1. Deep Coulee deposit Horizontal, sheet-like units; no sharp contacts
11 meters (35 ft) thick

2. Deep Coulee Rhythmically bedded silt and fine sand
with sheets of chaotically-oriented gravel

3. Deep Coulee
No buried soils
No significant scour and fill or ripples

4. Model for deposition of Deep Coulee sediments
Laurentide
ice
Hydrologic and physical controls cause semi-regular surges of water into and out of the lake
During lowstands, sheet-floods carried the local supply of gravel onto the shallow lake
Alternating beds of silt and sand were deposited into a relatively shallow lake
Bone collagen date!

5. Cat Creek deposit: a delta remnant
>24 meters (80 ft) thick

6. Cat Creek Well-sorted, imbricate gravels Finely laminated sands
Giant cross-beds throughout
Dip is 20-30o parallel to the long axis
No glacial erratics in or on the deposit
Local intrusives, cherts and limestones

7. Cat Creek deposit
Post-glacial topographic inversion left the valley fill deposit standing out in relief on the landscape
Formed as a valley fill delta triggered by downstream damming of Lake Mussel-shell

8. Conclusions and future work
Both Deep Coulee and Cat Creek are consistent with a dynamic, fluctuating lake
Suggests a spillway under or up against the ice
There is evidence for at least three stages of Lake Musselshell
Deep Coulee: ~2,600 feet
Cat Creek: ~2,850 feet
Highest erratics: 3,050 feet
Cosmogenic and bone collagen dates will help constrain the ages of the lakes

9. Erosion rates??