LET'S INVESTIGATE....THE ROCKS AND SEDIMENTS OF DELAWARE
Section overview
• To learn more about the rocks and sediments of Delaware, your teacher will assign to your group a particular rock, deposit or sediment formation that occurs in Delaware. Some of these rocks and sediments can be observed at, or very near, the surface in Delaware, most of them occur beneath the surface, some to as deep as several thousand of meters (up to 9,000 feet beneath the surface).
Note to teacher
• Assign groups of 2-4 students to the various rocks, deposits or formations. A list of 14 potential rocks, deposits or formations is provided below. From this list, be sure to assign those with an asterisk, as these are especially important to Delaware's geologic history.
List of rock formations
1. List of Potential rocks, sediments, or formations
2. *Iron Hill Gabbro3. *Baltimore Gneiss4. *Cockeysville Marble5. *Wissahickon Formation6. *Potomac Formation7. Magothy Formation8. Merchantville Formation
9. *Mount Laurel Formation10. Vincentown Formation
11. *Columbia Formation12. *Swamp and Marsh
Deposits13 *Beaverdam Formation14 *Calvert Formation15 Lynch Heights Formation
• For your assigned rock, deposits or formation, your teacher will give you either a geologic map of New Castle County or Kent County, Delaware to use for your investigation
Your job for lab
• Use the information provided on the geologic map with the geologic time scale and the generalized geologic map of Delaware to prepare a presentation to your class on your assigned rock, deposit or formation.
Your 5-10 minute presentation should include:
1. A description of your rock, deposit or formation (for example, what type of rock, igneous, metamorphic, or sedimentary is it?, what type of deposit is it?, what types of sediments are in your formation?)
2. Where does your rock, deposit or formation occur on your geologic map, is it exposed at or near the surface, or does it occur mostly beneath the surface
3. How thick is your rock, deposit or formation?4. What is the approximate age of your rock, deposit or formation?5. How was your rock, deposit or formation formed (for example, was it
associated with volcanic activity? was it associated with subduction? Was it deposited by meltwater flowing from a glacier? Was it deposited in a shallow (or coastal) marine environment?)
6. How does your rock, deposit or formation fit into the geologic history of Delaware?
7. Where does it occur within the geologic time scale?
Subsurface layers
• The Potomac, Magothy, Merchantville, Mount Laurel, Vincentown, and Calvert Formations primarily occur in the subsurface. So students might only find isolated locations where they are shown at the surface on the geologic maps of New Castle and Kent County. For these formations, it is essential that the students investigate and understand the cross-sections that are shown with the geologic maps.
Oldest rock formations
• The Baltimore Gneiss, Cockeysville Marble, Wissahickon Formation, and Iron Hill Gabbro are examples of Delaware's Piedmont Rocks. These are the oldest rocks in Delaware. They are evidence that Delaware was once at a convergent plate boundary and part of a large mountain chain, similar to the Andes Mountains or the Himalaya Mountains of today.
The Baltimore Gneiss, Cockeysville Marble, Wissahickon Formation, and Iron HillGabbro are examples of Delaware's Piedmont Rocks. These are the oldest rocks inDelaware. They are evidence that Delaware was once at a convergent plate boundaryand part of a large mountain chain, similar to the Andes Mountains or the HimalayaMountains of today.
Baltimore Gneiss• Have students use New Castle County geologic map for
presentation; 1) metamorphic rock; 2) found at/near the surface along the Pennsylvania/Delaware border; 3) one of the "basement" rocks in Delaware, thickness is not shown on the geologic map; 4) age is 600-570 million years; 5) gneisses are formed when sedimentary or igneous rocks are buried and subjected to high temperatures and pressures; 6) these are the oldest rocks exposed in Delaware, they were formed during the Precambrian or latest Proterozoic Eon, as we will discuss in the next part of this investigation, they were formed when Delaware was at a convergent plate boundary and igneous oceanic rocks were subducted and metamorphosed
Cockeysville Marble• Have students use New Castle County geologic map for
presentation; 1) metamorphic rock; 2) found at near the surface in valleys near Hokessin, DE; 3) one of the "basement" rocks in Delaware, thickness is not shown on the geologic map; 4) age is 570-438 million years; 5) marbles are formed when sediments or sedimentary rocks containing calcite are buried and subjected to high temperatures and pressures; 6) these are some of the oldest rocks exposed in Delaware; they were formed during the Cambrian to Ordoviciar Period within the Paleozoic Era within the Phanerozoic Eon, as we will discuss it the next part of this investigation, they were formed when Delaware was at a convergent plate boundary by subduction of oceanic sediments composed primarily of calcite and these sediments were then metamorphosed
Iron Hill Gabbro• Have students use New Castle County geologic map for
presentation; 1) igneous rock; 2) found at/near the surface at Iron Hill south of Newark, DE; 3) one of the "basement" rocks in Delaware, thickness is not shown on the geologic map; 4) age is 438-408 million years (Silurian Period within the Paleozoic Era within the Phanerozoic Eon); 5) gabbros are formed when magma slowly cools within the earth at mid-ocean ridge spreading centers; 6) these are some of the oldest rocks exposed in Delaware; they were formed during the Silurian Period within the Paleozoic Era within the Phanerozoic Eon, as we will discuss in the next part of this investigation, they were formed at a mid-ocean ridge, moved with a tectonic plate toward a subduction zone, and then were essentially "glued" onto the plate that wasn't being subducted (a process called obduction) when Delaware was at a convergent plate boundary
The Potomac, Magothy, Merchantville, Mount Laurel and Vincentown Formations
• The Potomac, Magothy, Merchantville, Mount Laurel and Vincentown Formations are Cretaceous to early Tertiary in age (144-58 million year old) sediments that were deposited when, after a long time period (260 million years) of continuing erosion of the large mountain range that was built during the formation of Delaware's Piedmont rocks (see above), Delaware was now near sea-level and occasionally the ocean would advance and Delaware would be below sea-level and marine sediments would be deposited.
