America's Volcanic Past -
Connecticut

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Volcanic Highlights and Features:
[This list is just a sample of various Connecticut volcanic features or events and is by no means inclusive.]

• Connecticut
• Connecticut Regions
• Early Connecticut
• Bronson Hills
• Central Lowland
• Central Lowland's Volcanic Rocks
• Connecticut River Valley
• Day Pond State Park
• East Rock Park
• Hampden Basalt
• Hanging Hills
• Higganum Dike
• Hogback Ridge
• Holyoke Basalt
• Kettletown State Park
• Mansfield Hollow State Park
• New Haven
• Osbornedale State Park
• Penwood State Park
• Sleeping Giant State Park
• Southfork Falls State Park
• Talcott Basalt
• Talcott Mountain
• Traprock Ridges
• Wadsworth Falls State Park
• West Rock Ridge State Park

Paleozoic Era:⁶
From 450 to 250 million years ago, during the Paleozoic Era, several crustal plates, including Africa and Eurasia, sequentially collided with the Proto-North American plate in a series of four mountain building events that ultimately created the Appalachian Mountains and the supercontinent Pangea. During these collisions, Avalonia, a small continent believed to have been a part of the African plate, was thrust against the continent of Proto-North America, closing and collapsing the intervening Iapetos Ocean. The collisions deformed and metamorphosed both the continental rocks of Proto-North America and Avalonia and the oceanic rocks and sediments of the Iapetos Ocean floor. This process created the schists, gneisses and granites exposed today in eastern and western Connecticut. Features of these metamorphic and igneous rocks show this complex geologic history, confirming the continental and oceanic origins and the processes of plate tectonics.

Mesozoic Era:⁶
Shortly after the collision ended, at the beginning of the Mesozoic Era or about 235 million years ago, plate tectonic processes reversed. Pangea began to break apart, initiating the opening of the Atlantic Ocean and leaving Avalonia welded to North America. In the early stages of this breakup, rift basins formed along and on both sides of the zone where the Atlantic Ocean finally opened. The Newark terrane in central Connecticut is the eroded remnant of one of these rift basins. It contains 200 million year old sedimentary rocks (brownstone) and lava flows and intrusions of basalt (trap rock).

Bronson Hills Terrane:²
The Bronson Hills terrane is believed to be partly the roots of an ancient 440-million-year-old "volcanic island arc" in the Iapetus Ocean, before the arc collided with the ancient continent. Its Ordovician rocks were re-metamorphosed during the Acadian orogeny about 380 million years ago. To View: from Wesleyan, head south on Route 9. The highway rises up the great eastern border fault of the Mesozoic Hartford basin, into early Paleozoic rocks (mainly gneisses) of the Bronson Hills terrane, which runs up the eastern side of the Connecticut River Valley into New Hampshire. The Ordovician Monson gneiss of the Bronson Hill terrane shows "s" folds.

Central Lowland:⁵
The central lowland of Connecticut is underlain by early Mesozoic arkosic sedimentary rocks locally intercalated with igneous basalt and diabase.

Newark Terrane, Hartford and Pomperaug Mesozoic Basins:⁶

• Buttress Dolerite (Middle? Jurassic), Intrusive Rock
  Dark-gray to greenish-gray (weathers brown or gray), medium- to fine-grained, commonly porphyritic, generally massive with well-developed columnar jointing, grading from basalt near contacts to fine-grained gabbro in the interior, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine

• West Rock Dolerite (Lower Jurassic), Intrusive Rock
  Dark-gray to greenish-gray (weathers bright orange to brown), medium- to fine-grained, grading from basalt near contacts to fine-grained gabbro in the interior, generally massive with well-developed columnar jointing, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine

• Hampden Basalt (Lower Jurassic), Stratified Rock
  Greenish-gray to black (weathers bright orange to brown), fine- to medium-grained, grading from basalt near contacts to fine-grained gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass

• Holyoke Basalt (Lower Jurassic), Stratified Rock
  Greenish-gray to black (weathers bright orange to brown), fine- to coarse-grained, grading from basalt near contacts to gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass

• Talcott Basalt (Lower Jurassic), Stratified Rock
  Greenish-gray to black (weathers bright orange to brown), fine- to medium-grained, grading from basalt near contacts to fine-grained gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass. Pillows in may places; volcanic breccia with fragmentary pillows in others

Connecticut Valley:³
Looking to the north in the central valley, a series of ridges can be seen to have a steep western face but a gentle slopes toward the east. These ridges are mainly basalt, also called traprock -- the lavas of great Mesozoic volcanic eruptions. They are surrounded by softer sedimentary rocks, mainly sandstones. Mesozoic sediments and lava flows of the Connecticut Valley were originally horizontal. As faulting tilted the rocks downward to the east, the asymmetric ridges came into existence.

