SOLUTION: Lab 5 geologic maps part 1 worksheet - Studypool - Free Printable

Let’s solve this step by step.

We are asked to construct a geologic cross-section along the transect line A-A’ on the map. The topography (the shape of the land surface) is already drawn in the profile box — we just need to add the rock layers underground, based on what we see on the map and how they dip (tilt).

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Step 1: Understand the map symbols

On the right side, there’s a legend showing which color/pattern equals which rock formation:

- Sc = Clinton Group (Silurian) → brownish
- St = Tuscarora Fm. (Silurian) → tan/orange
- Oj = Juniata Fm. (Ordovician) → purple
- Obe = Bald Eagle Fm. (Ordovician) → light gray/white
- Or = Reedsville Fm. (Ordovician) → greenish-gray

Also note: There are strike-and-dip symbols on the map — little T-shaped marks with numbers like “30” or “33”. These tell us how steeply the rock layers tilt underground.

For example:
- A symbol with “30” means the layer dips at 30 degrees.
- The long bar of the “T” shows the direction the layer runs (strike), and the short perpendicular bar points downhill (dip direction).

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Step 2: Trace the rock units along line A-A’

Look at where line A-A’ crosses each colored band on the map. From left (A) to right (A’):

Starting from the far left (near point A):

1. First, you cross On (not in legend? Probably older unit — but since it’s not in the key, maybe ignore for now or assume it’s beneath everything).
2. Then Obf → likely part of Ordovician, possibly same as Obe or Or? But again, not in main legend — focus on labeled ones.
3. Then Obl, Ocn, Or, Obe, Oj, St, Sc, then back through St, Oj, etc., toward A’.

But let’s simplify using only the formations listed in the legend that appear along A-A’:

From left to right along A-A’:

→ You start near Obf/Obl area (green shades)
→ Then hit Or (Reedsville)
→ Then Obe (Bald Eagle)
→ Then Oj (Juniata)
→ Then St (Tuscarora)
→ Then Sc (Clinton Group) — this is the big brown area in the middle
→ Then again St, Oj, Or, etc., going toward A’

Wait — actually, looking closely at the map, the line A-A’ goes diagonally across the map from upper-left to lower-right.

It cuts through:

Left side (A end): Starts in green areas (Obf, Obl) → then hits dark green (Or) → then white/light gray (Obe) → then purple (Oj) → then tan (St) → then large brown area (Sc) → then again tan (St) → purple (Oj) → green (Or) → ends near Ocn/Obl again?

Actually, let’s list them in order as you walk from A to A’:

Assume A is top-left corner, A’ is bottom-right.

Along A-A’, you pass through these units in sequence:

1. Obf (light green) — very edge
2. Or (dark green)
3. Obe (white)
4. Oj (purple)
5. St (tan)
6. Sc (brown) ← widest unit here
7. St (tan) again
8. Oj (purple)
9. Or (green)
10. Ocn (medium green) — near A’

So the pattern is symmetric-ish around the center (Sc), suggesting a fold — probably an anticline (arch-up) because oldest rocks are in the middle? Wait — no! In folded regions, if youngest rocks are in the middle, it’s a syncline; if oldest are in the middle, it’s an anticline.

Check ages:

Legend says:

- SILURIAN (younger): Sc, St
- ORDOVICIAN (older): Oj, Obe, Or

So Silurian rocks (Sc, St) are younger than Ordovician (Oj, Obe, Or).

In the center of our transect, we have Sc (youngest), flanked by St, then Oj, then Obe, then Or (oldest).

That means the youngest rocks are in the CENTER → so this is a syncline (a downward-folded trough).

BUT wait — look at the dip directions!

There are strike-and-dip symbols:

- On the LEFT side of Sc (in St layer), there’s a dip symbol pointing RIGHT with “33” → meaning the layer tilts 33° to the right (east?).
- On the RIGHT side of Sc (also in St layer), there’s a dip symbol pointing LEFT with “30” → tilts 30° to the left (west?).

That confirms it: both sides dip INWARD toward the center → syncline!

Perfect.

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Step 3: Draw the cross-section

You’re given the topographic profile already drawn — it looks like a valley with hills on either side.

Now, below that surface, draw the rock layers dipping inward toward the center.

Start from the surface down:

At any point along A-A’, the rock unit you see on the map is the one at the surface. Below it, the next older unit should be underneath, following the dip angle.

Since it’s a syncline, imagine drawing layers that curve downward in the middle.

Use the dip angles given:

- Where you see “33”, draw that layer tilting 33° toward the center.
- Where you see “30”, draw 30° toward the center.

Even where no number is given, assume similar dips unless contradicted.

