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A GEOLOGIC HISTORY OF AVON
The following essay was written by the late Mary F. Waterous, Avon conservationist and biology and geology teacher for over 30 years. It first appeared in Avon, Connecticut; an historical story by Mary-Frances MacKie published in 1988. We have received permission from the author to reprint the essay here.
The first people, Indians and whites, to look upon the area which has become the Town of Avon, as well as those who live in the town today, saw a panorama of mixed forests, ponds, brooks, the Farmington River and its floodplain, gently rolling hills, and some steeper cliffed regions. There was a great variety in the flora and fauna in the area due to the great number of biological niches available in such a diverse landscape. Its geologic history is a long one, and can be classified into four groupings in time: (1) preglacial time, when the whole region was part of a vast shallow sea with the sediments being deposited in it which would eventually become the rocks which make up the western part of the town, followed by long periods of erosion, faulting, the deposition of the sediments and volcanic rocks of
the eastern part; (2) glaciation, the period during which the glaciers advanced and modified the preglacial features; (3) deglaciation, the period during which extensive glacial deposits were laid down; and (4) postglacial time, a period of minor erosion and deposition.
This moment is but a vast continuum of time. Over five-sixth of geologic time had already passed when subsidence of our continent began so that Avon, along with vast parts of the continent, underwent the subsidence and the invasion of a great marine inland sea, several hundred feet deep. Sediments were deposited from land masses, and great volcanoes built up their cones. There was no life whatsoever on land, except possibly for some primitive algae which might have been able to live in moist areas. But life was teeming in the sea, with many strange forms which we do not have about us today, and they were all invertebrates. Living in the muds would have been eurypterids which could burrow and swim, and the shield fish, or ostracoderms. Shooting through the waters would have been long torpedo-like shelled
forms propelled by jets of water, the straight-shelled cephalopods, and their relatives those with curved shells and those with tightly coiled shells; their shells were chambered like their present-day descendants the chambered nautilus. Crawling over the bottom or swimming slightly above it would be enormous varieties of trilobites. Moving over the bottom there were marine snails and starfish, and buried in the mud were forms like today's mussels. The most abundant forms, however, were the brachiopods, which were supremely successful during those distant times; but they have been unable to compete today. In clearer waters there were many corals fas-tened to the bottom, and they built great reefs in their day. And waving gracefully in cur-rents of water were forests of beautiful crinoids. These are but a few of the life forms in that ancient sea which was accumulating thousands of feet of sediments. Subsidence ceased, and fold mountain building began. The rocks formed were the
metamorphic ones found in the western part of town; today we find them roughly running along a north-south line from Secret Lake in the north to Lovely Street in the south. They are about five hundred million years old and are schists, gneisses, and amphibolites. Today they are just remnants of what once were high moun-tains; hundreds of millions of years of erosion have taken their toll.
About two hundred and twenty five million years ago, erosion had reduced the area to one of relatively low relief. Faulting occurred and a basin began to form into which was deposited from mountains in the eastern part of Connecticut the sediments which would make our present-day arkoses, sandstones, and siltstones. Hundreds and sometimes thousands of feet of sediments collected. Then fissures split the earth and lava welled up to cover them. Three times the lava covered the sediments, like a triple-decker sandwich of lavas and sediments. Avon Mountain in the Talcott Range is made up of the last two lava flows, the Holyoke and Talcott basalts with the Shuttle Meadow sedimentary layer between them. The steep cliff of the range is the fault scarp formed about one hundred and eighty million years ago.
Underlying the surficial deposits of the center and eastern part of the town is the New Haven arkose; cutting through it is the diabase (a form of basalt) which makes up the ridge with its accompanying cliffs just to the east of Secret Lake, and running roughly in a north-south direction. Life had evolved, so that by this time the land had been invaded by both plants and animals, and the geologic period was the Age of Reptiles.
Sixty five million years ago uplift of the land began again, erosion was more vigorous, and the land would have appeared much as it does today. The ridges were probably somewhat higher and more rugged and angular prior to glaciation, and valleys probably were narrower with sharper profiles. The Farmington River ran to the Atlantic Ocean, and was not present as it is today in the eastern part of town. There were south-flowing streams in the eastern part of the town, eroding fairly easily in the softer sedimentary rocks.
One or two million years ago the Ice Age appeared with a series of glacier advances and retreats across the land. The number of advances is not known exactly, but there were at least four times when Avon was buried under the ice sheets. There are many glacially eroded surfaces exposed in the town, and glacier deposits cover nearly ninety percent of the area. In places, the main bedrock valley was cut to depths of more than one hundred feet below sea level by the glacial ice. As the glaciers overrode the area they removed most of the weathered mantle rock, some fresh bedrock, and any outwash which may have been laid down ahead of the ad-vancing ice. Much of the detritus picked up by the ice was smeared onto the underlying bedrock, particularly in valleys transverse to ice movements, and additional detritus was dropped on the
land surface as the ice melted. When stagnation of the ice was complete, the Farmington River picked its way along the edge of glacial moraines dumped in the south, thus taking a route from Unionville to Farm-ington, then running north through the eastern part of Avon and Simsbury through the Tariffville Gorge to become a tributary of the Connecticut River. Ice Age glacial deposits give us our present day swells and swales, little hills, bogs and swamps, kames and kame terraces. On top of these surficial deposits have been evolving the more than eighty soil types found in this town on which live the diverse forms of plants and animals.
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