TABLE OF CONTENTS Corrasion by Young Valley Brooks Potholes The Hudson River—A Fiord Rounded Mountain Summits Maturing Valleys Stream Deposits—Deltas Downslope Movement Swamps

EROSION IS THE PROCESS of loosening and carrying away the rock debris which have been produced by weathering. The process results in the eventual deposition of these materials in valleys, lakes and oceans. During the course of erosion fresh bedrock surfaces are exposed to weathering so that these two processes work together to wear down the upland surfaces.

The most important agent of erosion in the Hudson Highlands, since the end of the last ice age, is moving water. The cutting of stream beds by the sand, silt and rocks carried by moving water and the removal of loose materials by the running water has resulted in the major land forms throughout the region. From steep, young valleys with rapidly racing water cascades to the majestic Hudson River, slowly moving to the ocean through the fiord formed by a Pleistocene glacier, water erosion continues to shape the land in the present as it has throughout geologic history. Other agents of erosion include: wind, waves, currents in oceans and lakes, glaciers, gravity, avalanches, burrowing animals and earthworms.

CORRASION BY YOUNG VALLEY BROOKS

Streams of all sizes have a major influence on the shape of the land surface. Young valleys with steep walls and high gradients have been formed by the streams that flow rapidly down the hill sides carrying mud, sand, gravel and rocks with them. These materials cut into the floor and sides of the stream banks thus eroding the land. This process of corrasion causes modification of the young valley profile.

Shown in the photograph at top are rapids in Minisceongo Creek as it crosses Route 210 southeast of Lake Welch. Even a streamlet, flowing into Pine Swamp next to the Arden-Surebridge trail, slowly erodes its valley floor.

POTHOLES

Potholes are drilled into bedrock by the rock fragments carried by the fast moving water in a whirlpool. This action is common in fast moving streams and is observed today in many stream beds. As the glacial ice melted, at the end of the Pleistocene Epoch, torrents of water surged down through fissures or cracks in the ice mass. As this water reached bedrock it was possible for it to produce potholes.

The pothole pictured here is located on the west side of Seven Lakes Drive near the south end of Lake Tiorati. It is approximately ten inches in diameter and twelve inches deep. The rock surface into which the hole is cut is steeply sloping toward the road and shows no indication that it has recently been in a stream bed. It is possible that the pothole was drilled either under the ice sheet or by a meltwater stream. Subsequent down cutting of the stream left the remains of this pothole on the stream bank.

THE HUDSON RIVER„A FIORD

Before the Pleistocene ice ages the Hudson River followed a southerly course as it cut through the uplifted Highlands. As the land was covered with several hundred feet of southward moving ice, the glacial ice was channeled through the valley causing it to increase speed and cut deeper. As a result the bedrock of the river bottom was ground to a depth of as much as 1000 feet below sea level forming a fiord. The ridge ends projecting into the river were scraped off forming cut off spurs. As seen in the photograph above top, taken looking north from the scenic outlook on Bear Mountain, Anthony's Nose is an example of a truncated spur. Looking south one sees Iona Island where the course of the river was changed from its west side to the east side by the down-cutting glacier.

ROUNDED MOUNTAIN SUMMITS

The broad, rounded summits of the hills of the Hudson Highlands are the result of several geologic factors that have been present during the lifetime of these rocks. The composition of the Storm King granite/gneiss, that makes up many of the highest ridges, is relatively homogeneous over the region. Weathering of this rock tends to occur at the same rate over the entire summit and the formation of exfoliation plates favors maintenance of a rounded surface. The glaciers of the Pleistocene Epoch contributed to the rounded hills by their grinding and ice plucking actions. Water erosion, during recent times, has resulted in the gradual removal of weathered materials from the hillsides and valleys.

The hilltops observed when looking west from the top of Bear Mountain demonstrate the rounded conformation of the regional topography.

MATURING VALLEYS

As streams cut into their beds the slope of the valley floor is reduced and the rate of the water flow decreases. At this point in the evolution of the stream valley erosion of the walls becomes significant as the stream meanders, that is wanders from one side of the valley floor to the other, and slowly cuts into its banks. This lateral erosion is a slow process because of the erosional resistance of bedrock.

At the point where Stony Brook crosses Seven Lakes Drive on its way to the Ramapo River, its valley is in the process of maturing from a young V shaped valley, with rapids and waterfalls, to a broader, flatter valley with a meandering stream.

STREAM DEPOSITS-DELTAS

After the glaciers of the Pleistocene Epoch streams of the region removed glacial drift from hillsides and valleys. The erosive action of the moving water carrying silt, sand, pebbles and rocks also cut into bedrock. When these fast moving streams entered relatively quiet waters the rock debris they carried was deposited on the bottom of lakes or rivers. These deposits built up to form land masses called deltas. Some deltas were built in lakes formed as glacial ice blocked meltwater runoff. Much of the village of Haverstraw rests on a glacial lake delta.

The Iona Island delta is shown in the photograph above as it appears from Bear Mountain. This delta has been formed in recent times by deposits of mud from the Doodletown Brook as it enters an abandon channel of the Hudson River. This marsh land is now preserved as the Iona Island Bird Sanctuary and also serves as a breeding ground for fish and other local wildlife.

DOWNSLOPE MOVEMENT

Gravity is the driving force for stream erosion, which is currently the primary erosional mechanism in the Hudson Highlands, and also provides the debris carried by the water through the process of downslope move-ment. One form of such movement occurs when either frost wedging or exfoliation cause rock fragments to break from exposed rock surfaces in road cuts and fall to form talus. As this rock debris is carried by water it aids in the cutting of the stream bed and banks. This picture shows frost action at work along Route 6.

SWAMPS

As the fast flowing water of a stream enters a lake it slows down. The result of this change in velocity is that the mud and sand, carried by the stream water, falls to the bottom of the lake. At first this process builds deltas and as time goes on the entire lake fills and is converted to a swamp. Some glacial lakes have been partially drained, forming swamps, as the streams leaving the lake have eroded their valleys and thereby lowered the water level in the lake. Many of the glacial lakes that once dotted the landscape have been converted to swamps and eventually to dry land as erosion and deposition have progressed over time.

The swamp shown in the photograph is located northeast of the intersection of Lake Welch Drive and St. John's Road. There are many swamps throughout the parks and they serve as important breeding grounds and food sources for the wildlife that resides in the area.