Origin and development of true karst valleys in response to late Holocene sea‐level change, the Transverse Glades of southeast Florida, USA

The Miami Limestone is an oolite depositional body that is used as an analog model for geological interpretation of the rock record. Barrier‐bar complex, oolite banks, extensive bryozoan flats and tidal creeks, referred to as transverse glades, have been described. High‐resolution LiDAR data are used to produce unprecedented, detailed topographic maps of the transverse glades in the southern Atlantic Coastal Ridge. These maps were originally used to calculate historic discharge from the Everglades but revealed features inconsistent with the prevailing theory that the topography is of a depositional origin. Field observations verified an epikarst terrain truncated by collapsed subsurface conduits creating valleys, previously described as palaeotidal creeks. Observations of the 12 southern tidal creeks or transverse glades provided a sequence of six stages in the development of these karst valleys. After Late Pleistocene deposition of the Miami Limestone, sea‐level dropped producing conditions of: (a) epikarst which reduced the surface elevation preferentially, forming the southern Everglades Basin and modifying Biscayne Bay and Florida Bay precursor basins and (b) downward water movement producing vertical solution features. As Holocene sea‐level approached the South Florida carbonate platform margin a freshwater lens formed and groundwater movement became horizontal. Two sets of cavernous zones developed during the Middle and Late Holocene. Solution pipes and dolines provide a connection between the surface and groundwater along alignments which delineate subsurface conduits. Stages of valley formation are associated with the collapse of these conduits first forming a series of pocket valleys followed by narrow blind valleys and half blind or through valleys linking the Everglades to the coastal plain. Valleys then expanded in width until: (a) completion of cavernous zone collapse, (b) most boulder fields and valley margins reduced by weathering and (c) the valleys filled with wetland soils.


| INTRODUCTION
The surface topography of southeast Florida is widely accepted as the original depositional topography, which is reasonable based on the Late Pleistocene age (Halley, Shinn, Hudson, & Lidz, 1977;Hoffmeister, Stockman, & Multer, 1967). However, LiDAR data for the Atlantic Coastal Ridge and coastal plain were collected for storm surge analysis by the International Hurricane Center (Whitman, Zhang, Leatherman, & Robertson, 2003). The processed data are utilized to create the most detailed topographic map and associated elevation profiles of eastern Miami-Dade County, Florida (Zhang et al., 2003;Zhang & Whitman, 2005), for analysis of land loss associated with sea-level rise (Harlem & Meeder, 2008) and as a first step in the calculation of historic freshwater discharge from the Everglades to Biscayne Bay (Meeder & Harlem, 2010). The profiles revealed that topography is not as simple as typical tidal creek or river systems because of the steep, near vertical walls, irregular surfaces, boulder fields and the presence of escarpments and terraces. Field observations confirmed a surface characterized as epikarst interrupted by collapsed subsurface caverns forming valleys. This finding led to several years of further, more intense field investigations on the origin of South Florida topography.

| Previous work
Although karst features of South Florida are described in numerous works (Cooke & Mossum, 1929;Craighead, 1971;Davis, 1943;Duever et al., 1986;Harshberger, 1914;Parker, 1975;Parker & Cooke, 1944;Parker, Ferguson, & Love, 1955;Sanford, 1909;White, 1970), no studies focused on karst have been conducted except for two studies addressing subsurface cavern distribution (Cressler, 1993;Florea & Yuellig, 2007). In contrast to the world renowned north and central Florida karst systems (Fairbridge, 1968), the karst nature of South Florida is not well-known (Cunningham & Florea, 2009). The small scale (<6 m vertical relief), not the diversity of karst features, may be the reason for the lack of interest. The significance of karst in supplying water has been recognized (Parker et al., 1955) and is still the subject of research (Cunningham et al., 2004Cunningham, Renken, et al., 2006;Cunningham, Wacker, et al., 2009;Renken et al., 2005Renken et al., , 2008Sullivan, Price, Schedlbauer, Saha, & Gaiser, 2014). The role of karst in the origin and evolution of the Everglades ecosystem is not addressed in detail although the sedimentary record is fairly well-known (Gleason & Stone, 1994). The development of the Everglades basal marl (calcite mud) resulted from the precipitation of dissolved carbonate from the dissolution of limestone by biophysio-chemical processes (Gleason & Spackman, 1974). The basal marl is an important controlling mechanism in later Everglades ecosystem development (Gleason & Stone, 1994;Sklar et al., 2019).