The Potomac formationHave students use New Castle County geologic map for
presentation; 1) sediments - clays, silts and sands - some lignite (a type of coal); 2) exposed at the surface near Iron Hill, south of Newark, DE, occurs beneath the surface at increasing depths from northwest to southeast in Delaware; 3) from only 6 meters or less (20 feet) to over 500 meters (1,600 feet) thick in southern New Castle County; 4) age is -144-120 million years; 5) formed as deposits associated with a river; 6) they were formed during the early (lower) Cretaceous Period within the Mesozoic Era during the Phanerozoic Eon, Delaware was now near sea-level and these deposits are associated with a riverflowing toward the nearby ocean
Magothy formationHave students use New Castle County geologic map for
presentation; 1) sediments - clays, silts and sands - some lignite (a type of coal); 2) not exposed at the surface, occurs beneath the surface at increasing depths from northwest to southeast in Delaware; 3) it is 6 to 16 meters thick (20 to 50 feet); 4) age is -80 million years; 5) formed as deposits in shallow ocean waters; 6) they were formed during the later (upper) Cretaceous Period within the Mesozoic Era during the Phanerozoic Eon, parts of Delaware were now below sea-level and these deposits are associated with near coastal, shallow marine deposits, they are called channel-fill deposits because they fill-in rivers that
Gap in record
• gap in ume miiiion years) between the deposition of the Potomac and Magothy Formations
Merchantville FormationHave students use New Castle County geologic map for
presentation; 1) sediments - silts and sands with the mineral glauconite (a mineral that is formed in shailow marine environments; 2) not exposed at the surface, occurs beneath the surface at increasing depths from northwest to southeast in Delaware; 3) it is 6 to 60 meters thick (20 to 120 feet); 4) age is -80- 70 million years; 5) formed as deposits in shallow ocean waters; 5) formed as deposits in shallow ocean waters; 6) they were formed during the later (upper) Cretaceous Period within the Mesozoic Era during the Phanerozoic Eon, parts of Delaware were now below sea-level and these deposits are associated with a shallow marine environment
Mount Laurel Formation• Mount Laurel Formation - Have students use New Castle
County geologic map for presentation; 1 sediments - sands with marine fossils; 2) exposed at the surface in small areas near the Chesapeake and Delaware Canal, also occurs beneath the surface at increasing depths from northwest to southeast in Delaware; 3) it is 10 to 30 meters thick (30 to 100 feet); 4) age is -70 million years; 5) formed as deposits in shallow ocean waters; 6) they were formed during the later (upper) Cretaceous Period within the Mesozoic Era during the Phanerozoic Eon, parts of Delaware were now below sea-level and these deposits are associated with a shallow marine environment
Vincentown FormationHave students use New Castle County geologic map for
presentation; 1) sediments - sands with the mineral glauconite (a mineral that is formed in shallow marine environments); 2) exposed at the surface in small areas near Middletown and Odessa, DE, occurs beneath the surface at increasing depths from northwest to southeast in Delaware; 3) it is 1 to 30 meters thick (50 to 100 feet); 4) age is 66-58 million years; 5) formed as deposits in shallow ocean waters; 6) they were formed during the Paleocene Period during the Cenozoic Era within the Phanerozoic Eon, parts of Delaware were now below sea-level and these deposits are associated with a shallow marine environment
Calvert, Beaverdam, Columbia, and Holocene Marsh and Swamp Deposits• The Calvert, Beaverdam, Columbia, and
Holocene Marsh and Swamp Deposits are late Tertiary to Holocene in age (24 million year old to present) sediments that were deposited as Delaware continued to be near sea-level and occasionally the ocean would advance and Delaware would be below sea-level and marine sediments would be deposited, followed by glacial time periods (see Columbia Formation) when global sea level would be much lower and glacial to river sediments would be deposited.
