Each mountain in the chain consists of an erosional remnant of
Cretaceous intrusive igneous rock and associated
hornfels, which are more resistant to
weathering than the surrounding
sedimentary rock. All of the mountains have dark-coloured
mafic rock such as
gabbro and
essexite; some also have large areas of
pulaskite,
syenite, and other light-coloured rock. The Monteregian Hills are part of the
Great Meteor hotspot track, formed as a result of the
North American Plate sliding westward over the long-lived
New England hotspot, and are the
eroded remnants of intrusive
stocks. These intrusive stocks have been variously interpreted as the feeder intrusions of long
extinct volcanoes, which would have been active about 125 million years ago, or as intrusives that never breached the surface in volcanic activity. The lack of an obvious track west of the Monteregian Hills may be due either to failure of the plume to penetrate the
Canadian Shield, to the lack of recognizable intrusions, or to strengthening of the plume when it approached the Monteregian Hills region. However, there is evidence the hotspot track extends northwestwards, including
epeirogenic uplift,
mantle velocity anomalies and
kimberlitic volcanic features (e.g. the
Attawapiskat,
Kirkland Lake and
Lake Timiskaming kimberlite fields) that become older away from the Monteregian Hills. The shallow, rocky sandy loam soils of the summits are mostly covered in
forest. Where the underlying rock is rich in
olivine, as over large areas of Mont Saint-Bruno and Mont Rougemont, these soils are classed as
dystric brunisol.
Podzol tends to develop over rock which lacks olivine, although many of these podzols lack an eluvial (Ae) horizon. Lower slopes are covered with aprons of gravel or sand. The sandy soils are usually podzols with classic Ae development; they often have subsoil hardpan and are undesirable for agriculture. The free-draining gravels are preferred for
apple orchards, which grow in thermal belts where cold air can drain to the valley floor. ==Gallery==