June 12, 2011

Geology Word of the Week: B is for Brunton

Posted by Evelyn Mervine

Brunton “Pocket Transit” Compass. Image taken from here.

def. Brunton:
A fancy, highly-precise compass used by geologists (and surveyors, engineers, archaeologists, etc.) for navigation and also to measure the strike and dip of rock layers in the field.

Since I am waiting to board a flight to Wyoming, I thought it would be appropriate for this week’s geology word to be “Brunton.” Brunton compasses are made by the Brunton Company of Riverton, Wyoming. The first Brunton compass was made by David W. Brunton in 1894. Since then, the Brunton compass (often shortened to just “Brunton”) has become standard field equipment for the geologist. Today, the Brunton company makes a variety of compasses and navigational devices. Their “Pocket Transit” compass is the most common compass used by geologists. Personally, I own a Conventional Brunton Pocket Transit as well as a stand-alone Brunton inclinometer called the Clinomaster. Brunton compasses aren’t cheap (the Pocket Transit is about $500), but a Brunton is an important investment for the field geologist.

Geologists use Bruntons for general navigation (like a regular compass, the Brunton has a needle that points to magnetic north) as well as to measure the strike and dip of rock layers. Using a Brunton can be a little intimidating at first, but geology students generally learn all about the Brunton at field camp. After a few days of practice, measuring strikes and dips of rock layers using a Brunton becomes second-nature. Geologists use these strike and dip measurements to make geologic maps. Strike and dip measurements are also useful for understanding the geologic structure and history (e.g. uplift and deformation) of a region. Bruntons can also be used (along with camera lens covers, pencils, field notebooks, hammers, etc.) to provide a sense of scale in pictures of rocks.

Here is a website that teaches you how to use a Brunton to measure strike and dip:
MIT Website on Using a Brunton Compass

Figure showing strike and dip of rock layers. Figure taken from here.

Below are a few pictures of Bruntons in the field. Feel free to add more pictures of you with your own Brunton. Either put a link in the comments or email me the pictures, and I’ll add them to this post.

Bruntons can be mystifying at first to geology field camp students. The Stretch
(Dartmouth College’s geology field camp), Western USA, Fall 2005.
Using a Brunton inclinometer to measure a far-off mountainslope angle. A
regular Brunton rests on the rock to show me the direction I am looking,
Oman, January 2009.
Collecting a rock sample with Brunton safely stored in its leather carrying
pouch (at my waist), Oman, January 2009.