The Bedrock
Acton is underlain by the Nashoba Formation, an assemblage of old metamorphic rocks which were reshaped and covered during the continental ice ages. Original sandstones or similar sedimentary rocks were altered by heat and pressure over geologic time into metamorphic rock. The formation is largely gneiss, a relatively coarse-grained rock, which shows different layers of minerals upon close examination.

The mineral composition of the rocks in the Nashoba Formation varies considerably, and numerous subdivisions have been identified. Most of the formation in Acton is biotite gneiss, which contains small plate-like crystals of the mineral biotite, a black form of mica. The arboretum area is entirely underlain by this particular type of rock. Throughout the Arboretum, outcrops of bedrock show at the land surface. Several near Wood Lane are particularly easy to find, as is the outcropping just east of the reading circle.

The Nashoba Formation is relatively old, dating back to the Ordovician geologic period, which occurred between 430 and 500 million years ago. It has been subjected to extreme forces over geologic time as at least one mountain range rose and was eroded away. As a result, the formation is extensively folded and faulted. Hansen maps a fold axis through the Arboretum centered at the rock outcrop near Wood Lane. Elsewhere in Acton, the various subgroups within the Nashoba Formation are mapped as elongated bands that stretch from northeast to southwest, although no such bands are mapped within the Arboretum.

Faults separate the formation from the neighboring rocks to the northwest and southeast. These faults are minor and do not present a significant geologic hazard. Nonetheless, small magnitude earthquakes occur once every year or two. If residents even notice these earthquakes, they often mistake them for a large truck passing on the road, although sometimes they are accompanied by a sudden loud noise like a cannon shot.

The Nashoba Formation is punctuated in places by younger volcanic rocks, known as the Acton granite. Granite deposits were formed when molten magma intruded from the subsurface into the Nashoba Formation. The intrusions, which are relatively small features, were mined in the past in several small quarries in Acton. A group of cut granite stones lie off the Highland-Bog trail towards Wood Lane. These stones show the remnants of drill holes where the stones were broken off from a much larger boulder. The remaining part of the boulder is not granite. It shows a flat, rather crystalline surface typical of the Nashoba Formation and was apparently the wall of the fault into which granite intruded long ago. The cut granite stones remaining near the boulder are similar to the large foundation stones seen in colonial houses and barns around Acton. For some reason, these particular cut stones were never removed and used.

Glacial Striations and Deposits
The geologic character of Acton is largely determined by younger deposits that overlie the bedrock. These varied formations were deposited during the continental ice ages, which ended 10,000 years ago, a very recent time geologically. During the ice ages, sheets of ice over a mile thick in places blanketed Canada, the north central United States, and New England. The glaciers formed, wasted away, and reformed although only the effects of the most recent ice age are clearly discernible in the area's geology.

During each ice age, massive sheets of ice moved over the landscape, scraping, and re-depositing rocks and sediment. In Acton, the last glacier moved more or less due south. Glacial striations, marks scraped by the moving glacier and the rocks it carried, can still be seen on smooth rock outcrops. The high bedrock ledge on the edge of the meadow near Wood Lane shows glacial striations that point more or less south, roughly parallel to Wood Lane. The striations are difficult to see, but can be seen as rough indentations that all line up at an angle of about 45 degrees from the grain of the rock.

Glacial Till
The ice ages resulted in numerous and varied geologic deposits, formed when the glacier passed and also during the post-glacial period as the melting glacier produced torrents of water. Much of Acton is blanketed by glacial till, a compact mixture of sediment. Till is composed of a wide range of grain sizes, from very fine clay particles to large boulders. These various grain sizes were compressed under the moving glaciers into a poorly sorted mixture that is tight, almost impervious, to water. The widespread till deposits are also known as ground moraine.

The rocky soils that discourage farming in New England are typical of these glacial till soils. Colonial farmers removed the rocks from the soil on a yearly basis, forming them into stone walls like those seen at the Arboretum. In several places on the Arboretum, the soil was too thin or poor for farming; and rocks were simply dumped in large piles there. High water tables and poor drainage that interfere with planting are typical of till deposits as well.

Glacial Outwash
The lower elevations in Acton's landscape are generally occupied by glacial outwash deposits, sand and gravel deposited by water running from the melting glaciers. Fine-grained clay and silt were washed from these deposits by the running water, as a result these soils are more open and drain more readily than the till soils. All of Acton's public water-supply wells are located in sand and gravel outwash, and these deposits generally require greater protection from pollution than the areas covered by till.

Kame Terraces
The Arboretum lies at a transition from the upland ground moraine to lower-lying outwash deposits. South of Minot Avenue and extending south of Route 2 as far as Fort Pond Brook lie kame terraces. Kame terraces were formed as sand and gravel accumulated along meltwater streams that coursed along the margin of the melting glacier, between the glacier and adjacent higher land. Immediately south of Minot is a kame terrace on the site of Conant School.

Leading into the kame terrace south of Minot Avenue is an esker that courses through the Arboretum. Eskers, long sinuous gravel deposits, are found in several other locations around Acton. These deposits were probably made in tunnels under the wasting glacier or open crevasses in the glacier. Water in the meltwater tunnel or crevasse along the esker's path flowed southward to the kame terrace, depositing the sand and gravel that makes up the esker. Today, they stand as narrow causeways, 10 to 30 feet high, winding through the woods. Were it not for their tortuous path, one would mistake them for constructed road or railroad beds. A trail runs along the top of the esker at the arboretum. Over the years gravel was mined from the esker for the area roads leaving a large excavated bowl on its east side.

The meltwater streams that flowed along the course of the present-day esker and kame terrace discharged into glacial Lake Sudbury south of where Route 2 lies today. Glacial lakes formed when meltwater accumulated behind ice or earth dams. Lake Sudbury occupied an area covering parts of Sudbury, Maynard, West Concord, and South Acton. It may have consisted of several lakes rather than a single large lake. Where meltwater streams discharged into glacial lakes, they often formed kame deltas, deposits of sand and gravel that accumulated on the lake bottom. Kame delta deposits lie south of Route 2 as far as Fort Pond Brook.

Acton's geology continues to change. There are geologic formations that postdate the ice ages including swamp deposits, which are forming in wetlands throughout the town, and alluvium, which forms in streambeds. Swamp deposits are currently accumulating in the marsh on the Donald property on the west side of the esker, and even more dramatically in the bog. The bog was once open water, but has been filling in with sediment and sphagnum moss over the decades so that now it is a bog in transition to a meadow. Eventually, the bog will fill in sufficiently that all open water will be gone and a grassy meadow will take its place.