White Mountain Geology Part 3: The Last Two Million Years

Our glacial history began about 2 million years ago during the Late Pliocene. The earth entered into a cyclical period of warming and cooling periods. The cooling events resulted in ice ages while the warming periods experienced a Greenhouse Effect.

The climate cooled, glaciers formed high in the mountains and then flowed into the valleys and moved southward. A glacier is a permanent body of snow and ice. It forms when the winter snow pack fails to melt over the year allowing snow and ice to build up over the years. Glacial ice is similar to a metamorphic rock. It has interlocking crystals of the mineral ice. Its deformation is due to the weight of the overlying snow and ice. Over time the new snow and ice morphs into the dense impermeable mass that is found on glaciers. New snow is extremely porous and has a density less than a tenth that of water. Over time the delicate crystal points of a snowflake evaporate and disappear. The fragile snowflakes gradually become smaller, compact and reduce the pore space between each crystal. Snow that has survived a year or more increases in density and becomes impermeable to air.

Glacier Types

Two main types of glaciers would have been found in the White Mountains towards the beginning of these glaciations. The first was a cirque glacier and the second was valley glaciers. Cirque glaciers form high up in the mountains before overflowing into the valleys forming valley glaciers. The valley glaciers often followed old stream beds and scoured the sides and base of these valleys creating a U-shaped cross section. As these glacial periods progressed large ice sheets formed covering many of the mountains we see today.

Glacial Processes

While converging continents and volcanoes formed the White Mountains, glaciers shaped them. Two processes are responsible for the glacial features we see today. These are plucking and abrasion. Plucking occurred when melt water permeates cracks and joints in the rock as the glacier passed over the bedrock. This water then refroze, cracked the rock further, and fractured the rock. Glaciers often absorb loose rocks and carry them. Abrasion occurs as the rock picked up during plucking grinds against the bedrock like sandpaper.

Glacial Features in the White Mountains

Cirques are one of the most common and most distinct glacial landforms in glaciated mountains. Cirques are commonly found on the eastern slopes of the White Mountains where the snow would have been blown off of the summits and collected. Cirques are large bowl-like shaped valleys with a large steep headwall on one side. Many are bound on their down valley side by a bedrock threshold that impounds a small lake. Cirques start out as a large snow field. They are formed as a result of frost wedging, abrasion, and glacial plucking. Melted water works its way into the rocks under the snow, refreezes, and expands, fracturing the rock and breaking off pieces. Small rock fragments are carried away during thaws. A shallow depression in the rock is created and gradually enlarged. A glacier forms as snow accumulates in the deepening depression. Plucking then helps to enlarge the slope more and abrasion at the bed of the glacier then deepens the cirque further. Tuckerman and Huntington Ravines are both examples of cirques. Hermit Lake Shelter presently has 70 inches of snow in the bowl of Tuckerman Ravine. While both ravines see large amounts of snow it does not survive the summer and fails to form glaciers.

The notches in the White Mountains were carved by the valley glaciers. These glaciers followed V-shaped river valleys and as the glacier passed through it widened and deepened the valley leaving behind a U-shaped valley after the glaciers retreated.

The sheep’s backs or roche moutonnée are terms for asymmetrical landforms where the glaciers gradually wear down one side of a mountain and then pulls and plucks away the rock on the other side as it passes over. They were nicknamed sheep’s backs or whale backs because the resembled the backs of these animals. These formations can be used to determine the direction of glacial movement. Mount Willard in Crawford Notch and Mount Monroe are great examples of sheep’s backs.

Glacial striations are scratches or grooves found on the surface of the bedrock. As the glacier moved over an area the rocks that it carried at its base carves scratches or grooves into the rock as the glacier advanced. Examples of these can be found on the open slabs of rock outside Mizpah Spring Hut.

As the glaciers retreated they left behind large amounts of glacial till and erratics. Erratics are pieces of rock that are not native to an area. These rocks could have been carried a few feet or hundreds of miles from where they originated. They are usually sub angular; meaning that they are not jagged like newly fractured rock or perfectly round like the rocks found in rivers. They are instead somewhere in between because of the grinding they experienced while being carried by the glaciers. Glen bolder is a fantastic example of a glacial erratic and can be seen from Route 16 as you drive from Jackson towards Mount Washington. Glacial till is the poorly sorted rock and gravel that was left behind after the glacier melted and retreated.

Recent Geologic Events in the White Mountains

In the more recent history a number of rock slides have occurred throughout the White Mountains. Rock slides occur after periods of drought followed by intense periods of rain. This will liquefy the sediment causing in to flow down a slope. In the White Mountains the glaciers carved away any sediment, scraping down to the bedrock. As the glaciers retreated they deposited a layer of unsorted till that was later liquefied and slide down the slopes of many of the White Mountains. Two that come to mind are the Willey Slide and the rock sides on the slopes of Whitewall near Zealand Falls Hut.

Near the base of the Willey Slide stands the Willey House which is now run by the State Park during the summer. During the early summer of 1826 Crawford Notch received a sudden heavy rain storm which caused two rock slides close to the Willey House. A larger slide occurred on Monday August 29, 1826. A large storm caused the Saco River to flood and liquefied the loose rock, sediment, and vegetation that had been growing on the side of what would eventually be called Mt Willey. The Willey family was living in a small farm house near where the Willey House now stands. When they saw the rising flood waters they fled the house. Then a rock slide swept through. A large rock behind the house blocked the house from the rock slide and diverted it around the house. The whole family unfortunately perished. Some were buried in the rockslide while others drowned. The scar from the slide still remains up on the slopes and is a popular climbing route during the winter.

The slides on Whitewall are harder to date. There were about thirteen slides in this area that occurred both before, during and after J.E. Henry built his railroads through the area. Henry had his trains clanking along the Zealand Valley Railroad from 1884 through 1906. You can walk along the Ethan Pond Trail and follow the old railroad bed. As you pass below Whitewall you can see where slides had passed through before the railroad was built and where slides have come down since.

Up above tree line on the Presidential and Franconia Ridges you will find open areas of jagged rocks. These rocks were exposed after the glaciers retreated but were not deposited by the glaciers. These angular rocks are the result of frost wedging. Water will work its way into the cracks and joints in the rocks. When cold temperatures hit this freezes that water causing it to expand and fracture the surrounding rock forming the large rock boulder fields that you find above tree line.

While the glaciers, volcanoes, and colliding land masses are no longer shaping the rocks in New England the effects can still be seen and tell stories about what happened during our past. So get out there and see what the land around you is saying.

Photos by Tom Meagher and Kate Keefe