Tumalo Fault Geo-Exploration (26 Sep 2026)

Sat, 26 Sep 2026 15:30:00 GMT

Route and stops of the Tumalo Fault Geo-Exploration on the Geologic Map of the Bend 30- x 60-Minute Quadrangle, Central Oregon (I-2683) by D.R. Sherrod and others, 2004. Faults in the Sisters and Metolius Fault Zones are highlighted in red.

Saturday, September 26th

8:30 AM - 4:00 PM

Registration opens Thursday September 17th and is limited to 20 participants.

The Sisters, on the east, and Metolius, on the west, Fault Zones combine to form a trend of 50+ northwest-striking normal faults stretching some 38 miles long and 4-10 miles wide. The 28 mile-long Tumalo Fault is the longest fault in these zones. Recent graduate studies by John Wellik (2008), MacKenzie Mark-Moser (2018) and Daniel Mongovin (2022); earlier work by Sherrod & others (2004); and information in the USGS Quaternary Fault and Fold Database, provide the foundation for today’s Geo-Exploration. Please join Bob TImmer for a traverse along the Tumalo Fault.

The Geo-Exploration will start in the parking lot of the Pilot Butte State Park at the base of Pilot Butte. After a introduction, we will proceed west on Newport avenue, crossing a fault strand between 5th and 4th street, to 3rd avenue. We will turn right on 3rd and proceed to the north side of Bend and merge onto highway 20 towards Tumalo. We will stop at the Three Sisters Viewpoint (Stop 2) to group-up. From there, travel north to a wonderful exposure of a fault along Highway 126 (Stop 3). We will backtrack and head west of Highway 20 to start our traverse along the Tumalo Fault at Plainview (Stop 4). We will proceed to the south along the trace of the Tumalo Fault to Bull Flat, the site of the failed Tumalo Dam. We will explore the geology and the history of irrigation at 4 stops in the Bull Flat area. We will then leave the Bull Flat area and re-connect with the Tumalo Fault as we approach Shevlin Park (Stop 9). We will then follow the trace of the Tumalo Fault to our the last stop in Northwest Crossing (Stop 10). After a neighborhood walk along the fault, we return to Pilot Butte.

Today's traverse will be on the Geologic Map of the Bend 30- x 60-Minute Quadrangle, Central Oregon (I-2683) by D.R. Sherrod and others, 2004. While you can download this map, and very informative pamphlet, for free from the USGS website, your downloaded version will not be georeferenced. For those that want to follow today's traverse using the Avenza app, you can download a georeferenced copy of the map HERE.

Total walking mileage for the day will be about 3 miles with modest elevation changes. Some walking will be on sidewalks, some on gravel roads and some will be cross country. There is very little shade, but we will never be more than a half-mile from the vehicles. Please bring a sack lunch, water and shade protection. There are no restroom facilities between the first and last stops. Total driving distance is about 65 miles of which 10 miles is on well graded gravel roads and the rest on paved roads.

The designated ride share meet up location is the parking lot on the east, and lower side of Pilot Butte State Park in Bend.

Dogs are not permitted on COGS Geo-Explorations and Field Trips.

Please note that this event is limited to 20 participants and registration must be done via the registration button on the trip listing on the COGS website. Only COGS members and a guest may register. Registration will be confirmed on the webpage by a light green box indicating registration was successful and by a confirmation email. If you don't get this email you should try the registration process again. Please contact Bob TImmer ( bobtimmer@yahoo.com ) with any questions.

Harney Basin Field Trip (9 Oct 2026)

Sat, 10 Oct 2026 00:00:00 GMT

Harney Basin Field Trip

Led by Dave Sherrod

Friday, Oct. 9, 5:00 PM - Sunday, Oct. 11, 4 pm

Registration Opens Friday, August 28 at 8 AM

Please join Dave Sherrod on a two day exploration of the Harney Basin, south of Burns. We’ll assemble Friday evening (10/9, Day 0) at Malheur Field Station , with field Day 1 on Saturday and Day 2 on Sunday (10/10 & 10/11). Day 1 will explore the basin’s stratigraphy, structure, geomorphology, and lake history from middle Miocene time to present (road stops). Day 2 is devoted to Diamond Craters, a Holocene lava field with a fascinating complex history unlike any other in the western mainland U.S. The Diamond Craters day includes driving stops and a lunch stop at a viewpoint overlooking the craters. Participants have the option of ending the trip here but there is a recommended, but optional, walking tour of the central crater complex, in the heartland of Diamond Craters. This walk is cross country totaling about a mile with modest elevation loss/gain of 200'. The footing is generally good but footwear with good traction is recommended as well as hiking poles if you find them helpful. Please see the flyer prepared by Dave Sherrod for more details:

Harney Basin FT primer_Sherrod_20260727.pdf

The trip is too far from Bend to allow a Bend-based start each day. Malheur Field Station (MFS) , 30 miles south of Burns, will be our accommodations which feature rooms with two twin beds/room at $50/night/person and a full dinner ($25) Friday and Saturday nights. There is also an option for up to three self contained RV sites at $35/night. Participants could also stay or camp offsite. All of these prices will also have 3% added them to cover the credit card processing fee. Priority will be given to filling up the bed space as COGS has a financial commitment to covering that cost. If you would like to share a room with a particular person either register as participant/guest or contact me. If you would prefer to not share a room there is a single occupancy option (essentially you buy both beds and register yourself and a guest named 'Single Occupancy'. Please note that you must provide your own towels, linens or sleeping bag.

Participation in the group dinners is highly encouraged as it is integral to the field trip and introductions and some introductory info will be provided at that time. MFS can provide vegetarian/vegan and gluten free options, and accommodate dietary restrictions but please contact me on this so I can advise MFS. MFS does not provide alcoholic beverages but participants are free to bring and cosume their own. Breakfast, lunches and snacks are the responsibility of the participant. There are kitchen facilities with basic appliances and cooking equipment.

If you would like to use one of the RV camping sites please be aware that the RV must be self contained and no use of the bathroom or shower facilities is permitted. If you want one of these sites or plan to stay offsite please contact me prior to registering. There is a total of 20 bed spaces and total participation will be capped at about 25 participants.

In case of cancellation, refunds will be based on if COGS is able to fill your space or otherwise recover costs.

Contact Derek Loeb ( DerekLoeb@gmail.com ) with questions or to provide me with pertinent info. More information on the Malheur Field Station is provided on this info sheet:

Malheur Field Station Visitor Guide.pdf

Participants must be a member of COGS or a guest of a member. As always please leave your pets at home as they are not allowed on COGS field trips.

Ride Sharing: Details to be determined.

Registration Opens Friday, August 28 at 8 AM

Registration must be done via the registration button on the trip listing on the COGS website. Successful registration will be confirmed on the webpage by a light green box indicating registration was successful and by a confirmation email. If you don't get this email you should try the registration process again. If the trip is full, please join the wait list as this will be your best chance of securing a spot on the trip.

Central Oregon Geoscience Society upcoming events

Tumalo Fault Geo-Exploration (26 Sep 2026)

Saturday, September 26th

8:30 AM - 4:00 PM

Harney Basin Field Trip (9 Oct 2026)