A Detailed Geological History of the Cold River
The rocks in the Cold River Corridor tell a fascinating story of three distinct geological periods. The first period of mountain building occurred about 375 million years ago. The second occurred when magma was injected into the pre-existing mountain rocks. These first two chapters of the Cold Riverís geologic history built the bedrock of the riverís course. The third period played off these two by altering the surficial geology of the river through the glaciers that reached down from Canada and scoured the terrain. Melt water from these glaciers shaped the land surface both by eroding it and by depositing vast quantities of materials- sands, silts and large glacial erratics.
During the Ordovician Period, 430 - 500 million years ago, New Hampshire may have laid under the sea, bounded to the west by an ancient continental predecessor of North America, and bounded on the east by a volcanic island arc. Some of that periodís volcanism occurred under the sea and is now evident in the Ammonoosuc Volcanics. Such rocks are found in exposures between Alstead and South Acworth at Osgood Ledge, west of the Cold River.
Following this period of volcanic activity, sediments eroding from lands to the east buried the igneous rock under beds of mud. The rock from this period metamorphosed into schist known as the Partridge Formation. Towards the end of the Ordovician Period, the Taconic Orogeny occurred. With much folding and faulting, this mountain building event lifted the region of New Hampshire from under the sea. In the area of the Connecticut River, an upward arching structure formed as magma rose beneath the ground surface to form the Bronson Hill anticlinorium. Following this period of upheaval, erosion removed much of the Partridge Formation. What remains of this formation is also located between Alstead and South Acworth, although no good exposures have been located.
As the land subsided, the sea again covered western New Hampshire, depositing quartz sands and gravels in a relatively thin layer many feet thick. These sediments eventually became Clough quartzite, present now in a narrow band between the older Partridge Formation and the younger Littleton Formation. A juncture where these three rock types are in close proximity to one another is at the mine on the side of Beryl Mountain.
The Littleton Formation that formed nearly 400 million years ago during the Devonian Period is composed chiefly of fine to sandy muds, now metamorphosed into schists. Sporadic volcanism is also evident from this time, and lies about one half mile from the river mouth, under many yards of sand and sediment. Rock representing the Littleton Formation is visible at Deep Hole in South Acworth and at Vilas Pool in Alstead.
About 385 million years ago the sea once again retreated and a period of folding began which coincided with the injection of masses of molten rock in the shape of great mushrooms into the region of the Ammonoosuc Volcanics. These cooled and consolidated into the Olivarian magma series. One of these granite structures bisects the Cold River just west of Beryl Mountain and is called the Alstead Dome. At a later stage, magma was injected into the Littleton Formation. It cooled and formed Bethlehem gneiss, part of the New Hampshire magma series and visible in two areas along the River. First, the Bellows Falls Pluton reaches from Alstead south to the Connecticut River and is visible at Drewsville Gorge. Second, stretching upstream from Honey Brook to Crescent Lake is the Mt. Clough Pluton, exposed near Honey Brook where ledge bisects the Cold River.
Since this last episode of folding and magma intrusions about 30 million years ago, erosion has been the dominant force in the Cold River Corridor, although it is likely that episodes of uplift alternated with periods of erosion many times in the last 25 million years.
Along with the Ammonoosuc Volcanics at Osgood Ledge, the Clough quartzite, Partridge Formation and Littleton Formation juncture at Beryl Mountain, the Littleton Formation exposures at Deep Hole and Vilas Pool, the Alstead Dome just west of Beryl Mountain, the Bellows Falls Pluton at Drewsville Gorge, and the Mt. Clough Pluton at Honey Brook, are the amazing pegmatites in this area. Pegmatite is an extremely coarse-grained granite formed during the same period as the New Hampshire magma series as a watery fluid injected into rock fractures. Because of their formation process, pegmatites are found in almost any rock type. Extremely large crystals form due to the watery nature of the material. Gene Boudette, NH State Geologist, conveyed that this mineralization resulted in a valley noted for its beryl and feldspar deposits. Beryl Mountain, just south of South Acworth, was the site of a large pegmatite mining operation. Beryl crystals four feet long and feldspar crystals fourteen feet long have been found. Other valuable minerals associated with pegmatites are quartz, and books of white mica. Three other mines once operated in the South Acworth area and extracted primarily feldspar, which was shipped to the Midwest for use in the glass and ceramics industries. These features make the Cold River Corridor a geologist's and rock hound's delight.
