Saturday, December 8, 2007

Chasing birds and geology: Snow Buntings and Radnor kames

We look at glacial geology while chasing Snow Buntings and Lapland Longspurs, with mixed success, near the glacial kames* of Radnor, Ohio in northern Delaware County.

View of Radnor Cemetery gate where Radnor Road crosses the crest of a glacial kame.
Kames are glacial landforms, piles of sand and gravel left by water flowing on a glacier or leaving the glacier's ice margin. Pioneers often used glacial kames for burial because they are easy to dig. Fruit orchards were planted on kames because sand and gravel under shallow soils ensures good drainage. Professional aerial photo interpreters sometimes refer to cemeteries as "marble orchards" because these cultural practices were so common and widespread in the Midwest, and so easily seen when examining 3D stereo photo sets used in drafting topographic maps.
The Welsh "Lych" Gate to Radnor Cemetery was constructed in 1910 of locally quarried limestone, another geological resource influencing settlement patterns in central Ohio.

Wintertime is great for geologizing* landscape while chasing seasonal specialties--birds that occur in Ohio landscapes only seasonally. Absence of foliage opens our view to landscape and birds alike. Snow cover is not only pretty, it makes finding some of our bird species of open-field habitats a little easier. There's no better time than the first big snowfall of the season to explore new landscape.

Snow Buntings are a wintertime favorite of mine so a recent Ohio Birds Listserv post about Snow Buntings and Lapland Longspurs seen near Radnor, Ohio inspired me to chase them.

Snow cover makes these two species easier to see. They spend much more time along the roadside berm when their preferred bare farm fields are blanketed with snow. And it's easier to pick out longspurs and flocks of Horned Larks sitting still on distant dirt clods of plowed fields when background snow frames their silhouettes. Finding Horned Larks is the first step in chasing Snow Buntings.

Snow Buntings are occasionally found with larks and longspurs. It's fortunate they often hang-out together because Snow Buntings are cryptically colored: Perched atop small rocks in a vast farm field, they blend well with the snowy landscape. Their white, tan, and black patterned winter plumage hides them very well until they fly, flashing their broad white wing patches tipped with black.

I didn't find Snow Buntings this trip. I did find a half-dozen Lapland Longspurs and numerous small flocks of Horned Larks. As usual, they were uncooperative when I reached for the camera. The image below is of our more common Horned Larks.

Two Horned Larks seen here in typical habitat, farm fields.

Horned Larks are common in central Ohio farm fields in winter. They often gather in small flocks along the roadside when snow cover blankets fields. These larks were found along Price Road near Radnor, Ohio. Radnor Road and Hedely Road were productive drives as well.

Reasonable speculations. . .

I envision Snow Buntings as strongly associated with Ohio's pre-settlement prairie regions. They still occur most commonly in the lake plains and central till plains of Ohio where wet prairies once reigned. I think they follow ancient winter routes to the south, over Lake Erie and over the former Great Black Swamp country of northwest Ohio, taking them to former prairie country where snow blows less frequently and where bison once trammeled vegetation and moved snow, exposing wide swaths of prairie grass and bits of larval protein excavated by bison hooves. Today, buntings find open farm country along these ancient flight-paths, a habitat bonanza. Most Snow Buntings remain in the northwestern third of Ohio today. Scattered flocks of hundreds of wintering "snowflakes" may be found in the windswept fields of the lake plain some winters.

Imagine along with me a once wild Ohio where Snow Buntings follow bison to forage the seeds uncovered by their great snow-plow snouts, and to pick seeds from their dung. In the distance there is a band of Native Americans led by a sage hunter possessing the cherished natural wisdom of his clan. He sees a distant flash of white of Snow Bunting wings over tall Indian grass and pointing, whispers to his band, "There! Bison are there."

A little glacial geology. . .

