Delaware mineralogical society

A Delaware 501(c)(3) non-profit earth-science educational organization

 

Marl Pit, Middletown, DE

Minerals/Soils/Coastal Plain Geology

October 15: Marl Pit, Middletown, DE

Field Collecting Trip

Where: Marl Pit, Middletown, De

When: Sunday, October 15, 2017, 11:00 am to 2:00 pm

Contact: Tom Pankratz


Email me if you are interested in attending this trip. I’ll return a confirmatory email. If you do not receive my confirmation, please recontact me. tjpankratz@verizon.net

Details

This is a working quarry so Monday’s through Friday are unavailable, and the site is game hunted on Saturdays leaving Sunday as the only day we can collect. I selected the 11am to 2 pm time so families could schedule around other activities. Though the starting time is 11am, you can arrive any time after 11 you choose.
This trip is open to members of all ages. I think it’s a particularly family-friendly site.

The site is called a ‘marl’ pit. Marl is a lime-rich (calcium carbonate) mud or mudstone which contains variable amounts of clay and silt. I do not know if this site really has, or ever has had, marl of this definition. To me it looks mostly to be a sandy clay.  DGS Geosymposium 2017 Fieldtrip Guide.pdf

Two things make this site interesting. One is that it gives us a rather rare look at the Delaware Coastal Plain. We’ve all travelled over the coastal plain and even collected fossils at the canal and by Odessa, but we’ve only really seen the topsoil, and then that’s mostly covered with vegetation. Rarely do we get to see the stratification caused by periodic oceanic flooding or the ice age floods. I don’t really know which this site is due to. That it’s a ‘marl’ with calcium carbonate makes me think oceanic, but the absence of fossils and the presence of a lot of river rounded rocks makes me suspect ‘ice age’ floods. These rocks are the other thing that makes this site interesting. Their variety is quite amazing and I suspect just about every kind of rock in the greater Delaware River drainage basin is here in stratified layers. The other ‘rock’ that’s most prevalent is ‘bog iron’. These layers are inches to perhaps a foot or so thick and consists of a mix of goethite, hematite and limonite iron oxide rocks with a variety of configurations and colors. Very collectable! There may even be some ‘Indian Paintpots’ in the mix.

The site is easy to get into and we can drive right up to the collecting and viewing areas. The quarry walls that are being actively mined are perhaps 20 to 30 ft. high and consists of pretty loose sand and clay..we’ll want to stay well back from these. But elsewhere in the quarry the walls have been heavily eroded by rainfall and we can safely walk and climb up and down them.
There are quite a few ‘rain wash gullies’ in these walls. Vegetation cover is minimal in most places.

Safety:

Hard hats, safety vests and long pants are required for everybody.
Safety glasses are required if you are breaking rocks
Boots or ankle high shoes are required (safety boots are not required; no open toe or heel shoes please)

Collecting equipment:

The soil is soft and easy to dig. Shovels, picks, screens, perhaps a light weight hammer should be useful. I don’t think you find use for sledge hammers, chisels or rock-busting equipment. Buckets and wagons/hand carts will be useful.

I took and edited the following from the Delaware Geologic Survey website:

The State of Delaware is located within two physiographic provinces, the Appalachian Piedmont and the Atlantic Coastal Plain. Most of the state lies within the Coastal Plain; it is only the hills of northern New Castle County that lie within the Piedmont.

The rocks at the surface in the Piedmont are old, deformed, metamorphic rocks that were once buried in the core of an ancient mountain range. This range formed in a series of tectonic events that built the Appalachians between about 543 and 250 million years ago. All through that time and since, rivers and streams have carried the erosional products, mostly sand, clay, and gravel, from the mountains onto the Atlantic Coastal Plain.

Fall Line

Delaware’s Piedmont ends at the Fall Line where the metamorphic rocks dip under and disappear beneath the sediments of the Coastal Plain. The Fall Line roughly follows Kirkwood Highway, Route 2, across the state between Newark and Wilmington.

Atlantic Coastal Plain

Delaware’s Coastal Plain rises to about 100 feet above sea level. Its streams drain into the Delaware River or Bay, and for much of their length they are tidal. The Coastal Plain is made up of sediments, mostly silt, sand, and gravel, that have been eroded off the Piedmont and adjacent Appalachian Mountains. In cross section these sediments form a southeastward thickening wedge that increases from 0 feet at the Fall Line to over 10,000 feet along Delaware’s coast.

During the time represented by this unconformity, the rocks we see in the Piedmont today reached the earth’s surface as approximately 7 to 13 miles of overlying rock were removed by erosion allowing the buried rocks to rise to the surface in compensation. The oldest Coastal Plain sediments observed in Delaware are river-deposited sediments. These sediments were eroded from the Appalachian Mountains to the west, transported to the southeast by rivers, and deposited where the rivers met the ocean to form a delta. On top of the river sediments a sequence of marine silt and sand deposits records the rise and fall of the sea level many times during a period of over 80 million years, from the Late Cretaceous until the end of the Tertiary Period, about 2 million years ago.

On top of all of these sediments is a thin veneer of young sand and gravel that was carried into Delaware by glacial outwash during the Ice Age. Glacial ice did not advance into Delaware, but melt-water pouring off the glacier fronts carried great quantities of sand, silt, and gravel over southern Pennsylvania and Delaware. Delaware’s largest mineral resource is the sand and gravel deposited from the glacial outwash.
Make yourself at home.