MICHIGAN ACADEMY OF SCIENCE. 143 



comparatively rapid descents into Lakes Michigan and Huron above the 

 places named, while to the south of this diagonal line the outlets of river 

 valleys would be drowned. Thus we have along the south shore of Lake 

 Michigan what might be termed river-lakes at the outlets of the Grand, 

 Muskegon and Marquette ri^'ers, Avhere these river reaches have been drowned 

 by relative sinking and encroachment of the lake. Further northward 

 stream channels have a relatively rapid descent into the lake. 



The cause and effect of glacial-geologic action and resultant drainage, 

 has in many cases produced a striking form of drainage.^ In the Saginaw 

 Bay basin the Tittabawassee and Cass rivers are deflected by the Saginaw- 

 Port Huron moraine toward the south until meeting in the Saginaw river, 

 the concave area opening to the north.* This type of river flow has very 

 appropriately been styled willowy drainage, in this case the Saginaw river 

 forming the trunk of the tree, the Cass and Tittabawassee lateral branches. 

 On the west' coast the St. Joseph river forms a bow into northern Indiana. 

 There are, however, numerous streams which follow a more direct course, 

 the simplest type is where there is a smooth even sloping plain, the water 

 courses following nearly parallel and independent drainage lines, as in Bay 

 county.^ However, in the same county we have this willowy type of drain- 

 age characteristically developed. It is also characteristic of this type of 

 drainage that the main tributary streams come in from the south of the 

 trunk stream. In Bay county this is at least clue to the more abrupt descent 

 of the front of the moraine last deserted. 



TERRACES. 



These originate in various ways. 



1. Due to inequalities of hardness, the upper surface of the hard layer 

 marks the lower limit of the terrace. 



2. Due to flood plain deposition along the sides of a stream, and subsequent 

 down cutting of the channels by various operations as follows: 



a. The head advancing up stream may on reaching the head of the valley 

 plain lose so much of its load as to be able to sink its channel farther down, 

 forming cycles of erosion with alternate deposition, cutting and deposition 

 again. 



b. By the exchange of load dropping the course near the head of its valley 

 plain and taking up fine material, thus degrading its channel into the flood- 

 plain which the earlier and perhaps smaller stream had developed. 



c. As erosion and transportation vary accordingly to the grade of the 

 stream the flood-plain may be subsequently deepened due to stream develop- 

 ment which permits material to be removed which is temporarily left on the 

 flood plain. 



d. Any stream reaching the flood plain stage is apt to meander, the mean- 

 ders tend to migrate down stream and become relatively lower and more 

 capacious so as to hold the water of ordinary floods. At this stage or even 

 before, such parts of the earlier flood plain as remain are terraces. Other 

 causes are the uplifts in a region where the rivers are flats, the streams are 

 rejuvenated, and the remnants of their former flood plains become terraces. 

 Again if an alluvial flood plain has been built as the result of excessive sed- 

 iment, the exhaustion or withdrawal of the excessive supply would leave 

 the stream free to erode it where it had been depositing. An increase in 



1 A. C. Lane, Water-Supply Paper No. 30, U. S. G. S., p. 62. 



2 W. F. Cooper, Bay Co., Ann. Rep. Mich. Goel. Surv. for 1905, p. 373. 