As impressive as continental glaciation was to the land surface of the Cold River Corridor, the departure of these glaciers left the rich surficial geology we are familiar with today.
Glacial Lake Hitchcock, meltwater trapped behind an ice and rock dam near Middletown, Connecticut, stretched north to Lyme, New Hampshire, leaving evidence of its existence in the Cold River Valley. Melt water first flowing from (and on) the glaciers and eventually flowing down the Cold River valley deposited vast amounts of sand and gravel at the lake margin. Driving east on Route 123 towards Drewsville is an extensive plain with several small hills, including Drew Mountain, punctuating it. This was the top of a delta extending back to Cock Hat Hill, east of Alstead, that formed in glacial Lake Hitchcock over 10,000 years ago. The Cold River now bisects the remnants of that delta, and terraces on either side of the River are evidence that the top of the ancient delta lay at a relatively consistent elevation of 492 feet. Within this great deposit is Drewsville Gorge, where the Cold River eroded the delta down to expose the bedrock.
Over the course of deglaciation, several small glacial lakes formed along the Cold River. One was behind the falls at Vilas Pool; the falls area was probably plugged by ice and sediment. This area and other former glacial lakes like it are interesting meltwater deposits of sand and gravel. Most are relatively intact having been exposed to only minimal mining.
Just south of Vilas Pool is Cock Hat Hill. By following a woods road that leads along the west side of the hill along a trickle of a brook, there is an area with many glacial potholes. This area is well away and above the River channel. During deglaciation torrents of meltwater from Warren Brook must have roared through here. As the Cold eroded deeper into the surrounding sediments it captured Warren Brook's flow and the gorge around Cock Hat Hill was left dry.
The glacial legacy of the Cold River is most evident in the sand and gravel deposits scattered through the Corridor. These deposits are economically important for the region, allowing it to be self-sufficient for aggregate needed for construction, road building and winter road maintenance. Soil types classified as having some potential as sand and/or gravel resources encompass much of the corridor. The NRCS rates soils as being either "probable" or "improbable" as commercially useful sources of sand and gravel with a minimum of processing, basing these ratings on grain sizes, the thickness of the deposit, and the content of rock fragments. The criteria used are a deposit at least 3 feet thick and less than 50%, by weight, large stones. In reality, not all of these deposits would meet the stricter criteria of sand and gravel excavators looking for economically viable sites where the investment needed for site development and permitting must be weighed against the volume of good material at the site. Access is obviously another determining factor of economic viability. One large active sand and gravel operation, known as Cold River Materials, a Division of Lane Construction Company (formerly Whitcomb), is located in the corridor in Walpole and extends across the River into Langdon utilizing the vast amounts of sand and gravel left in the lower Cold River valley by glacial melt waters. Several other smaller active and abandoned excavations are located within the corridor in these soils as well.
Another aspect of the surficial geology of economic interest is the prime agricultural soils. As shown on the map entitled "Natural Resources - Cold River Watershed", prime agricultural soils cover much of the corridor from South Acworth to the Connecticut River. A comparison with the map "Land Use - Cold River Corridor" shows that much of the area encompassed by these soils remains in agricultural use today forming an important component of the local agricultural economy as well as an essential feature of the scenic value of the area.
As shown on the map "Natural Resources - Cold River Watershed", another economically significant result of the presence of Glacial Lake Hitchcock and the valley melt waters was the formation of stratified-drift aquifers. Stratified-drift aquifers consist of sorted sands and gravels deposited by glacial melt water. The large pore spaces permit these deposits to store and transmit groundwater readily. A series of stratified drift aquifers follows the Cold River corridor starting with a massive deposit along the Connecticut River in Walpole and stretching all the way to the village in Alstead. Other areas with aquifers with potential for municipal use are found in Langdon just upstream from Alstead, downstream from South Acworth where Great Brook joins the Cold, in South Acworth, upstream where Honey Brook joins the Cold and stretching along the River from East Acworth into Lempster. Although most of the Cold River valley aquifers have relatively low transmissivities, less than 1,000 square feet per day, they are nonetheless important as sources for existing individual private wells and potential future sources of small community water supplies.
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The 1983 New England Rivers Center New Hampshire River Protection and Energy Development Final Report noted the high significance of the geologic/natural features of the Cold River based on a state-wide analysis of river resource values.