This Cache is part of the Helena, Montana GeoExpedition. GeoExpeditionMT spans the scenic Helena area from the heights of MacDonald Pass, through historic downtown gulch then south to Boulder and the Elkhorn Ghost Town. View GeoExpedition rules and DOWNLOAD a PASSPORT here.
MacDonald Pass is located on Hwy 12 between Garrison Junction and Helena, MT and sits on the Continental Divide in west central Montana. It is a historical pass in the connection between the Missouri and Columbia River systems and the peoples who traveled between the two.
A continental divide is a line of elevated terrain which forms a border between two watersheds such that water falling on one side of the line eventually travels to one ocean or body of water, and the water on the other side travels to another, generally on the opposite side of the continent. Because the exact border between bodies of water is not usually defined, the continental divide is not always definite for any continent. Moreover, some rivers empty into inland seas, and thus do not end up in the oceans.
North America can claim 4 or 5 continental divides, depending on how drainage basins are categorized: the Great Divide, also called the Continental Divide, separates the the watersheds of the Pacific Ocean from those of the Atlantic or Arctic Ocean. The Northern Divide, or Laurentian Divide, separates the watershed of the Atlantic from those of the Arctic Ocean. The St. Lawrence River Divide separates the Great Lakes Basin from the rest of the Atlantic Ocean watershed. The Eastern Continental Divide separates the watershed of the Gulf of Mexico from the Atlantic Ocean.
Highway 12, the route between Garrison and Helena, crosses sedimentary rocks in the overthrust belt, granite (Cretaceous granite and granodiorite on and near the pass) that intrudes them, and two generations of volcanic rocks that partly cover them. The magma that became granite and volcanic rocks brought with it the gold and silver that made Helena a roaring mine camp in the early days of Montana.
Big slabs of sedimentary rock west of MacDonald Pass must have moved east on the overthrust faults sometime before the magma that became the Boulder batholith granite first invaded them about 75 million years ago, but probably not long before. Cretaceous brown sandstones and black shales are conspicuous within about 10 miles of Garrison. Sedimentary rocks exposed along the road near Helena, east of the Boulder batholith, are Precambrian formations.
In order to log this EarthCache, you must answer the following questions. 1) What is the elevation of MacDonald pass based on the sign on the pass itself? 2) What 2 river systems are affected by the Continental Divide at MacDonald Pass and which river(s) flow into which bodies of water? 3) What year was the Continental Divide near here first crossed by airplane and who was the pilot?
Some answers are found on the signage on the pass. The other answers are found in this information and by doing a little research on maps or by internet.
For those of you who are doing the Helena GeoExpeditionMT, you'll need to look on the back of the informational sign to get the needed password to get credit for your stop here.