The Geology of Mount Blue State Park

Geologic History

Of the bedrock that can be seen within Mount Blue State Park, the oldest was deposited about 330 million years ago at the bottom of a deep sea as sediments composed of black muds and clayey sands. They formed a sequence many thousands of feet thick. A large part of these were deposited in thick even-textured beds, but commonly very thin layers of mud were alternated with thin beds of clayey sand, producing a feature known as cyclic bedding, similar to a many-layered cake. It was possible to produce such a great thickness of sediments because the bottom of the sea sank at a rate close to that of the rate of accumulation of sediments. This phenomenon is not as strange and extraordinary as it perhaps sounds, but constitutes a very important manner of deposition.

Following the deposition of the black muds and clayey sands came a period of reduced erosion of land masses from which the sediments were derived. As a result, 300 to 500 feet of limy material interbedded with some mud and clayey sand was deposited in this area of accumulation under the sea.

Figure 1

After the deposition of these calcareous rocks, there followed a period similar to the earliest one, with the consequent deposition of approximately 5000 feet of cyclically bedded black mud and clayey sand. Younger sediments were deposited on top of these, but examples of these are not found within Mount Blue State Park. For a pictorial presentation of the events described so far see Figure 1.

With time the loose sediments became compacted and consolidated into solid rock. This process was aided by precipitation of various materials out of solutions which seeped through the sediments, as well as by the downward force exerted by above-lying younger sediments. The muds became transformed into shale, the clayey sands into micaceous sandstone, and the mud and limy ooze into micaceous sandy limestone.

Figure 2

About 300 million years ago, several tens of million years after all of the above-mentioned sediments were deposited and consolidated into rock, New England was subjected to great stresses from the northwest and southeast. As a result of these stresses all of the rocks were deformed into a series of great folds trending from northeast to southwest. This time of deformation in the Devonian period (see Figure 2 for breakdown of geologic time into periods) is known to geologists as the Acadian orogeny because it was first studied in Nova Scotia (Acadia).

Figure 3

Contemporaneous with the deformation, a hot liquid magma was intruded, i.e., injected, into the sedimentary rocks. With time this magma cooled and crystallized into a large mass of granite-like rock known as granodiorite. For this sequence of events see Figure 3. Heat from the injected magma raised the temperature of the rocks and brought about their metamorphism, or rock change. The shale was transformed into schist, the micaceous sandstone into micaceous granulite, and the micaceous sandy limestone into calc-silicate granulite. In addition to metamorphosing the sedimentary rocks and forming a large body of granodiorite, the cooling magma was injected into the solid rock and formed into various shaped smaller bodies, such as dikes, sills, and lenses. Such small bodies typically contain the following minerals: quartz, feldspar, muscovite, biotite, and occasionally tourmaline, apatite, garnet, and beryl. The size of the individual minerals is commonly much larger than in ordinary igneous rocks. Such coarse-grained rocks are known as pegmatite, and it is from such pegmatites that our semi-precious gemstones are obtained.

No younger bedrock than the granodiorite and related pegmatites is found within Mount Blue State Park. Following the igneous intrusion and deformation of rocks by folding, related to the Acadian orogeny, there was a long period of erosion. Any mountains that had been built were worn down to become hardly more than a gently inclined plain. Then about 100 million years ago, in the Cretaceous period (see Figure 2), the central part of what is now New England was uplifted perhaps as much as one and one-half miles. Rejuvenated streams began to cut into this more steeply inclined land producing a rugged mountainous terrain. Finally starting about one million years ago, a series of four ice sheets, or glaciers, spread from the north over all of New England. Between the times that the land was covered by ice, there were periods of warm, humid, almost tropical climate. The last ice sheet melted about 20,000 years ago, and we are now living at a time of warmer climate. Whether yet another ice sheet will come in the future, or whether the "Ice Age" is over, is a subject for speculation for most geologists. These glaciers came down laden with pieces of rock, which had been pried loose from the bedrock by the scraping action of the ice. Using these as one would use an abrasive, the glaciers scraped and shaped the countryside until the topography had the appearance that it has today. When the last ice sheet melted, it left behind an accumulation of sand, gravel, cobbles, and boulders, some of the last measuring more than 100 feet in diameter. The general name given to these loose sediments is drift. If these sediments were deposited directly from the foot of the glacier, without being subsequently washed by running water, the drift would lack stratification, that is bedding. It is then referred to as unstratified drift or till. Sizes of particles in till range all the way from clay to boulders. These particles were dumped by the melting glacier without being sorted as to size, so that clay-sized particles typically are found next to large pebbles and boulders. Water running over loose sediments tends to sort the sediments as to size and to deposit them in beds in which different strata are composed of particles of similar size. Thus wherever melt water from the remnants of the glacier flowed over drift, it imparted to it stratification and sorting. Such deposits are known as stratified drift. The rock fragments imbedded in dirt or lying on the surface, after being eroded out of the drift, are known as erratics. This term implies that they did not originate at the place where they are found, but were brought, possibly from a distant place, by the glacier. Most of the erratics in Mount Blue State Park are granodiorite or metamorphic rocks which look very much like the local bedrock. However, one occasionally finds blocks of unmetamorphosed sandstone, shale, volcanic rocks, or other types of igneous rock, which form the bedrock in areas tens of miles to the northwest. These constitute final proof of the carrying ability of glaciers.

After the last remnants of the glacier were melted and all of the glacially derived sediments deposited, young rivers began incising into the masses of drift. Some of the best exposures of drift are found in cuts produced by the erosive action of running brook and stream water. This running water removed the finer-grained material and left behind the pebbles, cobbles, and boulders which typically constitute the beds of streams in hilly terrain. The fine-grained particles, sand and clay, were carried downstream and deposited in the lower reaches of the brooks and in lakes. Typically sandy deltas form at the mouths of streams as they enter a lake. The wave action of the waters in the lake, prompted by wind, removes the sands from the deltas and redistributes them around the peripheries of the lake. If wind conditions are favorable, fine sandy beaches are produced.

The present-day scenery of Mount Blue State Park is the result of the sculpturing action of the glaciers. The valley stretching from north of Lake Webb to the Village of Dixfield in the south was formed by the abrasive action of the glacier. The bedrock underlying the valley is granodiorite, which proved to be softer and easier to erode than the metamorphic rocks that hold up the mountains surrounding the valley. Many outcrops within the park show evidence of glacial erosion, in the form of grooves and striations that were dug out and etched as loose rocks at the bottom of the ice were dragged across the surface of the bedrock. Excellent examples of this can be seen on the summit of Center Hill.

Outcrops of granodiorite are rare. When the glacier melted it left in many places on the floor of the valley as much as several hundred feet of drift. Consequently there is not one outcrop of granodiorite within the area of the lake in Mount Blue State Park. Instead one finds unstratified drift, locally dissected by erosion to a depth of as much as 20 feet by streams and brooks such as Swett Brook, which flows next to the winding drive to the beach in the park.

Introduction Geologic History Objects of Interest Glossary Geologic Map (pdf)

Last updated on January 11, 2008

Contents

The Geology of Mount Blue State Park

Geologic History