TABLE OF CONTENTS Bulldozing and Abrasion by Glaciers Glacial Lakes A Unique Glacial Pothole Erratic Boulders Propped Erratic Boulder Fields Plucking by Glacial Ice

CONTINENTAL GLACIERS or ice sheets are slow-flowing, huge, thick masses of ice. The glaciers that once covered what is now Canada and much of the northern part of the United States during the Pleistocene Epoch advanced and retreated across the Highlands four times.

Although the last of the continental glaciers left this region over 12 thousand years ago, the results of glacial action are still evident throughout the Hudson Highlands. Glaciers were agents of erosion as they bulldozed, scraped, ice plucked, and contoured the bedrock and soil of the area. Glaciers also deposited till (unsorted debris consisting of boulders, sand and silt) in terminal moraines at their edges and ground moraines as they receded during melting. The alternate freezing and thawing as ice sheets advanced and/or receded accelerated the mechanical weathering process of wedging by frost action. The melt waters that resulted during the recession of the glaciers cut deep valleys, including the Hudson River Valley. Lakes, such as Hessian Lake, and swamps were formed in the depressions that had been cut by glacial action.

BULLDOZING AND ABRASION BY GLACIERS

As the Pleistocene glaciers moved slowly southward they pushed and carried soil, rocks and other loose materials along with the ice. The result was to bulldoze large amounts of loose materials from the surface of the land leaving bedrock uncovered. Abrasion or scratching of the bedrock surface resulted as rock fragments, that were embedded in the ice mass, moved over the bedrocks. The photograph above and at top show two bedrock surfaces that have been bulldozed and abraded above the Indian rock shelter near Lake Tiorati and on Bear Mountain.

GLACIAL LAKES

As the glaciers of the Pleistocene Epoch moved over the land they deepened valleys by abrasion and plucking. Softer rocks were more easily eroded and depressions in the bedrock resulted. After the glaciers withdrew, these basins filled with run-off water forming lakes and ponds. Other glacial lakes resulted when the drift, dropped by melting ice, formed dams across valleys and caused the melt-waters to be trapped.

Picturesque Hessian Lake, shown in the photograph above, lies at the foot of Bear Mountain near the Inn. It is one of the few lakes in the parks that was formed wholly by glacial action. The lake basin was created when the softer Canada Hill granite, near the base of Bear Mountain, was eroded more easily than the resistant Storm King granite that makes up the mountain. Other lakes such as Tiortati, Sabago, Welch and Kanawauke have been created by man made dams.

A UNIQUE GLACIAL POTHOLE

Potholes were drilled by glacial streams as the meltwater swirled in circular basins. The sand and gravel carried by the stream served as an abrasive and cut into the bedrock thus forming the round, smooth-sided potholes. A unique pothole is found next to the Ramapo-Dunderberg Trail near the intersection with the Arden-Surebridge Trail on the south slope of Fingerboard Mountain. However, this location is not along a streambed but is in the face of a vertical rock wall. One explanation for this unusual structure is that the water of the stream was carried in an ice crevasse and encountered the bedrock near the summit of the mountain. Subsequent erosion has removed one side of the 8 foot pothole.

ERRATIC BOULDERS

One piece of evidence indicating that a region has been over-run by continental glaciers is the presence of large rock fragments lying on bedrock of different composition. These boulders, that have been transported by glaciers, are called erratics and are found in many locations throughout the Highlands. Often the source of the erratics is several hundred miles north of their eventual deposition location. It is common for these transported boulders to be scratched or striated as they were rubbed along bedrock and this process may have also flattened one or more sides of the rock and rounded their edges.

The photograph above shows erratics scattered on the glacier bulldozed surface near the top of Bear Mountain. Notice that the large boulder at the left is resting on a smaller stone at one end. This arrangement is the result of the erratic boulder being deposited on top of other glacial till, that is unsorted rock debris, which was later removed by water erosion.

PROPPED ERRATIC

The erratic boulder, shown above, rests on a smaller stone in the picnic area at the top of Bear Mountain.

BOULDER FIELDS

At the end of the glacial period, about 13,000 years ago, there was a period of rapid melting of the thick layer of ice. The drift, composed of soil, sand, pebbles, and rocks, that had been carried by the glacial ice was deposited over the entire landscape. Clay, sand and gravel were quickly washed to the valley floors by rain water and the water formed from melting ice. The larger boulders, that were left on hillsides, slipped or tumbled to the base of the slope sometimes completely covering the soil and bedrock and thereby forming a boulder field.

The boulder field pictured above is located on Long Path, near the point where it crosses St. John's Road. Notice that most of the rocks are somewhat rounded as a result of glacial abrasion and later erosion by stream action. These boulders were deposited from hillsides on the right and left sides of this valley as viewed in the picture.

PLUCKING BY GLACIAL ICE

As the glaciers of the Pleistocene era moved slowly in a generally southerly direction they acted like bulldozers as they pushed large quantities of soil and rock before them. They also worked like road graders as they dragged rocks embedded in the ice over the bedrock thus scraping and smoothing its north facing surface. The east and west sides of hills were also smoothed by this abrasive process.

The south facing sides of hills experienced the process called plucking. As ice, that was frozen to cracks in the bedrock, moved slowly forward blocks of the rock were broken free and dragged along by the glacial ice. This plucking process resulted in the south facing sides of hills being roughened to the extent that almost vertical rock walls were formed as illustrated by the rocks at the Indian shelter near Lake Tiorati shown in the photograph above.