Long Point North Rocks – follow Walker trail to shorelineThe Geology Parcourse for the Mind is sponsored by the Friends of Point au Roche State Park & through a grant from the Lake Champlain Basin Program. Professor Emeritus of SUNY Plattsburgh, Lawrence Gillett, prepared all the questions and answers along with the photographs.
When hiking along the shorelines of the Park one should know when water levels on the lake are high many rocks may not be accessible. Also the terrain is uneven and can be slippery where wet. Boulders may also have moved from a previous location as a result of winter ice flows.
Long Point North Rocks – follow Walker trail to shoreline
# and PHOTO |
SCIENTIFIC TERMINOLOGY |
LOCATION |
LAYMAN’S QUESTION |
1. <TOP>
|
Migmatite boulder |
44 46 31.6 N |
This kind of rock is generally not considered desirable as a building stone. On the contrary, I wish more building stones were like this. Can you guess why we disagree? |
2. <TOP>
|
Amphibolite boulder |
44 46 30.8 N |
Wouldn’t this look great as a lawn ornament? Where do you think this rock developed its exotic appearance? 1. by compaction of sediment near the Earth’s surface (a type of sedimentary rock). |
3. <TOP>
|
Monzonite gneiss boulder |
44 46 30.8 N |
Are the individual mineral grains (crystals) large enough to be separately distinguished here? |
4. <TOP>
|
Artifact |
44 46 30.2 N |
Do you think these bricks fell into a sediment or molten rock which was later converted to solid rock? |
5(a). <TOP>
|
Xenoliths |
44 46 30.0 N |
How did the light colored fragments wind up in this dark rock? |
5(b). <TOP>
|
Feldspathoidal (?) xenolith in boulder |
44 46 30.0 N |
The pale gray crystals here are weathering away faster than the whitish mineral (feldspar) surrounding them. Might this aid in the identification of the former? |
6. <TOP>
|
Meta-anorthosite boulder |
44 46 30.0 N |
Both the bluish and whitish minerals here are both plagioclase (90% or more of the rock). (The brown is just surface stain.) Can you guess which type of plagioclase formed first? |
7. <TOP>
|
Flaser gneiss |
44 46 29.8 N |
This rock (not the boulder) has been deformed. If we could restore it to its former shape, would it be longer parallel to the pen? |
8(a). <TOP>
|
Dolostone boulder |
44 46 29.8 N |
A barely visible feature here indicates that this boulder was once deposited as tiny sediment particles. Can you see this feature? |
8(b). <TOP>
|
Friction cracks on dolostone boulder |
44 46 29.8 N |
Assume that each curved crack here was made by a rock hitting this surface as the rock was being dragged along by a glacier. In what direction was the glacier moving? |
9. <TOP>
|
Charnokitic gneiss boulder |
44 46 29.7 N |
This type of rock is common in the Adirondacks. How might this boulder have arrived here? |
10. <TOP>
|
Fossiliferous sandstone boulder |
44 46 29.6 N |
Do you think the markings here are the remains of ancient organisms? Alternatively, could they be trails left by the organisms as they moved around? |
11. <TOP>
|
Migmatite boulder |
44 46 27.9 N |
Note the discontinuity in the rock marked by the ballpoint pen. What do you think happened along this zone? Give as much detail as possible. |
12. <TOP>
|
Zoned pegmatite dike in granitic boulder |
44 46 27.8 N |
The quarter rests on a rock type called pegmatite that is about ½ feet thick here and that cuts through a much finer grained rock of similar mineral composition. These minerals, most easily seen in the pegmatite because of its very coarse grain size, are quartz (gray) and feldspar (white or pinkish). Geologists puzzle over why crystals grow to such large sizes in pegmatites and why the quartz is largely segregated from the feldspar in some pegmatites. Any suggestions? (Hint: pegmatite generally contains at least some minerals with water in their chemical formulae) |
13. <TOP>
|
Fold(?) in gneiss boulder |
44 46 26.2 N |
The hammer pick seems to rest along the core of a tight fold. Can you think of any other way this pattern might have formed without folding all these layers? |
14(a). <TOP>
|
Garnet gneiss boulder |
44 46 25.9 N |
Garnet, the plums in this pudding, is practically restricted to metamorphic rocks. (metamorphic: formed by recrystallization of other classes of rock) Why do you suppose this mineral is rare in sedimentary or igneous rocks (sedimentary: originated as sediment at the Earth’s surface; igneous: formed by freezing of rock melt) |
14(b). <TOP>
|
Garnet gneiss boulder |
44 46 25.9 N |
The garnet crystals here (reddish brown lumps) grew in the surrounding rock at about the same rate in all directions. How do we know this? Could this tell us something about the arrangement of atoms within the crystals? |