Connecticut River Basin:⁴
The Newark Basin (in New Jersey, New York, and Pennsylvania) and the Connecticut River Basin are both "aborted rift" basins (rift basins that are no longer actively widening via rift-style tectonism and are no longer collecting sediments). Sediments began to accumulate in both basins during the Late Triassic. Both basins are half grabens which contain characteristic sedimentary conglomerates, sandstones, and mudrocks that usually bear a red or brownish appearance from an abundance of iron oxide minerals. ... Both the Newark and Connecticut River Basins contain "traprocks." The word, traprock, is derived from the Swedish word "trappa," meaning stair or step. In the mining usage, a traprock is any fine-grained igneous rock, ususally diabase or basalt, that can be crushed for building or road aggregate. Erosion creates their step-like appearance, producing abrupt termination of successive volcanic flows. During early Jurassic time, episodic rift-basin-style volcanism began to occur. Magma of basaltic composition migrated from the upper mantle to the surface along faults. Massive volcanic eruptions at the surface resulted in the formation of surface flows that spread for great distances across the low relief of the alluvial basins. After volcanism ended, the flows were buried beneath the more gradual accumulation of basin sediments. Because of the half-graben structure of these basin, the originally horizontal volcanic flows are now gently inclined. The resistance of traprocks to erosion (relative to the surrounding sandstone and shale) results in the formation gently dipping cuestas throughout the Mesozoic basins all along the Atlantic margin.

Day Pond State Park:⁷
Some light-colored boulders are found along the trail ... They are very coarse-grained and generally light in color. The major minerals are quartz, feldspar and mica. The coarse boulders are called pegmatite. There were once molten rock deep within the Earth, which cooled very slowly, thus allowing time for large grains to grow. When you reach the buried cable signs, walk uphill to your right about 50 feet. The freshly broken pieces of black and white banded rock are Hebron Gneiss (pronounced "nice"). Gneiss had its mineral grains sorted into dark and light bands by increased heat and pressure during metamorphism.

East Rock Park:⁴
The city of New Haven developed around a natural bay in the Connecticut shoreline where the Central Valley meets Long Island Sound. The Pleistocene glaciers helped carve away the softer sedimentary strata of the Connecticut River Basin, creating valleys between the higher, more erosion-resistant volcanic rocks that crop out in the Central Valley and the older metamorphic rocks of the surrounding Highlands. To the north and west of downtown New Haven is West Rock Ridge State Park, a park encompassing a great Jurassic diabase sill that intruded into the Late Triassic red beds. To the north of downtown, volcanic intrusions of Jurassic age crop out along the West River and Quinnipiac River valleys. These volcanic escarpments are responsible for the scenic landscape preserved in East Rock State Park and nearby Sleeping Giant State Park (located about 8 miles north of New Haven). East Rock Park encompasses another remnant of the massive diabase intrusion that formed West Rock Ridge. Although not as high (359 feet), the regional view from the top of East Rock is quite impressive.

Hampden Basalt:²
Route 372 roadcut between Routes 9 and 5/15, East Berlin. This is the East Berlin formation again, but here you can see the contact with the overlying Hampden basalt, the youngest of the lava flows. The Hampden basalt is named after the town in Massachusetts where it was studied.

Wadsworth Falls State Park:⁷
Beyond Little Falls the Blue Trail ends at the Orange Trail. Turn right on the Orange Trail and follow it to its end at Cherry Hill Road. To get to Large (or Wadsworth) Falls, turn right and follow Cherry Hill Road about 700 feet until you see the parking lot for the falls. Walk down the ramp and across the grass. A cement sidewalk will take you to the top of the fall. It is wheelchair accessible. Instead of taking the sidewalk, walk down the steps to the bottom of the falls. The rock here is dark gray, rather than red. This is the Hampden Basalt. Here at the falls two flows are exposed. The lower flow is massive, with widely spaced fractures. Near its top are a few holes where air bubbles escaped from the lava while it was still hot enough to be liquid. Above this flow the basalt forms columns, separated by closely spaced fractures. To the right of the falls, by looking closely at the top of the lower, massive basalt, you can see vesicles. These are holes in the basalt made when gas bubbles escaped from the basalt flow as it started to solidify. In some basalt these vesicles will be filled with a mineral which was deposited in the vesicle by hot fluids soon after formation, or later by groundwater with dissolved minerals moving through the rock. These gas bubbles are also sometimes elongated by the upward movement of the gas, forming pipe vesicles.

