# and PHOTO
LOCATION
QUESTION
1
Pyrobole porphyry, 2-foot diameter boulder
44 46 45.6 N
73 22 30.0 W
This boulder is igneous (cooled from a molten condition). The size of individual crystals in it depends in part on the rate of cooling (faster cooling yields smaller crystals). How could a magma start out cooling slowly and finish cooling more rapidly, as apparently occurred here?
2
Anorthosite boulder
44 46 45.8 N
73 22 29.9 W
This unusual igneous rock (crystallized from a molten condition) is almost entirely made of one mineral (plagioclase). Melt of plagioclase composition probably doesn’t exist. How might an igneous rock wind up having a different composition from the melt that formed it? (Note of interest: the brighter parts of the moon are mostly plagioclase.)
3
Gabbroic anorthosite boulder
44 46 46.1 N
73 22 29.7 W
What suggests that this is an igneous rock? (Crystallized from a molten condition.)
4(a)
Limestone boulder
44 46 46.3 N
73 22 29.0 W
Can you see any shells in this rock?
4(b)
Limestone boulder
44 46 46.3 N
73 22 29.0 W
What are the twig-like marks in this rock (twigs hadn’t evolved yet!)
5
Cuspate rib marks on bedrock
44 46 46.5 N
73 22 28.9 W
The curved marks near the quarter formed when the rock split. Can you guess the direction in which the crack traveled?
6
Limestone boulder
44 46 46.6 N
73 22 23.7 W
What is this spiral mark?
7
Quartz sandstone boulder
44 46 46.8 N
73 22 28.8 W
What indicates this boulder is of sedimentary origin? (formed from sediment accumulated on the Earth’s surface.)
8
Essexite boulder
44 46 47.1 N
73 22 28.2 W
What indicates that the composition of the large crystal at the point of the hammer pick (feldspar) changed as it grew?
9
Garnet in boulder
44 46 47.6 N
73 22 27.5 W
The red crystals here are garnet. Do you know of any uses for this mineral? Though garnet forms only deep below the Earth’s surface, we often see it at the surface. How come?
10
Dolostone boulder
44 46 48.0 N
73 22 26.9 W
What made the smile-like cracks in this rock?
11
Disharmonic folding on boulder
44 46 48.6 N
73 22 26.4 W
Layers here have folded independently of one another (disharmonically) and are partly tightly squeezed. Such folds usually form at very high temperatures when rock becomes mushy. However, high temperatures cause rock grains to grow larger as they recrystallize. This rock is fine-grained. Can you resolve this paradox?
12
Greenstone boulder
44 46 48.7 N
73 22 25.9 W
If you look at this texture with a magnifying glass, you can see that the crystals have irregular fragile shapes that couldn’t survive transport by water, ice, or wind. It is thus probably not a sedimentary rock. How else might it have formed?
13
Veins on boulder
44 46 48.7 N
73 22 25.9 W
Two types of veins occur here; white veins and a spotted vein. Study their intersections. Which type of vein formed first? (This problem is more difficult than usual, as you may discover!)
14
Dolostone boulder
44 46 48.8 N
73 22 25.7 W
This sedimentary rock (dolomite and clay in varying proportions) has a lumpy sort of layering as suggested by its shallowly pitted surface. How might such layering develop?
15
Flexural folding in bedrock
44 46 49.3 N
73 22 24.8 W
The layers here were originally deposited as horizontal beds of mud in an ancient sea. They are still horizontal on the right but slope down lakeward on the left. How come?
16(a)
Essexite boulder
44 46 50.2 N
73 22 23.7 W
Although this rock is igneous, it shows a crude layering. How might such layering have formed?
16(b)
Essexite boulder
44 46 50.2 N
73 22 23.7 W
How did the rounded dark blob get into this rock?
17
Slickenlines in bedrock
44 46 51.3 N
73 22 22.3 W
The scrape marks under the hammer are called slickenlines. What indicates these were not made by glaciers? In what direction did the surface beneath the hammer shift relative to the dark rock which was once immediately above this surface?
18
Fault ramp on cliff face
44 46 51.5 N
73 22 22.5 W
Exactly what happened here to cause the arching of the layers above the darkish lens of rock?
19
Fault breccia in bedrock
44 46 51.6 N
73 22 22.8 W
Breccia is rock composed of large sharp-edged fragments. What caused this fragmentation? How did these fragments later get surrounded by the white mineral (calcite)?
20
Fault bend fold in bedrock
44 46 51.6 N
73 22 22.8 W
Give all the evidences for faulting (slip of rock along cracks) here. Faulting occurred both before and after the white calcite was deposited. What suggests this?
21
Offset vein in bedrock
44 46 51.9 N
73 22 21.3 W
There are at least 3 ways this calcite vein could have been offset. What are they?
22
Bedding in bedrock cliff
44 46 52.7 N
73 22 20.8 W
Observe the fine alternating clay-rich (dark) and calcite-rich (pale) layers. Can you guess as to what was happening in the ocean to account for the repeated changes in the type of sediment that was being deposited? Calcite is carried in solution; clay as tiny particles.
23
Striated bedrock at
top of cliff
44 46 53.1 N
73 22 17.4 W
What caused the bedrock scratches that parallel the hammer handle?
24
Essexite boulder
44 46 53.3 N
73 22 19.9 W
An identical rock forms a vertical cylinder under Mt. St. Gregoire, 60 miles to our north. The cylinder cuts across the horizontal layers that surround the mountain. Knowing all this, how might you explain the alignment of the long white crystals (feldspar)?
25
Rhythmic bedding on cliff
44 46 53.4 N
73 22 19.8 W
The layering of sedimentary beds here seems to repeat in some fixed sequence. Can you suggest a corresponding sequence of environments that might explain the changing sediment types?
