Professional Paper No 50 Series B, Descriptive Geology, 79 DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY CHARLES D \\ALCOir, DlRECTOP THE MONTANA LOBE OF THE KEEWATIN ICE SHEET BY FRED. H. H. OALHOUISr OF Mm IARY APRS 1975 LIBRARY WASHINGTON ' GOVERN MBNT FEINTING OFFICE 1906 CONTENTS. Page Introduction................. ..----... - ..-..-- . - - - -. - --.... ... ............... 7 Genei al features of the area . - . __.-__. .-. ... ..... ....-- -. ........... ............... 8 Location..-......--... - .. .. ... .. ......... ..... ...... ................. 8 Problems presented. ... .......... ... ... ............ -.....-..-..--..-. 8 Physiography.. ....... .. ... - ........ ... ............ -. ................. 8 Structure............ .. ....... ............ .......... ... ....... ... .... .... 9 General geology..... .. ... .. ........ .. --........-. ........-..--...-..-.---. 10 Sedimentary rocks . - - - . ........................... .... ............---.- 10 Igneous rocks....... - .. -...-.--..-.----...-.....--.-. ... ....-.-.-..-.---...- 11 Surface formations.......... .. .. ... ............ ......... .. -..--..-. ........... 13 Quartzite gravels......... ........ .......... ... ....... .. .....'.. ........... 13 Character-. .... -..--...... .......... ................ ........................ 13 Distribution........................... ......... ... - .... ... ...-...-..-..--- 13 Drift of the mountain ice...... .....-.........--...-..-..--.---.--.- ............. ..... 14 Distribution of the mountain glaciers........................ ........................ 14 Detailed descriptions of the mountain glaciers.. ............ ..... ............... .... 15 Sun River Glacier--.-....-.-----..- .-..-... ... .-------...... .--...-..-- 15 Deep Creek Glacier........................... ........... ................. 16 Teton River Glacier............. ......................... ................... 17 Muddy Creek glaciers........ ............ ........ ...................... 17 Dupuyer Creek Glacier... ............... ............. .... ................... 18 Sheep Canyon Glacier...... ....... .. ... ... ......................... ..... 18 Blackfoot Glacier..... .................................................. , 5 ..... 18 Cutbank Creek Glacier.................. ..... .... .... ....................... 20 Milk River Glacier.......................... ........ ........................ 20 St Mary River Glacier ......--.----.---..------.-...---.......-................ 21 Drift of Keewatm ice sheet.... ............................................... ......... 22 Terminal moraine-........ ......---......--.--....... ..........................1 22 Later moraines.................................... t ................................ 26 Character of the drift... .......... ....----...-...-....--.... ..................... 27 Ice surface......... ........................................................... 28 Height........ ... ....................................................... 28 Slope......... ... ... ..................... ................................. 28 Drainage.... ............ ................................................................ 28 Lakes................................................................................. 28 Character of the lakes.....---.-----..... .................... ................. 28 Location of the lakes..................................... ...................... 29 ' Lake in St Mary Valley....................... ................................. 29 Lake in Two Medicine River Valley............. ................................. 29 Lake in Teton River basin....................................................... 29 Lake in Missouri River Valley.................................................... 30 4 CONTENTS. Drainage Continued - , Page Rivers................................................................................ 32 Effect of mountain glaciers.......................................................... 32 Effect of northeastern ice sheet,...................................................... 32 St Mary Rivei................................................................. 32 Milk River..................................................................... 32 Two Medicine River......................'...................................... 32 Birch Creek.................................................................... 33 Muddy Creek.................................................................. 33 Teton River................................................................... 33 Missouri River................................................................. 34 Interrelations of surface loimations........................................................... 