A very brief Primer: Continental glaciations have sculpted Ohio at least four times during the last 1.6 million years (probably seven, but as many as seventeen glaciations may have sculpted Ohio but sediments from only four events survive). Ice covered much of the Northern Hemisphere during each ice-time, for many tens of thousands of years each ice-time, with comparatively brief intervals of ice-free warmth in between the ice-times; like the ice-free warmth we now enjoy. Ice sheets were thousands of feet thick. They spread and thinned toward their margins from thick accumulation areas toward their centers. They reworked the landscape by scraping-up material at their bases and conveying it toward their margins; more like conveyor-belts than like bulldozers. Along the way, low areas were filled-in with drift and highs were scraped and flattened.

What's what: Drift is a general term for anything carried and deposited by glacial processes. The term "drift" includes ground moraine, mostly unsorted stony clay and silt materials deposited under the glacier; recessional moraine, more unsorted stony clay and silt materials dumped and mounded at the snout of the glacier as it paused its retreat for periods of time; and kames, piles of water-sorted sands and gravels deposited on, within, or against glacier ice by melt-water flow; and many more drift types we won't go into here.

Delaware County is a young landscape, likely less than 15,000 years young, formed of glacial drift. Thin ground moraine covers most of the county. Ground moraines and recessional moraines are dissected by several large river systems formed during deglaciation. Geologists have mapped the glacial geology of Ohio and of Delaware County.

The Glacial Map of Ohio includes county outlines. Look for Delaware County at the center of the map colored yellowish-green (ground moraine) with dark green arcs (recessional moraines) bending southward, and a few small pink blobs (kames) in the northwestern corner of the county. Do you see the cleft in the county's central moraine? Now look for that cleft on the shaded elevation map below and you have the county located. The shaded elevation map shows the moraines as raised elevation areas. We see the river systems cutting nearly straight north-south channels through the raised recessional moraines and ground moraines alike.

The Shaded Elevation map (large file (pdf)) of Ohio helps you see drainage at large scales. Click the "+" sign in the toolbar about eight or ten times and you will zoom in on the Radnor area. Do you see the shaded arcs indicating the recessional moraines? They are dark green on the Glacial Geology map above.

Now, look at the pink blobs mapped in the NW corner of the county on the glacial map. This is the terrain pictured below.

View of glacial kame from the north approach to Radnor, Route 203.

The flat ground moraine (foreground) abruptly terminates at the base of the kame where slopes rise steeply. Coarse deposits like gravel and sand piles hold steeper slopes than fine deposits like clay and silt. A home is built on the gravelly kame. Kames are elevated, easy to excavate, and are well-drained compared to ground moraine.

Now look at it on the topographic map found at this link:

This is the map view of the kame and it is obviously narrow and long like a segment of the deposits in a stream channel. These deposits probably were laid down by water running through eroded channels on the surface of the ice sheet. When the ice melted away the supra-glacial stream deposits were piled on the ground where they remain today.

The hill in this view is a glacial kame left forested. The flatish ground moraine in foreground surrounds the kame as seen at distant left.

Cows guard access to a borrow-pit dug into the end of the linear kame. Unconsolidated sand and gravel is easily borrowed from kames and borrow-pits are common in the area.
Huge glacial erratic seen from Route 203.

Glacial drift includes erratic boulders, chunks of bedrock carried from somewhere else. This one found south of Radnor along route 203 may be the second largest in Delaware County, exceeded only by the Sunbury boulder, largest in the state (a topic for a future post). It's height exposed above ground is around ten feet. Undoubtedly, as much as half remains hidden beneath ground. This large igneous rock was carried from central Canada.

Buttermilk Hill Road, west from the record bur oak tree pictured below at the entrance to Gallant Woods Preserve, ascends a recessional moraine. Many homes are built along the crest of the hummocky moraine. The ice sheet paused for a time at this position. Glacial drift piled at the snout of the ice sheet here.
State record (second place) bur oak tree at the entrance to Gallant Woods Preserve.

Ice sheets sculpted two-thirds of the state of Ohio. It's fascinating to think of the topographic impact of these behemoths on our state and beyond. Ice sheets reduced terrain, built terrain, and altered drainage patterns over huge areas of continents. The Ohio River is a product of drainage reversal caused by ice sheets!

