TABLE OF CONTENTS Chemical Weathering Exfoliation Flakes Weather Pits Frost Wedging Root Wedging Lichens and Mosses Exfoliation Plates Mass Wasting/Formation of Talus

WEATHERING IS THE PROCESS by which rocks are broken, worn, disintegrated, changed in color or composition by the action of water, air, plants and animals. The processes that cause weathering may be either mechanical, chemical or a combination of the two.

Mechanical weathering is the process of reducing massive, consolidated rock outcrops into boulders, small rocks, pebbles and smaller particles by physical means. Examples of mechanical weathering include: frost action, root wedging, exfoliation, running water and glacial abrasion, solar heating, and expansion caused by chemical change.

Chemical weathering or decomposition results when rock forming minerals react with carbon dioxide, oxygen, water and acids to produce a change in chemical and therefore a change in mineralogical composition. With the notable exception of silicon, the minerals that make up rocks react chemically with acids and oxygen to form products that are in general softer, more water soluble and larger than the parent

CHEMICAL WEATHERING BY GROUND WATER

Clear evidence of the chemical changes that result from the action of ground water on rocks is shown by the deposits of minerals on the bedrock surface of Bear Mountain exposed above Perkins Memorial Drive. As water is exposed to air some carbon dioxide is dissolved. This process results in the formation of carbonic acid. As carbonic acid reacts with the feldspars, which are contained in the granite bedrock, compounds containing carbonates and bicarbonates are formed along with clay minerals. As oxygen, contained in air, reacts with micas in granite iron oxide is an additional product. The sodium, potassium, calcium carbonates and bicarbonates readily dissolve in the ground water. The fine particles of the clay minerals and iron compounds are washed over the bedrock by ground water. The deposits that result, as the water evaporates from the bedrock surface, are show as white and dark streaks in the photographs above.

EXFOLIATION FLAKES

Exfoliation flakes are formed on exposed outcrops as two weathering processes break fragile, thin flakes from the surface of the rock. Feldspars and micas are minerals found in granites and gneisses. Chemical weathering produces new minerals as the oxygen from the air and carbonic acid, formed as carbon dioxide dissolves in water, react with these minerals. The new minerals formed by these chemical processes are more water soluble, and occupy a greater volume than the original mica and feldspars. The swelling process results in flakes of rocks being broken from the outcrop surface. The flaking of rock surfaces may, to a much lesser extent, be aided by the alternate heating and cooling and the resultant expansion and contraction caused by day-to-night and summer-to-winter changes in temperature.

The slickenside surfaces exposed in the road-cuts south of Lake Kanawauke Circle exhibit extensive exfoliation.

WEATHER PITS

The rate of chemical weathering of the minerals that make up granite depends upon the composition of the minerals. Quartz is the most resistant to chemical weathering and reacts extremely slowly. Hornblende and to a lesser extent feldspars react with carbonic acid, which is formed as carbon dioxide dissolves in water, to form clay minerals, soluble carbonates, and silica. Mica reacts quite readily with carbonic acid and oxygen to from clay minerals, carbonates, bicarbonates, iron oxide, and silica. As clusters of the more chemically active minerals in a rock decompose and erode depressions called weather pits are formed.

The surface of the Storm King granite, that makes up Bear Mountain, Dunderberg Mountain, West Mountain and other peaks in the park area, often shows pitting. The photograph shown below is of a rock surface near Perkins Tower along the Appalachian trail at the top of Bear Mountain.

FROST WEDGING

Dramatic evidence of the seepage of ground water over and through bedrock is provided by the ice flows that form on the face of rock cuts when the temperature drops below freezing. Whenever water freezes in soil or rock pores and cracks the 9% increase in volume that takes place during the solidification process provides an effective disruptive force. In order to break rocks water must be confined in the cracks or pores by freezing first at the rock surface and subsequently solidifying and expanding deeper inside, thus wedging the rocks apart. This frost action or wedging results in rock fragments breaking free from the bedrock surfaces and falling, under the action of gravity, to form talus at the base of cliffs.

The ice flows, pictured in the photograph above, demonstrate that at some locations in this rock cut in Mount Aramah, along Route 6, ground water is percolating through the bedrock as well as flowing over its surface.

ROOT WEDGING

Tree roots aid the weathering processes that break boulders and bedrock into smaller fragments. Water trapped in rock crevices wedges the cracks apart as it expands while freezing. The water also provides the moisture necessary to germinate seeds. The roots expand as they grow exerting additional pressure on the rock and wedging the crevices wider until the rock breaks. Acids formed during the course of growing also aid the chemical decay process of the rock.

The photograph, above left, taken along the trail from Lake Tiorati to Pine Swamp shows a young tree growing out of a steep bedrock outcrop. The other picture shows a rock that has been broken into two pieces by the root prying of two trees at the Indian Shelter near Lake Tiorati.

LICHENS AND MOSSES

Lichens are a unique group of plants because each species is a combination of two groups, namely, a fungus and an alga. These two groups live together in a mutually beneficial relationship called symbiosis. Lichens are often the first plant life to survive on exposed rock surfaces. Some lichens secrete acids which dissolve the minerals in the rocks. Others are able to break tiny flakes from the surface by expanding when wet and contracting when dry. Still others form chemical compounds with rock minerals that are absorbed by the lichen. The chemical and physical weathering caused by lichens and other sources produces a habitat for mosses and eventually higher plants which continue the breakdown of the rock surface.

The lichen covered boulder shown here is located near the point where Long Path crosses Route 210 east of the Kanawauke Circle.

EXFOLIATION PLATES

Exfoliation plates are large layers of granite that break away from the underlying rock and conform to the surface contour of the parent rock. These plates or sheets of granite break free from the bedrock without any significant amount of weathering. Exfoliation plates may be several inches to several feet thick and as much as hundreds of feet long. The explanation of the formation of the plates is that the rocks that exhibit the property of shedding large plates are those that have been formed deep underground under tremendous pressure. When the overlaying burden is removed by erosion, and the pressure released, the rocks expand upward and cracks parallel to the surface are formed.

The exfoliation plates shown in the photograph above are on Bear Mountain near Perkins Tower. The process of plate exfoliation of the granite mountains in the Hudson High-lands helps to account for the rounded mountain tops of the region.

MASS WASTING/FORMATION OF TALUS

As weathering disintegrates the exposed rock in the face of a cliff loose pieces fall under the action of gravity. The process is one from of mass wasting and results in the accumulation of rock fragments called talus. Much of the talus seen at the foot of cliffs in Bear Mountain and Harriman State Parks is the result of the wedging action of ice during the thaw-freeze period at the end of the ice ages. The size and shape of talus fragments is determined by the jointing present in the bedrock and the weathering conditions. Closely spaced joints and chemical weathering produce small talus fragments while wide jointing and physical weathering produce large talus blocks.

The talus pile, shown in the photograph above, is located on Rockhouse Mountain along Route 210 between Lake Welch and the Kanawauke Circle.