| Geological setting
Only strata of the Late Pleistocene Miami Limestone composed of oolite, skeletal and minor amounts of quartz sand and silt out crop (Cunningham, Renken, et al., 2006;Parker et al., 1955). Three depositional facies are recognized, namely cross-bedded ooids, bioturbated ooids and bryozoan (Hoffmeister et al., 1967). A distinct disconformity between the Late Pleistocene Miami Limestone and the underlying Pleistocene Fort Thompson Formation, an arenaceous skeletal packstone, is described (Hickey et al., 2010;Perkins, 1977). The historic theory of the origin of the South Florida terrain is that of a depositional origin; the transverse glades had their origin as tidal channels in an oolite shoal (Hoffmeister et al., 1967;Shaler, 1890). The active oolite shoal (present Atlantic Coastal Ridge) developed ca 15 km west of a contemporaneous coral reef, the Key Largo Limestone, that extends as far north as Soldier Key in central Biscayne BayKey in central Biscayne Bay. Further offshore is a recent coral reef that also extends as far north as Soldier Key. Halley et al., (1977) proposed that the outer portion of the Atlantic Coastal Ridge is a later barrierbar complex deposited against the antecedent oolite shoal system. Neal, Grasmueck, McNeil, Viggiano, and Eberli, (2008) documents two depositional units along the barrier-bar complex using ground penetrating radar, the basal mottled oolitic facies and the upper bedded oolitic facies which is subdivided into two depositional units representing oolitic shoal deposits and barrier-bar formation through alongshore progradation. This paper focuses on karst valleys produced by collapse of cavernous zones between the Everglades Basin and coastal plain along the water table surface. Numerous valleys in karst terrains have been reported but none of them were created by karst processes alone. Valleys of karst origin are generally considered non-existent and the term karst valley is considered misleading (Sweeting, 1973). The question of the existence of valleys of true karst origin is based upon two well-established concepts: (a) karst drainage is characteristically downwards and (b) valley formation is generally of fluvial origin involving the erosion and transport of materials at the surface. Valleys in karst landscapes are generally classified as either: (a) allogenic or through valleys, formed by rivers with origins in impermeable rocks adjacent to limestone; (b) blind valleys; (c) half-blind valleys; (d) pocket valleys; or (e) dry valleys which form under conditions of glaciation, permafrost, structural or tectonic activity (Sweeting, 1973). The other geological settings forming valleys in karst regions are not observed in South Florida.

| Purpose and objectives
The purpose of this paper was to document the karst origin of transverse glades by presenting a developmental sequence of morphologies found by LiDAR and verified in the field. The karst origin of the transverse glades offers a new paradigm in contrast to tidal origin. The following objectives are addressed: (a) production of Atlantic Coastal Ridge and transverse glade topography maps; (b) production of Atlantic Coastal Ridge transverse and longitudinal profiles of the transverse glades; (c) production of topographic maps of smaller scale, specific sites; (d) develop an origin for the karst valley model based upon repetitive and sequential topographic profiles; and (e) document the relationship between sea level, freshwater table and karst valley development.

| METHODS
The study area included northern Monroe, eastern Collier and southern Broward Counties, but was focused on Miami-Dade County south of the Miami River. The best outcrops were found along the transverse glade valley walls and on remnant exposures ( Figure 1). Bodies of limestone left behind after erosion were termed remnants. Most of the Atlantic Coastal Ridge was covered by urban development and a great deal of the transverse glade area was in agriculture or lower density housing thus reducing available outcrops to parks and isolated natural areas. More than 67 field sites were investigated, but only 12 sites were used in this report and cover the variation found in the development of karst valleys ( Figure 1 and Table 1).
LiDAR data were collected by the International Hurricane Center for the purpose of studying storm surges along the east coast of South Florida (IHRC {International Hurricane Research Center, Florida International University} 2004). LiDAR data were reduced, removing non-ground measurements and released for public use Zhang et al., 2003;Zhang et al., 2005). These data were used to produce topography maps with >30 cm resolution, far superior to previous topography maps, with precision eliminating the need for most surveying. The site-topography maps produced by LIDAR were used with Atlas GIS software to produce elevation profiles of the Atlantic Coastal Ridge, transverse glades and remnants. These profiles were instrumental in characterizing valley morphology and in the comparison of sites. As many as 12 elevation profiles were made across each transverse glade in order to document topographic trends from the Everglades Basin to the coastal plain. Only one set of transverse elevation profiles are presented as representative of all transverse glades. Karst features such as escarpments and terraces, dolines, hanging valleys and valley margin features were identified. The Atlantic Coastal Ridge longitudinal (long axis) profile was made along the topographic high trend. The longitudinal profiles down the transverse glades established valley bottom topography and slope.
High-quality historic aerial photography (1938), taken prior to most urban development, was utilized allowing observation of many features presently undetectable. Recent aerial photographs were used for easy site location. The source for all aerial photographs was cms.uflib.ufl.edu/maps/collections. Digital cameras were used to document karst features found in the field and for comparison with the LiDAR produced topography. Field measurements consisted of measuring solution feature dimensions for confirmation of LiDAR topography and for correlation of subsurface conduits.