An immense amount of drift is carried within creeping ice sheets. Many people imagine that ice sheets operated like bulldozers surging south from points north and pushing large chunks of Canada ahead of them before quickly melting away. A venerable local county extension agent wrote a history of my home county terrain that employs this misconception; and it remains assigned reading for school children today.

I read this account as a youth and I was thrilled to adopt the misconception that our latest ice sheet, the Wisconsinan age Laurentide Ice Sheet, had bulldozed spruce trees, nearly intact, all the way from Canada to my backyard. These spruce logs were uncovered just a few miles from my home!

Of course, we know this is not what happened. These spruce trees grew nearby along the ice-front for tens of thousands of years as the ice-front dominated the south-central Ohio region for tens of thousands of years.

The dynamic ice-front moved back and forth over meters and miles during this long span of time. When the ice surged forward occasionally it bulldozed the locally grown spruce trees into jumbled piles of logs buried in glacial drift. We use radiocarbon dating to determine the age of the drift in which we find these logs! In my area they date to around 17,000 or 18,000 years ago, the time when the Laurentide Ice Sheet reached furthest south just before receding northward permanently.

An ice sheet transports drift by picking it up at its base (called entrainment) and moving it toward its margin. Drift begins its journey as part of an ice-saturated mass of bedrock blocks, boulders, gravel, sand, silt, and clay frozen to the base of the ice sheet. The mass scrapes against bedrock along the bottom of the flowing ice sheet until the mass is thrust into the basal ice layers along shear-planes and zones of deformation rising downstream from the base.

The mass is raised into the ice slowly as it is pushed from behind by the inexorable flow of ice. The resistance of the ice downstream as it is scraping along the bedrock and freezing to it in places, pushes back enough to force upstream drift masses to shear upward at low angles into the ice sheet. The drift then rides the ice south.

Often, earth scientists refer to this process as a conveyor-belt. This is helpful for visualizing the mechanism by which ice sheets convey drift from places north to places south. The giant boulder seen south of Radnor, pictured above, was conveyed to its position in Ohio from its original position somewhere in Canada by this mechanism.

Ice sheets move forward toward their margins in a fairly steady way (though punctuated by surge-pulses) pushed by accumulating volumes of ice near their centers. The ice flows away from the accumulation zone by deforming under its own weight. The accumulation zone of the eastern portion of the Laurentide Ice Sheet was at about the position of Hudson's Bay.

Far to the south, ice sheets melt back along their margins at the same time the ice is flowing forward. If they flow forward faster than they melt back, the ice-front surges ahead over virgin terrain. When the ice sheet flows forward at about the same rate the ice-front melts back, the ice-front will hold an average position for as long as this continues.

The ice sheet is not sitting still even when the ice-front is not moving forward. The ice continues to move forward all the while until the very final stages of deglaciation when large areas of ice stagnate and just melt away in position.

Generally, as the ice-front melts, the material inside the ice sheet is exposed as melt-water drips away and flows along drainage outlets as sediment-laden runoff. This runoff deposits vast outwash plains along drainage valleys like today's Scioto River Valley.

Much of the drift remains along the ice-front as broad mounds. It piles up as if it were coming off of a conveyor-belt. This is how end moraines and recessional moraines form. Their position coincides with the position of the ice-front at the time they formed. End moraines tell us how far the ice sheet flowed. Recessional moraines tell us where the ice-front paused before continuing to melt back toward its center.

When ice sheets melt back faster than they move forward, they leave behind the material frozen inside as ground moraine.

*Geologize: Applying geological principles to deduce and hypothesize geological process and history through direct observations of landscape, drainage patterns, bedrock, soil, land use, and so on.
*Kame: Steep-sided mounds or terraces of sand, gravel, and boulders. Glacial sediments laid down by water flowing on a glacier or leaving the glacier's margin, in contact with ice: Ice-contact stratified drift. Long kames are sometimes called eskers. One definition suggests that a kame ten-times as long as wide is an esker. The long narrow Radnor kames appear to have been deposited on the glacier surface as supra-glacial stream deposits, then dumped onto the ground as the ice melted away. A process definition would rank some of the Radnor kames as eskers.

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