YARDLEY GEOLOGY
Delaware River Valley at Yardley... Geology & Topography
By Larry Hale - January 22, 2004
At one time perhaps 3,500 years ago the Delaware River shoreline ran along Canal Street, and the river frequently flooded five or ten feet above Main Street. However, right up to the arrival of the Europeans the long-term trend has been for the average river discharge to diminish each year. Simultaneously, the relative ocean level has been rising, but at a diminishing rate of rise. We are supposedly within 2,000-years of the end of a typical 100,000-year glacial cycle, in which the global ocean level varies from hundreds of feet below its present level, to 30-feet above its present level. Actual measurements appear to show that in recent centuries the ocean level has been rising about 1-foot each 100-years. If the ocean level reaches 30-feet above its present level, Yardley would be located at the top end of the Delaware Bay, and the high ground along the river bank (between Brock Creek and Morrisville) would become an island.
“THE ISLAND”
Actually, the high ground along the river already was an island at one time.
It’s difficult to know the exact sequence or put a time line on it because the
river valley has been formed by countless cycles of “deposition and erosion”
and the present river may have migrated sideways. But in its most recent configuration
it’s evident based on the terrain that there were two channels divided by an
island. Multiple channels are accounted for by the fact that the earlier deeper
river, i.e. the river valley, widens out at Yardley (Ref: “A View of the River”
by Luna B. Leopold, Publ. by Harvard University Press, 1994, Page 22). There
is today a ridgeline along the river between Morrisville and Yardley, marking
the top of the former island, formed thousands of years ago by a broad sand-and-gravel bar. The boulders
and some of the sediments are direct remnants of the glacier (carried down by
floating ice). Stratified glacial drift (sand and gravel) extends down the river
from Belvidere (the furthest advance of the glacier) all the way to below Trenton.
Much of this must have been reworked and in addition, some deposits between
the canal and the river also result from erosion of tributary watersheds.
A recent Letchworth Avenue storm drainage project provided an opportunity to observe conditions directly under the ridgeline referred to above. There was undisturbed unconsolidated sediment still existing directly under River Road, and I recorded the following: The top 18” consisted of 4” of Blacktop over a 10” base, underlain by 4” comprised of two dark bands (presumably the 1800s road cart-way). From 18” depth to 5½ feet depth there is four feet of clay and silt. From 5½ feet depth to 7½ feet there is two feet of gravel and cobbles. Below the gravel there was tightly compressed sand. The contractor pulled out a huge glacial boulder measuring 6 ft. x 4 ft. x 3 ft.
Higher elevations in Yardley are also alluvial. Some along the railroad-cut may be very ancient, while others along Canal Street and Main Street are much more recent... my guess for Main Street would be 5,000 years B.P. (Before Present). These are former floodplains, now classified as a terrace.
“A terrace is a former floodplain
which may only occasionally still flood.”
(Ref: “A View of the River” by Luna B. Leopold, Publ. by Harvard University
Press, 1994)
Terraces laid down by the river may also be subject to local flooding from creeks. A very rare creek flood occurred in the Brock Creek watershed in June 1996. This provided an opportunity to observe the results of “erosion” of what is clearly a river terrace behind the Meeting House on North Main Street. I photographed the resulting washout from Brock Creek... mounds of rocks extended well out into the Delaware River in front of the old bridge abutment.
DRAINAGE ALONG THE CANAL
The original natural drainage along where the canal is located today was in
the same direction and at the same gradient as the river. But eventually, uneven
deposition and dry weather vegetation sometimes tended to block the flow toward
Morrisville, so that swamps behind Macclesfield Park may have been naturally
formed. At other times alternate periods of erosion likely cleared-out and re-established
the waterway. There still exists a wide drainage swale starting in Lower Makefield
behind Belmondo, and a wide floodway below the railroad behind Macclesfield
Park ---but floodwaters along this natural swale are blocked by man made structures.
The most significant of these is the canal itself, which destroyed the slope
and (in Yardley, below White's Creek) usurped what was probably the deepest and best part of the
waterway. The canal was followed by the construction of the railroad, which
blocked just about all of whatever waterway remained. Below the railroad, drainage
remains in the same direction and discharges into the Delaware River via Potato
Creek just below Ferry Road ---I prefer to stick with the original names of
these creeks.
Most lands along the entire length of the canal drain in the same direction as the river. This pattern is interrupted at Yardley where erosion from Brock Creek appears to have built up much of the land between the canal and the river. Below Brock Creek there is higher ground along the northeast side of the canal and reverse drainage from College Avenue to Brock Creek via a several hundred feet wide swale, including a narrow deeper swale and brook immediately behind the homes along Bell Avenue. All drainage above College (between the canal and Bell Avenue) is toward Brock Creek.
THE DELAWARE RIVER FLOODPLAIN
“A floodplain is a level area near
a river channel, constructed by the river in the present climate and overflowed
during moderate flow events.”