Order of layers from top to bottom (youngest to oldest):

Surface → Sc → St → Oj → Obe → Or → ... deeper old stuff

But remember: in a syncline, the layers bend down, so at the center, Sc is at the surface, and as you go outward, you expose progressively older layers.

Underground, beneath Sc, comes St, then Oj, etc., curving downward.

So in your cross-section box:

- Start at the surface trace of each unit along A-A’.
- For each boundary between units, extend it underground at the appropriate dip angle.
- Make sure the layers converge downward in the middle (syncline shape).
- Don’t forget to label each layer with its code (Sc, St, Oj, etc.)

Also, note: some units may pinch out or repeat due to folding — but in this case, since it's a simple syncline, each layer should appear twice except the central one (Sc).

Wait — actually, Sc appears only once in the middle? No — looking again, Sc is only in the very center. Flanking it are two bands of St, then two of Oj, etc. So yes — symmetric syncline.

Therefore, in cross-section:

Draw a U-shape (or V-shape depending on tightness) with:

Top layer: Sc in the middle, thinning out to zero at edges? No — Sc doesn't reach the edges. Actually, Sc is only exposed in the center. So underground, Sc forms a lens-like shape sitting atop St.

Better way: Think of stacking pancakes that are bent into a U.

Bottom-most visible layer might be Or, then above it Obe, then Oj, then St, then Sc on top in the center.

But since Sc is youngest, it must be on top.

So vertical sequence (from bottom up):

Deep underground: Or → Obe → Oj → St → Sc (only in center)

And all layers dip inward.

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Step 4: Sketch it mentally

Imagine the cross-section axis from A (left) to A’ (right).

At far left (A):

Surface = Obf/Obl → but since not in legend, perhaps treat as equivalent to Or or Obe? Let’s stick to mapped units along A-A’.

Actually, re-tracing carefully:

Looking at the actual path of A-A’ on the map:

It starts in the northwest corner (point A), goes southeast to A’.

Crosses:

- Near A: mostly Obf and Obl (both light green) — assume these are part of Ordovician, maybe correlatable to Obe or Or. Since Obe is Bald Eagle and Or is Reedsville, and Obf/Obl aren’t defined, perhaps they’re lateral equivalents. To avoid confusion, use only the legend-defined units that clearly intersect A-A’.

Clear intersections:

Moving from A to A’:

1. Crosses Or (Reedsville) — dark green stripe
2. Then Obe (Bald Eagle) — white stripe
3. Then Oj (Juniata) — purple stripe
4. Then St (Tuscarora) — tan stripe
5. Then Sc (Clinton) — wide brown area
6. Then St again — tan
7. Then Oj — purple
8. Then Or — green
9. Ends near Ocn — medium green (assume similar to Or)

So the repeating pattern is:

Or – Obe – Oj – St – [Sc] – St – Oj – Or – ...

With Sc only in the very center.

Thus, in cross-section, we’ll have:

From left to right at surface:

Or | Obe | Oj | St | Sc | St | Oj | Or | ...

Underground, each layer continues downward, dipping toward the center.

Dip angles:

- Left side: St dips 33° to the right (toward center)
- Right side: St dips 30° to the left (toward center)

Assume other layers have similar dips unless specified.

So when drawing:

- At the contact between St and Sc on the left, draw Sc overlying St, and both dipping 33° rightward until they meet their counterparts from the right.
- Similarly on the right, dip 30° leftward.
- The Sc layer will form a keystone shape in the middle, thickest at center, tapering off.
- Beneath St, draw Oj dipping similarly, then Obe, then Or.

Make sure the layers don’t cross — they should stack neatly, bending smoothly.

Also, since topography is already drawn, match the surface exposure: where the map shows Sc at surface, your cross-section has Sc touching the ground line in the middle. Where it shows St, St touches the ground, etc.

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Final Check

Is this consistent?

Yes — synclinal structure, youngest in center, layers dip inward, dip angles match symbols, sequence matches stratigraphic column (Silurian over Ordovician).

No faults shown along A-A’, so no offsets needed.

All good.

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Final Answer:
Construct a geologic cross-section along A-A’ showing a syncline with the following features:
- Surface exposures from left to right: Or → Obe → Oj → St → Sc → St → Oj → Or
- All layers dip inward toward the center (Sc)
- Left-side dips approximately 33°, right-side dips approximately 30°
- Layer sequence underground (bottom to top): Or → Obe → Oj → St → Sc (central only)
- Layers curve downward in a U-shape, maintaining thickness and non-crossing geometry
- Label each layer with its formation code (Sc, St, Oj, Obe, Or)
- Match surface contacts to the existing topographic profile