45 The quartzite gravels and the two drift sheets.............................................. 45 The northeastein and the mountain drift sheets............................................ 46 Deductions ft om interrelations................................................. ......... 49 Age of the quartzite gi avels.......................................................... 49 Age of the northeastern duft........................................... ............. 52 Evidence from erosion............................/............................ 52 Evidence from topography of terminal moi aine..................................... 53 Evidence from relations of the two drift sheets..................................... 53 The drift outside the terminal moraine.................................................... 54 Summary...........-...------...-..-.......-..........-.-...--...--.--.--..-.---...-...... 56 Index................-.....--........--......-..-...---.-..-.--.-...---.....--.......-.... 59 ILLUSTRATIONS. < " Page. PLATE I Map of nortnein Montana, showing the glaciated area.................................. 7 II A, Perched concietion weathered from Cretaceous sandstone, B, The Shonkin Sag.......... 12 III A, Summit Pass and adjacent valleys, B, Sun River Canyon...-......--.-.-----........ 18 IV. A, Eagle Butte nearly encircled by moraine of northeastern ice sheet; B, Till section 100 feet high on Muddy Creek......._.._..........-....--.-....--..-...---.-...-------.-- 26 V. Map showing area covered by the Great Falls Lake and the position of the terminal and later moraines......--.-......-.......-.......-.......-.......---.....-----.-.------- 30 VI A, Falls of the Missouri, B, One of the falls of the Missouri, showing amount of post-Glacial erosion-..........-.........-...............-.......---..--.--..-.-.--...-.--... 34 VII. A, Water gap formed by Missouri River; B, Wind erosion near Dry Fork of Marias River.... 36 FIG 1 Section in the Canadian Rockies, showing thrust-fault relations of Cretaceous to Algonkian and Cambnan................................................................... 9 2. Section along the 49th paiallel, showing thrust-fault relations of Cretaceous to Algonkian and Cambrian................................................................... 10 3. Cross section showing relations between the two lake deposits in the Missouri Valley west of Great Falls..................................................................... 30 4 Longitudinal section showing relations of the lake deposits to the drift dam and the ice dam.. 30 5. Section showing relation of the lacustrine clay to the giavels and till of the noitheastein ice sheet........................................................................... 31 6. Section on Belt Creek, showing the moraine covered by lake deposit..................... 31 7. Relative size of post-Glacial and pre-Glacial valleys of Two Medicine River............... 32 8 Cross sections showing relative sizes of Two Medicine, Cutbank Creek, and Marias valleys.. 33 9. Valley of the Missouri, showing post-Glacial cutting in sandstone and drift................ 34 ILLUSTBATTONS. 5 Page FIG. 10 Section showing the pre-Glacial valley of Missouri River............................... 35 11 Cross section showing the drift in the valley through which Highwood Creek now flows..... 37 12 Map showing the pre-Glacial course of the Missouri between Sand Coulee and the mouth of Highwood Creek................................................................. 37 13 Relations of the post-Glacial and the pre-Glacial channels of the Missouri between mouth of Highwood Creek and mouth of Little Sandy Creek................................... 38 14 Map showing outlines of the great pre-Glacial valley between Missouri and Milk nvers..... 39 15 Map of the vicinity of Havie, showing the relation of the pre-Glacial Missouri to the valleys of the post-Glaual streams...._.--.....-........_..._....__..._.__................... 39 16 Map showing pre-Glacial river system of northern Montana............................. 40 17. Map showing drainage of northern Montana, when ice had maximum extent and also pre- Glacial river system......----.-.-------------.----.-......-.-..-.-.-..-.-..-..... 41 18 Map showing drainage of northern Montana at stage of younger moraine................. 41 19 Map showing post-GIacial drainage system of northern Montana, with a part of the pre- Glacial system...-__..-....-._.--._-----..--..-----.-...._......-............... 42 20 Cross section of the Shonkin Sag at the time the ice was present-......---..-.-..-.-.-... 42 21 Section across the Shonkin Sag, showing how it was formed on the slope of Highwood Moun­ tains. __._.__._-_.._-_-..---.--.-.----..----.--..-.---.-..--..--..----.--...-._. 43 22. Sketch