Hanging Hills:²
The Hanging Hills are basalt ridges, the edges of 20-million-year-old lava flows.

Hanging Hills:⁴
The Hanging Hills are a 1,000 foot high broken escarpment of traprock where Metacomet Ridge bends eastward across the Central Valley just north of Meriden, Connecticut. Scour by moving glacial ice has plucked away the basalt from the steep southern end of the crest of the broken ridge, creating the overhanging cliffs. The cap rock of the Hanging Hills escarpment is the massive Early Jurassic Holyoke Basalt; it is nearly 700 feet thick. The exposures along the ridge tops display an irregular polygonal columnar jointing pattern that formed as the massive volcanic surface flows gradually cooled. Most of these fractures are tightly cemented by minerals that formed long after the flows were buried by younger sediments. The tell-tale grooves and scratches from rocks embedded in the bottom of the glacier are still visible in patches on the barren bedrock along the cliff tops. The Early Jurassic Shuttle Mountain Formation (red beds) and the Talcott Basalt crops out along the forested hillsides at the base of the ridge. In the Meriden region, numerous northeast-trending normal faults offset the volcanic flows and intervening sedimentary rocks. Several of these faults break the Metacomet Ridge north of Meriden. Stream erosion and glacial ice carved canyons along these faults, dividing the ridge into the finger-like promontories of the Hanging Hills (West Peak, East Peak, South Mountain, and Cathole Mountain, west-to-east respectively.). Merimere Reservoir was built in the fault-controlled valley between East Peak and South Mountain.

Higganum Dike:²
The Higganum dike at Exit 9, Route 9 is the root of a huge volcanic fissure eruption that produced a flood basalt 200 million years ago. To View: Exit 9, park along Route 81 and walk back. The Higganum dike is well exposed here. Examine the columnar cooling fractures and other features of this great basalt dike, which has been dated as 201 million years old.

Hogback Ridge:³
The hook-shaped N-S ridge just north of New York's Long Island is a cuesta or hog-back ridge, one of several tilted and eroded lava flows in the Connecticut Valley. It is contained within coarse Triassic-Jurassic sediments that were deposited rapidly during rift-faulting of North America, about 180-200 million years ago, as the supercontinent of Pangea was pulled apart and Europe and Africa separated from North America. When you are visiting Normandy in western France, look for the matching rocks!

Penwood State Park:⁷
In the park, follow the road to the picnic area. Then walk up the closed road. All along the road you will see outcrops of Holyoke Basalt, the middle of the three basalts (Hampden, Holyoke, and Talcott). In some places it forms columns. Holyoke Basalt outcrops occur in many places along the road as you ascend toward the circle in the road. At the circle, follow the blue trail to the "Pinnacle". Along the way you will cross some Shuttle Meadow Formation siltstone in the trail. Siltstone is thinly bedded (breaks in thin layers) and here is red in color because of the iron cementing the grains together.

Kettletown State Park:⁷
This part of Connecticut is all metamorphic rocks (except for the Pomperaug Valley in Woodbury and Southbury). The rocks in western Connecticut have been changed from the rocks they originally were by high heat and pressure. The rocks of Kettletown were originally Iapetos ocean floor sediments, mixed with a few basalt flows.

Glacial Erratic:⁷
The William Miller (blue blazed) Trail, to your right from the road heading to the Youth Group Camp Area, provides a good introduction to a variety of rocks. At the T in the trail, turn right. About 500 feet down the trail you will encounter a large, lumpy brown boulder at the edge of the trail. This glacial erratic came from the northwest, from the Pomperaug Valley. Erratics are boulders left behind by the glaciers when they melted. They are sitting on bedrock different from the type of rock they are. We know which way this erratic came from because it is a chunk of basalt, and the nearest basalt bedrock is found three miles to the northwest in Southbury.

Pillow Basalts:⁷
Sometimes lava flows into water, causing the surface to solidify instantly, while the inside of the flow is still liquid. This results in blobs, which then break open, allowing the lava to flow a little farther, then form another blob. The result is pillow basalts, and it appears this boulder and several more a little farther along the trail are pillow basalts. At the time basalt was flowing into the valley, Connecticut was much farther south and much drier than it is now. But there were lakes here and there, especially during the monsoon season.

