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Thunder Rocks in Allegany SP, NY — © Dave Spier

Allegany State Park, in the southwestern part of New York State, is unique to the area in never having been glaciated during the Wisconsin ice advance. The glacier stopped just short of the Allegheny River Valley that surrounds the park. Besides lacking the typical deposits of ground moraine, the park’s hills were spared the scouring effect of a bulldozing ice sheet.

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“Snout-nose” is one of many rock creatures at Thunder Rocks. What animal does this one remind you of? A higher-resolution copy can be found on National Geographic’s Your Shot at http://yourshot.nationalgeographic.com/photos/2242557/

Thunder Rocks is one of several “rock cities” on hilltops near the Pennsylvania line. It is made of huge, joint-fractured Olean conglomerate blocks from the Pennsylvanian period, the same massive rock type that forms its more-famous cousin, Rock City, southwest of Olean. Other outcrops littering hilltops in the region come from different conglomerate layers in other geologic periods including the Mississippian Pocono group. West of Jamestown, the Wolf Creek conglomerate at the base of the late Devonian Conewango group forms Panama Rocks. All of the photos in this blog were taken at Thunder Rocks with the exception of the Salamanca conglomerate closeup. Higher-resolution copies of the first two photos can be found on National Geographic’s Your Shot here and here.

ThunderRocks_ANP_©DaveSpier_D018126blog

ThunderRocks_ANP_©DaveSpier_D018128blog

Conglomerate is essentially nature’s concrete. It contains numerous pebbles or even cobbles plus sand and silt cemented together by either limestone, iron oxide, silica or clay. It is sometimes called “puddingstone.” Conglomerates can occur in massive beds resistant to erosion. In the Allegany region, they are underlain with soft shales that easily erode and allow the conglomerate to break along joint planes. Soil creep then slowly carries the blocks downhill.

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Olean conglomerate blocks from the Pennsylvanian period form Thunder Rocks. Note the cross-bedding in the middle layer.

Salamanca conglomerate, found chiefly at Bear Caves in Allegany SP and Little Rock City northwest of Salamanca, is from the late Devonian Conewango group.

Salamanca conglomerate, found chiefly at Bear Caves in Allegany SP and Little Rock City northwest of Salamanca, is from the late Devonian Conewango group. This closeup is part of a boulder at a parking area elsewhere in the park.

Maps and non-technical information on visiting Thunder Rocks can be found on the Enchanted Mountains – Cattaraugus County website. A separate page lists several other nearby sites on the Cattaraugus County Geology Trail. There is a brief mention of Thunder Rocks on page 173 in Roadside Geology of New York, by Bradford VanDiver, PhD, 1985/reprinted 2003, published by Mountain Press.

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These photos were taken during the annual Allegany Nature Pilgrimage held the first weekend after Memorial Day. There’s usually at least one geology hike during the event. For most of these photos, camera white balance was set to “cloudy” for the overcast day, but light filtering through the spring canopy gives some scenes a slight greenish cast.

Corrections, comments, and questions are always welcome at northeastnaturalist@yahoo.com or connect through my Facebook page and photo page. For topics in the northeast, there is a separate community-type page at The Northeast Naturalist. Other northeast nature topics can be found on the parallel blog Northeast Naturalist.

Thunder Rocks

Chippewa Falls, Ontario

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approaching the lower Chippewa Falls

Chippewa Falls (introduction) — © Dave Spier

At the east end of Lake Superior, Highway 17, the Trans-Canada Highway, crosses the Harmony River on the east side of Batchawana Bay near Harmony Beach. (This is the second bay on Lake Superior north of Sault Ste. Marie.) Just upstream from the bridge, a wayside park provides access to Chippewa Falls, which is actually two cascades 150 feet apart. The original bedrock is 2.7 billion-year-old pink granite, but, at the lower falls, it is still covered with a remnant of a 1.1 billion-year-old lava flow called the Keweenawan basalt that was extruded during the Grenville orogeny. The contact between the two rocks types represents 1.6 billion years of erosion that brought the granite to the surface by the time of the volcanic activity. A lateral fault cuts through both layers on the north side of the present-day lower falls (out of sight in the two opening photos).

A remnant of the gray, 1.1 billion-year-old lava flow called the Keweenawan basalt, extruded during the Grenville Orogeny (mountain-building episode), covers the 2.7 billion-year-old pink granite next to the lower Chippewa Falls (hidden right rear).

A remnant of the gray, 1.1 billion-year-old lava flow called the Keweenawan basalt, extruded during the Grenville Orogeny (mountain-building episode), covers the 2.7 billion-year-old pink granite next to the lower Chippewa Falls (hidden right rear).

Potholes in the Keweenawan basalt are geologically-recent erosion features resulting from swirling eddies carrying abrasive sand, gravel and cobbles in a circular motion that grinds them down into the base rock.

Potholes in the Keweenawan basalt are geologically-recent erosion features resulting from swirling eddies carrying abrasive sand, gravel and cobbles in a circular motion that grinds them down into the base rock.

A small remnant of the gray Keweenawan basalt covers the pink granite as we climb toward the lower Chippewa Falls.

A small remnant of the gray Keweenawan basalt covers the pink granite as we climb toward the lower Chippewa Falls.

erosion along several of the many joints in the pink granite provides a partial view of the lower Chippewa Falls

erosion along several of the many joints in the pink granite provides a partial view of the lower Chippewa Falls

the lower Chippewa Falls

the lower Chippewa Falls

several visitors provide scale next to the crest of the lower Chippewa Falls

several visitors provide scale next to the crest of the lower Chippewa Falls

view across the pink granite bedrock toward the upper Chippawa Falls at upper right

view further upstream looking across the pink granite bedrock toward the upper Chippawa Falls at upper right

The upper falls was created by a vertical diabase dike cutting across the granite. Here the fault displaces the dike by 30 feet upstream on the northwest side of the river. The upper falls can be reached by an 800 foot trail from the parking area although we didn’t have time to try it. Apparently the trail continues another 500 feet upstream to a bed of large boulders. If you were able to travel six miles further upstream, you’d reach the confluence with the Chippewa River.

The upper Chippewa Falls was created by a vertical, gray, diabase dike cutting across the pink granite. Here the fault displaces the 65-foot thick dike by 30 feet upstream on the northwest (left) side of the river.

The upper Chippewa Falls was created by a vertical, gray, diabase dike cutting across the pink granite. Here the fault displaces the 65-foot thick dike by 30 feet upstream on the northwest (left) side of the river.

Corrections, questions and suggestions are always welcome at northeastnaturalist@yahoo.com or connect through my Facebook page and photo page. There is a separate community-type page for The Northeast Naturalist. Other nature topics can be found on the parallel blog Northeast Naturalist.

calcite fills veins in the basalt at the contact with the original granite bedrock as we return to the lower level

calcite fills veins in the basalt at the contact with the original granite bedrock as we return to the lower level

Logs become trapped in erosional features that often result from joints and other fractures. Note the dark basalt at the top overlying the granite underneath.

Logs become trapped in erosional features that often result from joints and other fractures. Note the dark basalt at the top overlying the granite underneath.

reference: Roadside Geology of Ontario — North Shore of Lake Superior by E. G. Pye, 1997, pgs. 130-131

“A plaque erected by the Ontario Motor League highlights Batchawana Bay (at Chippewa Falls) as the mid-point in the longest national highway in the world — the Trans-Canada Highway.” (from the Batchawana Bay PP page )