(Ref: “A View of the River” by Luna B. Leopold, Publ. by Harvard University
Press, 1994)
Flood Insurance Maps for Yardley identify the lands between the canal and the river as subject to a 100-year flood. This is very useful information for setting floor elevations, but does not reveal detail information about the land. Actual flooding varies widely depending on the precise location, and taking into account the various ridgelines, terraces, and swales.
River flooding in Yardley from moderate flood events is limited mostly to a fairly narrow band of low-lying area immediately adjacent to the northeast side of the canal (the side toward the river), and located mostly in the upriver one-half of the Borough. Much of the land between the canal and the river actually consists of terraces of varying heights, well above the flood level caused by moderate flood events. And there’s a much higher terrace along Main Street, where a flood 5-feet deep would be an extremely rare event... my guess, a 1000-Year Flood, occurring on average once every thousand years, not including possible floods caused by an ice-jam at the railroad bridge. The term “floodplain” should never be confused with other terms such as “flood zone” and/or related terms used by regulatory agencies for various good purposes, but not providing detailed information or scientific understanding of the actual topography.
WHITE’S CREEK (SCENARIO)
White’s Creek at one time entered a
much larger Delaware River and/or a branch of the river at about the same location
(behind the church) where it enters the canal today. Over time the river channel
at that location was diminished by deposition, but the creek still flowed toward
Morrisville. Eventually uneven deposition and vegetation tended to block the
route to Morrisville, so that the creek intermittently may have flowed to Brock
Creek. The ridgeline along the river prevented White’s Creek from flowing directly
to the river. Over time the river withdrew and a floodplain was formed, setting
the stage for the creek to meander. Almost all creeks have a natural tendency
to migrate sideways forming a sine-generated curve (like the Mississippi River)
and once the land built-up to near its present level the creek would have constantly
changed its location. However, at the specific point in time when the Europeans
arrived, traces found on the ground show where it was located. From the rear
of the church the creek curved northward, then curved eastward, then northward
again to a point on Bell Avenue located 400 feet south from College Avenue... by this time surface
erosion from the leading edge of the “island” had allowed the outlet of the
creek to either migrate southward and/or otherwise find an opening across the ridge. Once past the ridge, the location where
it actually entered the river had by that time been pushed
all the way down to just inside the lower end of Janney’s Landing (at the lower property line of the red brick apartments).
As mentioned previously, there is a ridgeline along the high ground for almost the full length of the “island.” It passes through the center of the fairly new Macclesfield housing development, right up through the Eastburn Mansion and the Belville House, and the upper side of the original Livezey lot, where the ridgeline starts to bend away from the river. From there it continues toward North Bell Avenue, dips down slightly at College, rises partway back up along Bell, tapers down toward Ferry Street, and terminates just past Ferry Street. My scenario of what must have happened is based partly on a close study of this ridgeline.
In modern times man-made structures including the railroad and the canal, have combined with the higher lands along the river to create what amounts to a large basin. With the route to Macclesfield blocked, the resulting basin naturally overflows to Brock Creek.
RARE LOCAL STORM (JUNE 12, 1996)
An extremely rare local storm occurred in the Yardley area on June 12, 1996.
The runoff from this storm was more severe than produced by any other storm...
probably for hundreds of years. I base this mainly on the fact that it washed
away a fairly solid over 100-year old steel footbridge to the bleachery. This
storm led to a special study by Pennsylvania D.E.P. Although this study focused
mainly on White’s Creek (D.E.P. uses the recently coined name “Silver Creek”),
their findings written and/or verbal statements included the following:
1) The worst part of the storm fell on the Brock Creek Watershed
2) The Brock Creek flood discharge was blocked by the Canal Aqueduct
3) A large part of the Brock Creek flood overflowed into the canal.
The combined flow from the Brock Creek and White’s Creek is believed to have overflowed much of the canal’s length between Lock #5 and Lock #6. The height of the towpath varies widely so that some areas received more overflow than others. A large part of the overflow was trapped in what amounts to a large man-made basin above the railroad, between the canal and the natural high ridge along the river. These waters then flowed north to Brock Creek with some escaping through the opening at Ferry Street directly to the river at the low spot near the Medical Center.
Continuing to walk for exercise, I’m now focused on Yardley’s topography and geology. As a civil engineer, I find this a lot more interesting than working out in a gym.
Larry Hale, January 22, 2004
References:
1. A View of the River, by Luna B.
Leopold, published by Harvard University Press, 1994.
2. Geology of the Delaware Valley,
by Horace G. Richards, Mineralogical Society of Pennsylvania, 1956.
3. Soil Survey, Bucks County Pennsylvania,
by R.T.A. Burke, U.S. Department of Agriculture, 1946.
4. Pennsylvania Geology, by George
H. Ashley, Topographic and Geologic Survey, 1931.
5. Keywords: Wisconsin deglaciation, relative sea level, surficial geology,
alluvial plain.