Pegmatite Erratics:⁷
Continuing on down the trail, you will see more of the pillow basalt erratics. Farther along some pegmatite erratics have been left behind. They are lighter colored than the basalt and very course-grained. These are another type of igneous rock (rock which was once melted, like the basalt). But pegmatite formed from molten rock very far below the surface. Because it cooled very slowly, the crystals were able to grow very large. Pegmatite intrusions are generally small. Often they are found as thick or thin white or pink veins in other rocks. Usually pegmatite has the same composition as granite, only coarser. The predominate minerals in this pegmatite are feldspar and quartz. Feldspar is the duller one, quartz a little shinier.

Amphibolite Boulders:⁷
The blue trail continues past two trails to the left (shortcuts to the other side of the loop). Later it makes a sharp left turn. Soon you will see many amphibolite boulders, especially after crossing a small stream. These are a type of gneiss (pronounced "nice") that is black with some white banding here and there. Amphibolite was once basalt flows on the ocean bottom, but have since been metamorphosed so that the original minerals converted to amphiboles (a group of minerals). The amphibole making up this rock is called hornblende. Its dark green to black color makes the rock very dark.

Pegmatite Outcrop:⁷
A pegmatite outcrop lies approximately 30 feet from the pothole on the Nipmuck Trail. Pegmatite is an igneous rock that formed from molten rock buried deep below the surface. Since the molten rock was well insulated beneath the surface, it cooled very slowly, allowing the crystals to grow very large. Pegmatite intrusions usually have the same composition as granite, only coarser. Furthermore, pegmatite intrusions are of great interest to mineral collectors because they may contain a variety of rare minerals. This pegmatite outcrop contains large quartz, feldspar, and mica crystals.

New Haven:⁴
The city of New Haven developed around a natural bay in the Connecticut shoreline where the Central Valley meets Long Island Sound. The Pleistocene glaciers helped carve away the softer sedimentary strata of the Connecticut River Basin, creating valleys between the higher, more erosion-resistant volcanic rocks that crop out in the Central Valley and the older metamorphic rocks of the surrounding Highlands. To the north and west of downtown New Haven is West Rock Ridge State Park, a park encompassing a great Jurassic diabase sill that intruded into the Late Triassic red beds. To the north of downtown, volcanic intrusions of Jurassic age crop out along the West River and Quinnipiac River valleys. These volcanic escarpments are responsible for the scenic landscape preserved in East Rock State Park and nearby Sleeping Giant State Park (located about 8 miles north of New Haven).

Basalt Dike:⁷
Further down the red trail, there is an abrupt change in the geology. The rocks along the trail change from gneiss to basalt and then back to gneiss. This transition of rock types is due to a dike, which is a sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes. The dikes located in Osbornedale State Park are all feeder dikes, which help supply the large lava flows in the Hartford Basin 200 million years ago. This particular dike is composed of diabase, a fine-grained igneous rock that cooled rapidly near the earth's surface. ... In some areas along the trail, the basalt dike is nearly 100 feet wide. After you cross the dike and the stonewall along the trail, you will reach another outcrop of gneiss, indicating you have left the basalt dike area.

Pegmatite Outcrop:⁷
Following the trail down the hill, you will encounter a large pegmatite outcrop. Pegmatite is an igneous rock that formed from molten rock buried deep below the surface of the Earth. Since the molten rock was well insulated beneath the surface of the Earth, it cooled very slowly, allowing the crystals to grow very large. Pegmatite generally has grains larger than 1 cm in diameter, by definition. This pegmatite contains feldspar, quartz, and biotite. Pegmatite intrusions are of great interest to mineral collectors because they may contain a variety of rare minerals. However, this pegmatite outcrop does not contain any rare minerals.

Holyoke Basalt:⁷
In the park, follow the road to the picnic area. Then walk up the closed road. All along the road you will see outcrops of Holyoke Basalt, the middle of the three basalts (Hampden, Holyoke, and Talcott). In some places it forms columns. Holyoke Basalt outcrops occur in many places along the road as you ascend toward the circle in the road. At the circle, follow the blue trail to the "Pinnacle". Along the way you will cross some Shuttle Meadow Formation siltstone in the trail. Siltstone is thinly bedded (breaks in thin layers) and here is red in color because of the iron cementing the grains together.