| RESULTS
The results from the different methods employed in this study of karst valley genesis are presented by landscape feature and development stage. This method of presentation was selected for ease of use and a better understanding of the development of karst valleys by collating all supporting evidence for each stage, reducing redundancy by presenting all photographs F I G U R E 1 LiDAR topography map of southeast Florida with the major geomorphic features labelled. The location of the enlarged LiDAR image is delineated in red with a red arrow in the State image. The transverse glades are numbered in black from south to north, 1-12. Major field sites are numbered corresponding to Table 1. Two red circles with arrows indicate that these sites are located off the map in the direction of the arrow the distance indicated. The location of the longitudinal elevation profile down the axis of the Atlantic Coastal Ridge is indicated by the line A-A′. Water bathymetry is indicated by shades of blue from near shoaling (lightest blue) to ~5 m (dark blue) | 561

MEEDER anD HaRLEM
and LiDAR in separate sections then summarizing for each stage. With the exception of historic and aerial photographs, all photographs, LiDAR images produced from source data files and GIS results were original products.

| Atlantic coastal ridge
A topographic map of the Atlantic Coastal Ridge was made documenting an elevation range from 8.6 m above mean sea level adjacent to the Miami River to little more than 1 m in Everglades National Park (Figure 1). The Atlantic Coastal Ridge was segmented by 12 transverse glades in the study area. The topography between valleys was typical uneven epikarst terrain (Klimchouk, 2004) with abundant doline development ( Figure 2). Pillars of outcropping remnant limestone, indicated by very narrow, sharp peaks in the red line between transverse glades, delineated the original depositional surface. Holocene wetland sediments covered both the Everglades Basin and coastal plain on either side of the Atlantic Coastal Ridge (Davis, 1946).

| Transverse glades
Everglades Basin water delivery to Biscayne and Florida Bays was historically determined by transverse glades.
Transverse glades either crossed or nearly crossed the Atlantic Coastal Ridge and appear as numbered green bands running across the tan Atlantic Coastal Ridge ( Figure  1). Transverse glades were irregular in nature and were described in terms of transverse and longitudinal elevation profiles and orientation with respect to the Everglades Basin.

| General characteristics
The Atlantic Coastal Ridge topographic highs and transverse glade floor lows increased in elevation northwards along the profile A-A′ ( Figure 1). Maximum bank full stage was determined from valley wall elevations and indicated by the upper green line (Figure 2). The historic Everglades stage was determined by the highest elevation of wetland soils along valley flanks (dark blue line, Figure  2). The elevation of the Everglades Basin floor (light blue line, Figure 2) was higher than transverse glade floors (lowest light blue, Figure 2).

| Elevation profiles
Considerable valley morphology variability between and along transverse glades was documented based upon  Figure 3A). However, three distinct reaches were found in most transverse glades; the western (Everglades Basin margin), central and the eastern (highest portion of the Atlantic Coastal Ridge). The western reaches of transverse glades exhibited broad, shallow profiles, with an irregular epikarst floor frequently modified by several shallow channels present within the glade ( Figure 3B, Profile A, arrows). The limestone surface was reduced by >1 m from the Everglades Basin floor creating the floor of Transverse glade 2. The central reach exhibited two distinct channels and had steeper walls with a terrace ( Figure 3B, Profile D, arrows). The Atlantic Coastal Ridge reach exhibited a single channel with steep walls ( Figure 3B, Profile F, arrow). The broad and shallow western valley's transition into steeper and deeper valleys provided evidence of valley wall maturation and further documented valley development towards the east. Some valleys were compound with two steep-walled channels that exhibited lateral expansion by collapse and epikarst deterioration of the limestone ridge separating them, as well as, along the outer valley walls ( Figure 3C). These individual channels developed over separate groups of subsurface conduits with the same floor elevations (between 1 and 1.3 m) and were easily observed along the Deering Drain valley floor walls as discontinuous cavernous horizons at the base of escarpments. This process resulted in irregular valley floors covered with boulders and remnants as observed by the very uneven elevation profiles indicative of high relief ( Figure 3D).

