12 GROVE KARL GILBERT-DAVIS [Memoies [ vol TI xxi i ; 



and gives a monotonous appearance to the smooth landscape. He prepared separate reports 

 on several counties, following a standard plan for the State as a whole; but in addition he dis- 

 covered and solved a delicate problem regarding the surface features of his apparently unpromis- 

 ing district. The results thus gained were so novel and so significant that Newberry generously 

 permitted their publication in the American Journal of Science in 1871, two years in advance 

 of their appearance as a chapter of Volume I of the survey reports in 1873. The area concerned 

 is a plain of faint relief, which presents a general and very gentle slope northeastward to the 

 southwestern end of Lake Erie; its materials were described as lacustrine clays, from 50 to 100 

 feet in thickness, and explained as the deposits from the expanded predecessor of Lake Erie 

 which overflowed southwestward to the Ohio-Mississippi system. The nearly level plain is 

 interrupted by two low and concentric swells or "ridges," both of curved outline, convex to 

 the southwest; the outer and larger one being from 25 to 50 feet high, from 4 to 8 miles wide, 

 and some 200 miles long around its curve, the chord of which measures about 120 miles. 



At the close of a second season's field work, after Gilbert had detected the divergent ar- 

 rangement of the glacial striae on occasional exposures of bedrock, the occurrence of a south- 

 westward outflow channel for the expanded body of clay-depositing water which proved it to 

 be a lake and not an arm of the sea, and several beaches that mark shore fines temporarily 

 occupied as the expanded lake fell to lower levels, he came upon a fruitful explanation for the 

 curved ridges and the arrangement of the neighboring streams, concerning which he made a 

 concise entry in his diary on November 10, 1870: "Invented the moraine hypothesis for St. 

 Jo and St. Marys rivers"; this brief statement being one of very few of its kind in his long 

 series of annual records; and it is this "moraine hypothesis" that forms the main subject of 

 his special report. It is interesting to note that, as if already unconsciously developing the 

 well-balanced and candid style of presentation which characterized so many of his later writings, 

 Gilbert opens the chief passage concerning his hypothesis, not with a confident assertion of 

 his conclusion as if it were a fact, but with a frank announcement of it as an opinion: "I con- 

 ceive," he wrote regarding the larger one of the two swells of the surface, "that this ridge is 

 the superficial representation of a terminal glacial moraine, that rests directly upon bed rock, 

 and is covered by a heavy sheet of Erie clay, a subsequent aqueous and iceberg deposit"; yet 

 while he inferred the moraine to be thus buried, he thought that the clays "so far conform to 

 its contour, as to leave it still visible on the face of the country — doubtless in comparatively 

 faint relief, but still so bold as to exert a marked influence on the hydrography of the valleys." 



The context shows he had seen that all the little brooks run down the faint slope of the 

 plain on courses which converge northeastward toward the lake; but that on reaching the 

 exterior side of the curved morainic swells, the brooks are gathered into streams that flow along 

 the base of the swell to the axis of the curves, where, uniting in the Maumee, they resume their lake- 

 ward flow through open gaps in the swells. A rational treatment was thus accorded to the 

 disposition of drainage lines, and that at a time when the courses of streams were usually 

 treated simply as matters of course, for which the current methods of orthodox geology sug- 

 gested no explanation. More briefly expressed in the terminology of to-day, the Maumee 

 drainage would be called consequent upon the inclination of the plain and the slopes of the 

 morainic swells. Yet although the arrangement of the streams was discovered to be generi- 

 cally explainable, neither Gilbert nor his later associate, Marvine, who made the same dis- 

 covery for various streams on the eastward slope of the Front Range of the Rocky Mountains 

 a few years afterwards, thought of giving the streams a generic name indicative of their origin; 

 that happy idea sprang from the inventive, systematizing mind of Powell when he explored 

 the Colorado River of the West. 



Gilbert's statement closes with the first announcement of a conclusion of far-reaching 

 importance concerning the lobate margin of the great continental ice sheet, the pattern of a 

 small part of which he had detected: 



We are here furnished partial outlines of the great ice-field, at two" stages of its recession. Though but 

 small fractions of the entire outlines, they yet suffice to indicate that the margin was lobed or digitate in con- 

 formity with the topography of the country that it traversed. 



No finer instance of a mental leap from a particular instance to a broad generalization 

 can be found. It is true that priority in the recognition of drift ridges as terminal moraines 