Talcott Basalt:⁷
At the "Pinnacle" the basalt looks similar to the Holyoke Basalt, but this is the Talcott Basalt, the oldest of the three basalt flows. It is very rough on the top, more so than the Holyoke. Although the Talcott is the bottom flow, it outcrops higher than the Holyoke, the middle flow, because the flows are tilted toward the east.

Sleeping Giant:²
Through central Connecticut and Massachusetts, Interstate 91 runs past a series of long, prominent ridges. These elongated mountains with their dramatic rust colored cliffs have long caught the eye of Connecticut's residents and visitors. Adrian Block, the first European to sail Long Island Sound and see the Connecticut coastline, named one of his anchorages Rodenberg (red mountain) for the great red cliffs that dominated the landscape, That anchorage is now the port of New Haven, and the mountains that so impressed Adrian Block are East Rock and West Rock. These landmarks, as well as Sleeping Giant, the Hanging Hills of Meriden, Talcott Mountain, and other mountains of Connecticut's Central Valley are traprock ridges, so named for the hard rock of which they are made.

Sleeping Giant:⁴
The city of New Haven developed around a natural bay in the Connecticut shoreline where the Central Valley meets Long Island Sound. The Pleistocene glaciers helped carve away the softer sedimentary strata of the Connecticut River Basin, creating valleys between the higher, more erosion-resistant volcanic rocks that crop out in the Central Valley and the older metamorphic rocks of the surrounding Highlands. To the north and west of downtown New Haven is West Rock Ridge State Park, a park encompassing a great Jurassic diabase sill that intruded into the Late Triassic red beds. To the north of downtown, volcanic intrusions of Jurassic age crop out along the West River and Quinnipiac River valleys. These volcanic escarpments are responsible for the scenic landscape preserved in East Rock State Park and nearby Sleeping Giant State Park (located about 8 miles north of New Haven).

Basalt Pieces:⁷
Continuing on around to the east side of the park, the trail passes through a valley. Along the trail about half way to the pond, you may start to see dark brown potato-shaped stones in the trail. Some are as large as a foot across. These are pieces of basalt moved south by the glacier from somewhere north of I-84 where there are the remains of small basalt flows from about 200 million years ago. Basalt tends to break up easily when transported by water or ice, so it is unusual to see basalt erratics, as rocks moved by glaciers are called. Basalt is dark gray, but weathers to a rusty brown color because of its iron content.

Pegmatite Vein:⁷
Before reaching the pond, if you walk through the woods to the ridge on your right, you will see an example of a rock type known as granofels and identified on the state bedrock map as the Taine Mountain Formation. It is more massive than the gneiss, with little or no banding of different colored minerals. The minerals in this rock are the same as in the Collinsville Formation, but arranged randomly. There is a pegmatite vein running through the granofels in one place. It is lighter colored and much coarser grained than the granofels, and formed when hot, melted rock was injected into a fracture in the granofels.

Penwood State Park:⁷
At the "Pinnacle" the basalt looks similar to the Holyoke Basalt, but this is the Talcott Basalt, the oldest of the three basalt flows. It is very rough on the top, more so than the Holyoke. Although the Talcott is the bottom flow, it outcrops higher than the Holyoke, the middle flow, because the flows are tilted toward the east.

Talcott Mountain:²
Through central Connecticut and Massachusetts, Interstate 91 runs past a series of long, prominent ridges. These elongated mountains with their dramatic rust colored cliffs have long caught the eye of Connecticut's residents and visitors. Adrian Block, the first European to sail Long Island Sound and see the Connecticut coastline, named one of his anchorages Rodenberg (red mountain) for the great red cliffs that dominated the landscape, That anchorage is now the port of New Haven, and the mountains that so impressed Adrian Block are East Rock and West Rock. These landmarks, as well as Sleeping Giant, the Hanging Hills of Meriden, Talcott Mountain, and other mountains of Connecticut's Central Valley are traprock ridges, so named for the hard rock of which they are made.

