DAVENPORT PAST AND PRESENT
GEOLOGICAL SKETCH OF THE CITY OF DAVENPORT.
The external features, and internal resources of any given district of country, are intimately connected with its future history. From its external features we gather in the main a knowledge of those habitable qualities which render it more or less desirable for civilized abodes. In its geographical position we learn the commercial advantages which attach to its location, as being accessibel or more remote, from business centres.
From a knowledge of its internal resources, we obtain the clearest insight to its productive capacities, determining in great measure the extent and character of its future population.
Hence it is that an accurate geological view of any district, affording information, both as regards external features and interanl resources, is important and useful as a key to its future history.
The Geological substratum upon which the city of Davenport is located, is a white or light gray limestone, characterized by its fossils to belong to the Hamilton group of Devonian Rocks. This limestone crops cut along the river banks, of the upper portion of the city. It presents, near East Davenport, perpendicular cliffs, varying in height from 15 to 25 feet above low water mark; thence occupying the bed of the Mississippi river, it forms the lowest chain in the course of the Rock Island rapids, re-appearing again, similar in character, on Rock Island proper, and the corresponding left bank of the Mississippi. The shores of both banks of the river are here strewn with water-worn pebbles of this white limestone, variously mixed with smaller fragments of transported igneous rocks, including agates, cornelians, and numerous froms of porphyry.
This bed of limestone underlies the whole city of Davenport, appearing on or near the surface at its south-eastern border, extending from East Davenport to Perry street. Thence to the western limits of the city it is more deeply covered under alluvial deposites. This rock, together with its alluvial covering, forms a gentle ascending slope from the river bank to the irregular line of bluff hills, which here bound the valley of the Mississippi. Where this rock is largely developed in steep mural faces, as adjoining and just below East Davenport, the bluffs approach near the river bank, leaving little or no space for bottom levels. This gives a somewhat rugged character to this locality. In following the western course of the river the limestone dips lower beneath the surface, and the bluffs recede, thus giving greater width to the valley portion of the city towards its western border.
The bluff formation, attaining an average elevation of 150 feet above the river level, presents on its outer edge abrupt slopes and rounded crests, commanding extensive views of the course of the river above and below. Extending back from the river, this formation is cut up with deeply trenched valleys, variously branched and thence emerging on the upland prairie beyond.
These several features collectively, combine a pleasing variety of external scenery, and offer grading facilities easy of application, and well suited for the purposes of drainage.
DEVONIAN LIMESTONE FORMATION.
Referring more particularly to the special characters of the formations above alluded to, we notice the underlying limestone strata to be composed of a series of distinct beds, varying considerably in structure and composition.
First of these in a descdending order is an irregular shaly bed, containing the geater part of the fossils which serve to characterize this fomation. These strata are more largely developed to the south and west, being the common surface rock on both sides of the Mississippi, come eight or ten miles below the city, at and adjoinging the town of New Buffalo. In this latter locality the rocks are replete with fossils easily procured, and in fine state of preservation. Within the limits of the city this bed is exposed at only one locality, formerly known as LeClaire quarry, now foot of Farnam street. The rock here crops out just at the foot of the bluff, at an elevation of about forty feet above the river level.
To this fossiliferous bed succeeds the more common surface exposure, consisting of a white or light colored rock of slatey texture, weathering on exposure into thin irregular fragments. This character of rock shows a variable thinkness of from five to twenty feet, and is well exhibited at the lower point of Rock Island, forming the greater part of the exposed rocky cliff on which old Fort Armstrong was built.
To this slatey rock succeeds a more compact bed, mostly massive and heavy bedded. Its texture varies from that of a close irregular breccia of light color, and exceedingly brittle to loose strata of blue argilaceous rock, readily disintegrating on exposure to the atmosphere. Intermediate to these we generally notice several seams of a more earthy gray rock occurring in even beds, and frequently containing masses of fibrous gypsum. These latter seams furnish the best quality of building rock in this vicinity, being in fact the only rock suitable for dressing under the hammer. This seam is of very variabel thickness, being in some places entirely wanting, while in adjoing localities it attains a thickness of several feet. The main bulk of the limestone quarries, being made up of the heavy bedded and irregular seamed rock, is only suitable for foundations or rough ashlar work.
Aside from building purposes this limestone contains no minerals of any economical value, occasional spangles of sulphurate of zinc or moderate sized crystals of calcareous spar being the only minerals worthy of note. The slatey surface layers are employed for conversion into quick-lime, but the product is of rather indifferent quality.
One peculiarity of this limestone formation deserves more than a passing notice, both from its singularity and also its connexion with the subterranean distribution of water. This peculiarity consists in the frequent occurence of fissures filled in with clay, evidently infiltrated from above. These fissures or clay seams may be frequently noticed in the perpendicular face of quarries, here they are seen interrupting the regular series of rock strata with masses of grayish, very adhesive clay. These seams vary in width from a few inches to several feet, and are frequently bottle-shaped, narrowing above and bulging out below. Prof. Hall, State Geologist, is inclined to the opinion that this clay is cotemporaneous with the underlying fire-clay of adjoining coal measures, and that these fissures were filled up at the same period that coal was in process of formation. These clay seams are frequently met with in digging wells or deep cellar foundations, in which situations they are often accompanied with living springs of water. From such sources are evidently derived the supplies of water from artesian borings, which have been made with partial success in various parts of the city.
The bluff formation constitutes a well marked step in the series of quarternary deposites, succeeding the drifter boulder era, and anterior to the recent surface alluvium. This formation, generally of considerable thickness, corresponding to the height of the bluff hills, forms the substratum of the upland prairies. It is composed of a great variety of earthy materials, including finely pulverulent marls, beds of coarse sand and gravel, aggregations resembling hard-pan or pudding-stone, overlaid by a variabel layer of yellow clay, and gradually blending with the present surface soil. These several features indicate this formation as resulting from the deposition of extensive fresh water lakes, having variabel currents and mostly shallow waters. Not unfrequently well excavations bring to view a buried soil of rich vegetable mould now covered up by twenty feet or more of lacustrine deposites, containing fresh water shells. This earlier surface soil supported a rank arborescent vegetation, and is proved by buried remains, to have been the roaming places of the now extinct tribes of the gigantic Mastodon and Northern Elephant. The upper clay in the bluff series, is everywhere extensively used for the manufacture of brick.
A fine sectional view of the general features of this bluff formation may be seen in the cutting along the west side of Harrison street, opposite Sixth street.
It would be interesting, did space allow to present some facts, in regard to the supply of coal in this district, but this must be left for another occasion. It will be sufficient here to state, that the only reliable supply of coal for this section of country, is to be obtained from the Rock River coal basin. This has been recently opened to market by two Railroads, and is successfully worked by three distinct mining companies. The present facilities are sufficient to meet the local demand, and the source of supply is ample for all future wants.