Definition of Large-Scale Structural Features

The structure of the Adirondack Mountains has puzzled geologists

for decades. This is due to the polyphase deformation

that complexly deformed the region during the Ottawan

orogeny (1.1–1.0 billion years ago). In 1936 J. S. Brown was

one of the first investigators who recognized that the stratigraphy

of the Northwest Lowlands is repeated by a series of

folds. Later workers, including Ynvar Isachsen, suggested

that there are five sets of large-scale folds that occur throughout

the Adirondacks. In addition, rocks of the central and

southern Adirondacks are strongly foliated and lineated. The

large-scale folds and rock fabrics suggest northwest directed

tectonic transport, which is consistent with other kinematic

indicators in the rest of the Grenville province.

Even the most generalized geologic maps of the Adirondacks

reveal that this region possesses multiple large-scale

folds. Delineating the various fold sets is difficult, due to the

fold interference patterns, but at least five sets of folds are

recognized. The timing of these fold sets has remained

obscure, but at least some are related to the Ottawan orogeny.

It is also not clear whether these folds formed as a progressive

event or as part of distinct events.

Fold nomenclature, i.e., anticline and syncline, is based

on structural evidence found in the eastern parts of the

Adirondacks. The shapes of igneous plutons and orientation

of igneous compositional layering have aided structural geologists

to determine fold superposition in this region. The earliest

fold set (F1 folds) are reclined to recumbent folds. Mainly

minor, intrafolial F1 folds have been documented, with rare

outcrop-scale examples. The presence of larger F1 folds is suspected

based on rotated foliations associated with F1 folding

in the hinge areas of F2 folds. Many F1 folds may have eluded

detection because of their extremely large size.

The F2 folds are the earliest mappable folds in the

Adirondacks, an example being the Wakely Mountain nappe.

In general the F2 folds are recumbent to reclined, isoclinal

folds. The F2 folds are coaxial with the F1 folds and have fold

axes that trend northwest to east-west. Both of these fold sets

have been suggested to be associated with thrust nappes.

The F3 folds are large, upright-open folds that trend westnorthwest

to east-west. Therefore, they are considered coaxial

with F1 and F2 folds. F3 folds are best developed in the southcentral

Adirondack Highlands. Examples of these folds are

the Piseco anticline and the Glens Falls syncline. Northwest

trending F4 folds are best developed in the Northwest Lowlands

and are rare in the Highlands, except in the southern

regions. North-northeast trending F5 folds are open, upright

folds except near Mount Marcy where they become tight. F5

folds are better developed in the eastern parts of the Adirondacks.

Due to the spatial separation of F4 and F5 folds, distinguishing

relative timing between the two is difficult.

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