Traprock Ridges:²
Through central Connecticut and Massachusetts, Interstate 91 runs past a series of long, prominent ridges. These elongated mountains with their dramatic rust colored cliffs have long caught the eye of Connecticut's residents and visitors. Adrian Block, the first European to sail Long Island Sound and see the Connecticut coastline, named one of his anchorages Rodenberg (red mountain) for the great red cliffs that dominated the landscape, That anchorage is now the port of New Haven, and the mountains that so impressed Adrian Block are East Rock and West Rock. These landmarks, as well as Sleeping Giant, the Hanging Hills of Meriden, Talcott Mountain, and other mountains of Connecticut's Central Valley are traprock ridges, so named for the hard rock of which they are made. Two great geologic forces formed the traprock ridges: volcanism and erosion. Two hundred million years ago, when dinosaurs still roamed the earth, volcanoes forced out great flows of lava through long cracks in the floor of the Connecticut Valley. After the volcanic activity stopped, the whole region was fractured and tilted to the east. Since then, erosion has eaten away at the bedrock of the Connecticut Valley. Thousands of feet of brownstone have been washed to the sea, but the dense, hard, volcanic traprock eroded much more slowly, leaving the traprock layers as long ridge backs standing out far above the surrounding landscape. Traprock is a dark, fine grained rock. When newly broken open it is dark grey, but after exposure to weathering, the iron contained in the rock rusts, causing it to turn a reddish color. The rock usually fractures into angular blocks or columns along a network of cracks that formed when the lava first cooled and shrank into rock.

Wadsworth Falls State Park:⁷
The youngest rocks in Connecticut lie in the central part of the state. Wadsworth Falls State Park is made up of those young rocks, only about 200 million years old. About 250 million years ago all of Earth's land made up one huge continent called Pangea. It began to break up about that time, with large pieces of continental crust moving in various directions. What is now North America broke away from present-day Europe and Africa. As this occurred, tension fractures formed in the land, such as happen if you try to stretch cookie dough or modeling clay. Two such fractures formed in central Connecticut, allowing a long narrow valley to drop below the level of the surrounding land. Sediments from the surrounding highlands washed into the basin. Deep fractures formed in some places and lava flowed up to the surface from the upper mantle. Three such lava flows filled the valley and covered the surrounding uplands. In between the flows, sediments continued to flow into the still dropping valley. We now have a pile of sedimentary rock (made from the sediments), lava flow, sedimentary rock, lava flow, sedimentary rock, lava flow, sedimentary rock. Finally, the eastern side of the valley dropped faster than the western side, so now the rocks all dip toward the east. Over the intervening 200 million years, the higher uplands have eroded down so they are now much lower than they were and the basalts have all been eroded off of them. Basalts are now found only in the valley, where their lower elevation protected them from erosion. Sedimentary rocks are made up from sediments, usually deposited by water in low areas such as streams, lakes and oceans. Over a long time these sediments can become so thick the pressure compresses the grains close together. Groundwater moving slowly through them gradually deposits dissolved minerals, such as calcium, silica or iron, between the grains, cementing them together. The lava flows cooled and solidified into basalt (also called traprock).

Hampden Basalt:⁷
Beyond Little Falls the Blue Trail ends at the Orange Trail. Turn right on the Orange Trail and follow it to its end at Cherry Hill Road. To get to Large (or Wadsworth) Falls, turn right and follow Cherry Hill Road about 700 feet until you see the parking lot for the falls. Walk down the ramp and across the grass. A cement sidewalk will take you to the top of the fall. It is wheelchair accessible. Instead of taking the sidewalk, walk down the steps to the bottom of the falls. The rock here is dark gray, rather than red. This is the Hampden Basalt. Here at the falls two flows are exposed. The lower flow is massive, with widely spaced fractures. Near its top are a few holes where air bubbles escaped from the lava while it was still hot enough to be liquid. Above this flow the basalt forms columns, separated by closely spaced fractures. To the right of the falls, by looking closely at the top of the lower, massive basalt, you can see vesicles. These are holes in the basalt made when gas bubbles escaped from the basalt flow as it started to solidify. In some basalt these vesicles will be filled with a mineral which was deposited in the vesicle by hot fluids soon after formation, or later by groundwater with dissolved minerals moving through the rock. These gas bubbles are also sometimes elongated by the upward movement of the gas, forming pipe vesicles.

West Rock Ridge State Park:⁴
The city of New Haven developed around a natural bay in the Connecticut shoreline where the Central Valley meets Long Island Sound. The Pleistocene glaciers helped carve away the softer sedimentary strata of the Connecticut River Basin, creating valleys between the higher, more erosion-resistant volcanic rocks that crop out in the Central Valley and the older metamorphic rocks of the surrounding Highlands. To the north and west of downtown New Haven is West Rock Ridge State Park, a park encompassing a great Jurassic diabase sill that intruded into the Late Triassic red beds. ... West Rock Ridge ... rises to over 600 feet and extends for nearly 15 miles along the eastern side of the Central Valley. A six mile portion of the ridge is protected in West Rock State Park.

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