26
Dolostone boulder
44 46 53.5 N
73 22 19.8 W
What carried this big boulder to this place?
27(a)
Arkose and graywacke boulder
44 46 53.9 N
73 22 19.4 W
The rock under the quarter (arkose) contains many white feldspar crystals. When crystals grow freely in a liquid (such as magma, molten rock), they develop geometric shapes bounded by flat surfaces. When crystals are carried and deposited by water, they bump together and may become more irregular or rounded in shape. Is this rock igneous (crystals formed in magma) or sedimentary (eg. deposited by flowing water)?
27(b)
Arkose and graywack boulder
44 46 53.9 N
73 22 19.4 W
What change in the conditions of sediment deposition accounts for the change in sediment color from light to dark? (The hammer head rests on a darker part.)
28(a)
Migmatite
44 46 54.3 N
73 22 13.5 W
The paler rock layer here pinches and swells unlike a molten rock wedging a crack open. How might the mismatch in the opposing crack walls be explained, assuming that the paler rock really was molten (is an igneous rock).
28(b)
Migmatite
44 46 54.3 N
73 22 13.5 W
Most of this boulder is composed of metamorphic rock that has been compressed. How do you know?
29
Migmatite
44 46 54.8 N
73 22 18.3 W
The pinkish layers here probably crystallized directly from a molten condition (are igneous). The darker rock contains aligned crystals indicating squashing of a solid (metamorphic rock). Could the minerals in the pink layers (quartz and feldspar) have once been scattered through the darker rock? (Hint: quartz and feldspar melt at a lower temperature than the dark rock.)
30
Fracture cleavage in bedrock
44 46 55.8 N
73 22 18.0 W
The “cracks” perpendicular to the hammer are not sedimentary bedding planes. How can careful field observation prove this?
31
Fracture cleavage
In bedrock
44 46 56.2 N
73 22 17.4 W
Note that the thin vertical seams, one of which is shown here, are filled with clay-rich rock. The adjacent rock is much richer in the more soluble mineral calcite. All the bedrock has responded to horizontal compression as indicated by its tilting and folding. Can you guess how these seams may have contributed to horizontal shortening during this compression, knowing that solution of some minerals can occur under stress?
32
Flexural folding in bedrock
44 46 56.2 N
73 22 22.5 W
How could such brittle rock as this have developed these gentle bends? (It is assumed the layers were all originally deposited as horizontal beds.)
33
“Sponge rock” boulder
44 46 56.4 N
73 22 16.8 W
The pits mark places that have been dissolved by water after this boulder was broken away from its bedrock. Why is it so deeply pitted compared to most of the other boulders around here? (Note…we aren’t sure either!)
34
Graywacke boulder
44 46 57.0 N
73 22 16.8 W
How did the aligned pits develop on this boulder?
35
“Sponge rock”
largest boulder
44 46 57.3 N
73 22 16.6 W
Where could this boulder have come from? We know of no bedrock like this within hundreds of miles!
36(a)
Gabbro
44 46 57.3 N
73 22 16.6 W
How can you tell that this boulder wasn’t torn from nearby bedrock?
36(b)
Gabbro
44 46 57.3 N
73 22 16.6 W
All crystals in this rock are large. Its overall composition is like that of basalt, the most common type of lava, but the crystals are larger than is found in lava. (Lavas are at least partly fine-grained due to rapid cooling.) Where, in or on Earth, would cooling be slow?
37
Limestone boulder
44 46 57.4 N
73 22 28.6 W
What are the strange-looking shapes in this rock?
38
Graywacke boulder
44 46 57.4 N
73 22 16.4 W
This is a sedimentary rock deposited in water as broken bits of older rock. The sizes of these particles ranges from tiny clay-size to coarse sand-size. In what sort of environment might such a poorly sorted sediment be deposited? (Choose one… On a beach? On a sand dune? In the deep ocean made murky by landsliding?)
39(a)
Granitic gneiss boulder
44 46 57.6 N
73 22 15.6 W
The alignment of grains indicates this rock has been squashed while remaining solid. What other deformation besides squashing has occurred?
39(b)
X-cutting dikes on boulder
44 46 57.6 N
73 22 15.6 W
Cracks in this rock have been intruded by molten rock (magma) which then solidified to produce two structures called “dikes”. Which dike solidified first; the dike with a pen along its edge, or the narrower dike trending left-right?
40(a)
Coarse gneiss boulder
44 46 57.9 N
73 22 15.9 W
This rock was once probably igneous. Why is it now best-considered metamorphic (changed by heat and/or pressure while remaining solid)?
40(b)
Coarse gneiss boulder
44 46 57.9 N
73 22 15.9 W
What was the direction of squashing of this rock?
41
Quartz-sandstone boulder
44 46 57.9 N
73 22 15.9 W
What might account for the numerous small pits in this rock?
42(a)
Syenite boulder
44 46 58.0 N
73 22 15.8 W
The nearest known bedrock like this boulder is over 100 miles away. Apart from this, what else suggests this boulder came from that far away?
42(b)
Syenite boulder
44 46 58.0 N
73 22 15.8 W
What indicates that this rock has never been squashed?
43
Dolostone boulder
44 46 58.1 N
73 22 15.5 W
How do you think the irregular crack-like marks on this rock formed?
44
Dolostone boulder
44 46 58.5 N
73 22 15.3 W
Look at all the peculiar markings on this boulder. How might they have formed?
45
Gravel ridge
Looking S.
44 47 0.4 N
73 22 13.5 W
Where did all the pebbles in this ridge come from? Almost all are the same rock type.
46
Gravel ridge. Looking N.
44 47 0.6 N
73 22 13.2 W
How can you tell that this ridge was not deposited by a glacier?