500 PLEISTOCENE OF INDIANA AND MICHIGAN. 



But it might easily have happened that one among the several distributaries then existing 

 should have afforded a lower passage than that over the barrier above Amherstburg. 



Spencer notes the depth of 90 feet in the north channel of the St. Clan- delta, a little south 

 of Algonac, and takes this as an indication of a northward-flowing stream, and in order to have 

 a northward gradient from this point, supposes the contemporary beach north of Point Edward 

 to be at least 110 feet below present lake level. But, as has been pointed out above, this assump- 

 tion and the interpretation on which it depends are not justified by sound theory. Some of 

 the deep holes in Mississippi River above New Orleans are 150 feet deep, but to account for this 

 it is not necessary to suppose that the Gulf of Mexico once stood 180 or 200 feet lower than 

 now. Deep holes of this kind in great rivers flowing in alluvial or other soft sediments are 

 common enough and their cause is well known. All of them are in bends where the current 

 is swift and is strongly deflected against the concave bank. The deepest holes in Rouge and 

 Clinton rivers as well as the one south of Algonac stand in this relation. They are not relics 

 of a past condition but are made by the rivers as they are to-day. It is not these exceptional 

 deep holes that constitute evidence of deeper erosion in the past, but the average depth of 

 the lower courses of the tributary streams. 



As shown on the charts, the present distributary channels in the St. Clair delta have depths 

 ranging commonly between 25 and 50 feet, and at their extremities the North and South chan- 

 nels have least depths of 8 and 10 feet, respectively, not counting the artificial canal. It seems 

 scarcely possible to explain the overdeepening of the tributaries of Detroit River and Lake St. 

 Clair except by northward drainage from the barrier at Amherstburg through some one of the 

 distributaries in the St. Clair delta. 



Sydenham River stood in a different relation to the delta, for it enters the Ecarte dis- 

 tributary 6 miles above its mouth and it is only at their mouths that the distributaries grow 

 shallow. It would seem certain therefore that the Sydenham at that time went northward. 

 But this and the other tributaries of St. Clair River show no more effects of drowning than do 

 the streams farther south; indeed, they show less, for a reason given below. 



PRESENT DROWNING OR FLOODING. 



When the transitional or two-outlet stage of the Nipissing Great Lakes ended and the pass 

 at North Bay was uplifted above that at Port Huron, the great outlet river returned once more 

 to a southward course and the abandoned beds of St. Clair and Detroit rivers and Lake St. 

 Clair were refilled and the enlarged mouths and lower courses of all the tributaries were drowned. 

 That was the beginning of the present condition; there have been no changes of outlet nor any 

 notable change in volume since that time. The level of the lake waters was then about 15 feet 

 above the present level at Port Huron, 9 or 10 feet above Lake St. Clair, and 6 or 7 feet above the 

 present river between Detroit and Grosse Isle. Hence, the drowning effect, as it appeared at the 

 first return of the large river, has been considerably reduced by the lowering of the river surface. 

 This is especially true of the tributaries of St. Clair River, where this lowering has amounted 

 to 12 or 15 feet, but the amount is progressively less in Lake St. Clair and Detroit River. This 

 accounts mainly for the somewhat greater amount of drowning seen now in the Rouge and the 

 Thames as compared with Black River at Port Huron. The lowering of the river surfaces has 

 been due to the recent erosion of the barriers and of the river beds — to the gradual sinking of 

 the great connecting rivers more deeply into the soft sediments of the plain; and, as already said 

 (p. 499) , the amount of this deepening (to which drowning now gives the effect of overdeepening) 

 might be taken as a measure of the time during which the southward discharge was absent. 



Other factors, however, such as lowering of the river level and silting up or aggradation of 

 the overdeepened parts, operate to reduce the apparent deepening and complicate the calcula- 

 tion. Each tributary has been bringing more or less sediment to its drowned lower course 

 ever since the great volume returned. The finer parts are partly deposited in the estuary 

 and are partly carried into the larger river. Sand and gravel are deposited in the heads of 

 the estuaries, where the first deep water is met, and are gradually accumulating as inset 

 delta deposits. It is interesting to note that the amount of this refilling with sand and gravel 