Calvert, Beaverdam, Columbia, and Holocene Marsh and Swamp Deposits• Today sediments are being deposited in the
marshes and swamps of Delaware as well as some locations in Delaware Bay.
Calvert Formation
• Calvert Formation - Have students use Kent County geologic map for presentation; 1 sediments - silts, clays, and sands with marine and some land plant and animal fossils; 2) beneath the surface at increasing depths from northwest to southeast in Delaware; 3) up to 80 meters thick (250 feet); 4) age is 24-5.3 million years; 5) shallow marine deposits; 6) they were formed during the
Calvert?• witnin tne i-inanerozoic ton, parts orDelaware
were now below sea-level and these deposits are associated with a shallow marine environment; this formation is associated with some of the major aquifers in Delaware - during the building of Route 1; excavation cut thru the Calvert Formation exposing abundant fossils, including river porpoises, and whales. A good reference describing this fossil site is:
• http://www.dgs.udel.eduipublicationsipubs/SpecialPublicationsisp2tpdf
Beaverdam Formation• Have students use Kent County geologic map for
presentation; 1) sediments - mostly sands with some silts; 2) exposed at the surface north to sound along the western portion of Kent County – see generalized geologic map for distribution throughout Delaware; 3) 25 to 30meters thick (75-100 feet); 4) age is 5.3 to 1.6 million years; 5) sediments associated with river deposits and possibly estuarine (Delaware Bay) deposits; 6)they were formed during the Pliocene Period during the Cenozoic Era within the Phanerozoic Eon, global sea level was lower at this time, exposing more of Delaware as a land surface, these sediments are from rivers flowing acrossDelaware
Columbia• - early Pleistocene - Have students use Kent County geologic map for
presentation; 1 sediments - sands and gravels with some silts; 2) exposed at the surface north to sound along the middle portion of Kent County see generalized geologic map for distribution throughout Delaware; 3) -16 meters thick (50 feet); 4) age is 146 to 1 million years; 5) sediments deposited by meltwaters flowing away from glaciers - the glaciers extended down into the central portions of Pennsylvania ; 6) they were formed during the early Quaternary Period during the Cenozoic Era within the Phanerozoic Eon, theglobal climate was much colder during this time period, glacial ice covered more Df earth's surface (up to 30%) and global sea level was much lower Lynch Heights - later Pleistocene - Have students use Kent County geologic map for presentation; 1 sediments - sands with beds of gravels and silts; 2) exposed at the surface north to sound along the middle portion of Kent County - see generalized geologic map for distribution throughout Delaware; 3) 16 metersthick (50 feet); 4) age is between 1.0 million and 10,000 years; 5) sedimentsassociated with river deposits and possibly estuarine (Delaware Bay) deposits; 6)they were formed during the later Pleistocene Epoch during the QuaternaryPeriod within the Cenozoic Era during the Phanerozoic Eon, global sea level waslower at this time, exposing more of Delaware as a land surface, these sedimentsare from rivers flowing across Delaware
Swamp and Marsh Deposits• Swamp and Marsh Deposits - Holocene - Have students
use Kent County geologic map for presentation; 1 sediments - silts and clays with organic material (decomposing plants) and some sands; 2) found mostly along the coastline of Delaware Bay and near some rivers in Delaware - see generalized geologic map of Delaware for distribution; 3) less than 1 to 1 meters thick (1-50 feet); 4) age is 10,000 years and younger; 5) deposits occurring in swamps and marshes; 6) they were formed during the Holocene Epoch during the Quaternary Period during the Cenozoic Era within the Phanerozoic Eon, these are the most recent sediments in Delaware and they are being deposited today in swamps and marshes, mostly near the coastline of Delaware Bay
NOTE TO TEACHERS
Review for the class the origin of igneous rocks (i.e., formed by the cooling and subsequent cooling of magma) and how the texture and composition can be used to infer the composition of the magma and how the magma cooled
NOTE TO TEACHERS
• Introduce to the class the origin of metamorphic rocks (i.e.,formed by exposing pre-existing (igneous, sedimentary, or other metamorphic) rocks higher pressures, higher temperatures, and/or the passage of fluids; this exposure causes these rocks to change (or re-crystallize) without melting)
Questions based on Student Presentation
1. What are the names of the oldest rocks (ages between 600 and 400 million years) that occur in Delaware?
2. What types of rocks are they?3. How could the Iron Hill Gabbro have formed?4. How could the Baltimore Gneiss, Cockeysville Marble and
Wissahickon Formation have formed? 5. At what type of plate boundary, divergent, convergent or
transform, are you likely to have metamorphism of oceanic sediments and oceanic crustal rocks? Justify your answer
6. If there was a convergent plate boundary in the Delaware area 600 to 400 million years ago, what must Delaware have looked like? Based on what you learned about convergent plate boundaries earlier in this unit, sketch in your journal what you think were the major features in the Delaware region.
