged crags on the down-ice or lee face of the roche moutonnée
Strong structural control over roche moutonnée form. Here the steep rock steps face into the former direction of ice movement,
Definition: rock hills shaped by the passage of ice to give a smooth up-ice side and a rough, plucked and cliffed surface on the down-ice side. The upstream surface is often marked with striations.
The hills of Central Park have been streamlined by glacial erosion. Many also show a pronounced asymmetry, with smoothed and rounded up-ice slopes and craggy down-ice slopes - these are classic roches moutonnées. Some are rounded on both up- and down-ice slopes - these are whalebacks.
The smoothing of these rock slopes is a product of glacial abrasion. The glacier sole was studded with stones and the tremendous pressure of the overlying ice meant that the rock surface was ground down and all sharp edges were removed. A sandy sole led to glacial polishing; a rocky sole gave striations and grooves. The cliffs of roches moutonnées have been steepened by the progressive loss of blocks. Water generated by pressure melt on the up-ice side of the hills migrated to the lower pressure zones on the lee side. Here the water froze again, allowing joint-bounded blocks to become stuck on to the ice at the glacier base and plucked from sockets.
Rock structure exerts a powerful control over the form of ice-moulded hills. Roche moutonnées and whalebacks reflect former directions of ice flow most faithfully in strong rocks with simple systems of cuboidal fractures, like granite. In the highly folded and locally sheared schists of Central Park the rock hills vary greatly in outline and form. Several factors complicate the basic relationship between hill orientation and patterns of former ice flow. Gordon (1981) has shown that hill asymmetry can be an unreliable indicator of former ice flow where structural influence is pronounced. Good examples exist in the Park of hills with steep rock slopes that face into the former direction of ice flow because the rock layers dip in the opposite direction. Only the rounding of the projecting surfaces distinguished these faces from the sharp-edged crags of true roches moutonnées.
Of special interest in Central Park are the cross-cutting relationships between different sets of glacial lineaments observed by Merguerson and others. Some ice-moulded hills carry two sets of glacial grooves. Some roches moutonnées are aligned NE-SW yet carry a set of NW-SE trending grooves relating to a later phase of ice movement. These relationships remind us that glacial landforms are often compound features, with elements derived from different phases of glaciation. In Central Park the last glacial advance may have done little more than scrape the surface of these massive, resistant hills.