372 GLACIAL GEAVELS OF MAINE, 



clay begins. The only way to secure accuracy is by micrometric measure- 

 ments of the size of the grains. 



An interesting marine delta is in the valley of the west branch of the 

 Union River, in Aurora. It is locally known as the Silsby Plains, and is 

 elsewhere described. The valley of Union River was at one time occupied 

 by an arm of the sea from 1 to 8 miles broad. Into this inlet the great 

 Katahdin osar river for a time poured nearly at right angles. A delta of 

 gravel and sand formed in front of its mouth. This delta extended across 

 the whole breadth of the valley then under the sea, and for 4 miles south- 

 ward and nearly a mile north of the mouth of the glacial river. The last- 

 named fact indicates strong tidal currents on the coast of Maine at that time. 

 If the Bay of Fundy was at this time' a strait connecting the Gulf of Maine 

 with the Grulf of St. Lawrence, the tides would probably not be so high in 

 eastern Maine as now; yet here is evidence of considerable tidal action. 

 Tidal currents sweeping along the coast would tend to mix the clay portions 

 of the deltas of the glacial rivers. 



II. Another class are here termed "ice-bordered" or "narrow marine" 

 deltas. They are usually much longer from north to south than from east 

 to west, having but little of the fan shape. At their southei'n ends they pass 

 by degrees into clays having the same level, like the delta-plains above 

 described. They are found in valleys or level regions much broader than 

 they are, where there is no topographical reason why a delta, if deposited 

 in the open sea, should not have spread outward in fan shape. Except at 

 the southern extremity, the sand and gravel end abruptly. The east and 

 west flanks commonly form a steep bank or bluff rising sometimes as much 

 as 20 feet above the marine clay which here covers the base of the grave] 

 plain. The transition from the coarse matter of the plain, such as sand, 

 gravel, cobbles, etc., to the clay at the sides of the plain is very abrupt. 

 Very evidently the clay was deposited later than the coarse matter at all 

 points of the plain except on the south. Evidently the glacial rivers flowed 

 in channels which were open toward the ocean, but at the sides were bor- 

 dered by ice which covered the rest of the valleys and prevented the delta 

 from spreading out into fan shape. At one time I described these deltas as 

 being formed in bays within the ice, into which the tidal waters extended 

 as they do into an estuary. They are all situated below the contour of 230 

 feet, and if they were deposited at the time the sea stood at its highest level 