Answer to 3rd question• Formed by cooling of magma — evidence of large
crystals in gabbros show that this cooling must have been relatively slow (origin not from a volcano, but rather from slow cooling within the earth's crust); the dark color of the gabbro suggests that the magma from which gabbros form is mafic in composition (relatively lower amounts of silicion and oxygen, relatively higher amounts of iron, magnesium, and calcium) Evidence suggest that the Iron Hill Gabbro must have an oceanic origin with slow cooling within the Earth's crust — from plate tectonics have learned about the formation of gabbros at mid-ocean ridge spreading centers
Answer to 4th question
• By exposing pre-existing rocks to higher pressures, higher temperatures and/or the passage of fluids. As described in your student presentations, metamorphic rocks like the Baltimore Gneiss, Cockeysville Marble and Wissahickon Formation show evidence that they have been formed by metamorphosing oceanic sediments (e.g., the Cockeysville Marble) or the oceanic crust (e.g., Baltimore Gneiss and Wissahickon Formation).
Answer to the 5th question
• Convergent plate boundaries. Subduction takes oceanic sediments and crustal material and brings them down into the earth exposing them to higher pressures and higher temperatures.
• Based on processes that are occurring at modern day plate boundaries, geologists can infer that the metamorphic rocks in Delaware were formed at a convergent plate boundary that must have existed in the Delaware area between 600 and 400 million years ago when these rocks were formed. Remember that one of the basic principles in geology is that "the present is the key to the past".
NOTE TO TEACHERS
Below is a diagram that illustrates what some of the major features must have been. Note the occurrence of subduction, a deep oceanic trench, volcanoes and high elevation mountains on the plate above the subduction zone. Small to large magnitude earthquakes at shallow to deep depths would be associated with these features. Delaware at this time would have been very similar to the either the modern day Andes or Himalaya Mountains.
Delaware 600-400 MYA
Why don’t we have rocks from every different section of time?
• Based on the evidence from your class presentations, between 408 and 144 million years ago, no rocks or sediments are present at the surface or in the subsurface in Delaware.
More Questions
7. What do you think has occurred during this time period in the Delaware region?
8. According to your student presentations, at what time did sediments begin to be deposited near this coastline? What type of sediments were they? In what type of environment were they deposited?
9. What is the evidence for glaciers playing a role in Delaware's Geologic History?
10.What types of sediments are being deposited today in Delaware?
Answer to question 7
• Weathering and erosion of the large mountain range stripped away the rocks that made up most of this mountain range. The sediments that were produced by this weathering were carried away by rivers, streams and deposited in large basins.
NOTE TO TEACHERS
Land surfaces, especially mountain ranges, are dominated by erosion, as opposed to deposition. Areas of deposition occur either in association with rivers/streams or glaciers on land, but by far, most of the depositional zones for sediments are in coastal (i.e., swamps/marshes near coastlines, beaches along coastlines) or marine environments.
• Beginning about 200 million years ago, the North American tectonic plate began to separate from the European and African tectonic plates. At the divergent plate boundary between the North American and European and the North American and African plates a new ocean basin began to form — the Atlantic Ocean.
Late jurassic
Delaware was now located near the coastline of the newly forming Atlantic Ocean. To the north and west were the Appalachian Mountains that were continuing to be eroded. As more and more of these mountains were eroded away, Delaware's Piedmont Rocks (e.g., the Baltimore Gneiss, Cockeysville Marble, Wissahickon Formation, and Iron Hil Gabbro) moved closer to the surface, eventually they were exposed at the surface where we see them today.
Answer to question 8
• Beginning in the Cretaceous Period 144 million years ago. Sands, silts and clays. Deposited in river (fluvial) to shallow marine environments.
Answer to question 9
• Columbia Formation sediments indicate glacial activity in the region. Most likely deposits that are associated with melt water flowing away from glaciers that were located in central Pennsylvania
Answer to question 10
• Fine-grained silts and clays, along with organic material, in the swamps and marshes of Delaware.
Check in
• Based on the evidence from the rocks and sediments that occur at or near the surface and beneath the surface, describe in your journals the geologic history of Delaware.
Answer to check in• Answers should include old (600 to 400 million
years) metamorphic and igneous rocks formed at a convergent plate boundary, followed by a long period of erosion with no deposition of sediments in the Delaware area, followed by river to shallow marine deposits starting in the Cretaceous period (144-66 million) continuing into the Cenozoic Era. In the past 1.6 million years, there is evidence from the Columbia Formation of glacial activity in the Delaware region, followed by modern deposits occurring in the swamps and marshes of Delaware.