| Longitudinal elevation profiles
A high degree of variability in transverse glade slope and continuity was documented by the longitudinal elevation profiles ( Figure 4). Slopes varied from a continuous slope from the Everglades Basin to the Bay (Transverse glades 3, 4 and 8), sloping to the Everglades from the east (Transverse glades 9 and 11) or were compound, with reversals of slope along the course (Transverse glades 2, 5, 6, 10 and 12). Eight of the 12 transverse glades terminated prior to reaching the coast. These transverse glades were termed half-blind valleys as they have one topographic open end, in contrast to blind valleys which were closed at both ends forming a closed contour (Sweeting, 1973). The elevation of transverse glade floors, at their head at the eastern edge of the Everglades Basin, was 1.8 m in the north and decreased southwards to ca 1.2 m. The elevation at the coastal plain ranged between 1.2 and 0.9 m, respectively. Variation in slopes included: continuous slopes from the Everglades Basin to the Bay (Transverse glades 3, 4 and 8), slopes from the Coastal Ridge to the Everglades Basin (Transverse glades 9 and 11), or were compound, with reversals of slope along their courses (Transverse glades 2, 5, 6, 10 and 12).

| Orientation of transverse glades
The transverse glades exhibited a general NW-SE orientation along the Atlantic coastline and trended in more southerly directions in the westernmost Atlantic Coastal Ridge reach ( Figure 5). Most transverse glades appeared to radiate outwards from the centre of the southern Everglades Basin.

| Documentation of karst valley development
A six-stage conceptualization of karst valley development was formulated ( Figure 6 and Table 2) and described and discussed in chronological order (Figures 7-12). Each developmental step in each stage has been located on the model ( Figure 6) by a corresponding numbered dashed line box. Field, (2002) and Sweeting, (1973) provided karst definitions.

F I G U R E 4
Longitudinal elevation profiles were made along the bottom of each transverse glade from the Everglades Basin (left) to the coastal plain (right). Transverse glade (TG) numbers correspond with locations presented in Figure 1 and

| Stage A
Alignments of dolines and solution pipes were identified and located over subsurface conduits ( Figure 6A). The relationship between the dolines and solution pipes with subsurface conduits was not observable in expansive wetlands because of sediment burial and overlying scrub cover. However, this relationship was confirmed in uplands such as the Castellow Hammock and Camp Owaissa Bauer ( Figure 1, Sites 8 and 10) and exposed in later stages of development after collapse of subsurface conduits. Horizontal conduits developed with a slight dip towards the coast. Such alignments of dolines were found throughout the Big Cypress Swamp (Duever et al., 1986), Southeast Saline Everglades (Ross, Sah, Meeder, Ruiz, & Telesnicki, 2013) and the central and southern Everglades Basin and were usually filled with Holocene sediment and identified by the overlying tree islands ( Figure  7A). Dolines were observable as topographic depressions throughout the eastern Everglades Basin and Atlantic Coastal Ridge. Both solution and collapse dolines were very common in linear alignments; some dolines exhibited radiating fractures ( Figure 7B). Subsurface cavernous zones were not readily observed in Stage A and ranged from simple ovate shaped conduits like those reaching Biscayne Bay ( Figure 7C) to wide, irregular stratiform cavernous horizons, usually with little headroom, best observed at the base of collapse valley walls ( Figure 7D). Alignments of collapse dolines were also found on large remnant limestone outliers ( Figure 7E). The profile (A-A′) down the axis of aligned dolines displayed the epikarst surface and depths and distribution of dolines over the conduit ( Figure 7E and F). Doline floor elevations varied little (0.7-1.3 m) with most variation the result of the presence of boulders or minor sediment fill. Foot caves were visible in many instances.

| Stage B
Expansion of dolines and coalescence into elongate, frequently barbell shaped, pocket valleys established Stage B valleys from their precursors ( Figures 6B and 8A). Stage B valleys were long, narrow, discontinuous steep-walled valleys with floors covered by large boulders ( Figure 8A). The development of fractures between solution pipes and dolines was common ( Figure 8B). A profile ( Figure 8C) displayed valley floor roughness ( Figure 8A, A-A′). Coalescing dolines were easily separated from individual dolines by their greater width and topographic irregularity. An elevation profile displayed the steep collapse walls and the deep epikarst at the surface ( Figure 8D). Horizontal cavernous zones were first observed during this stage as foot caves ( Figure 8E), in many instances buried under collapse boulders and debris ( Figure 8F).

| Stage C
Further collapse resulted in widening and coalescence of pocket valleys forming longer, steep-walled half blind to blind Stage C valleys ( Figure 9A). The Snapper Creek site exhibited Stage C valley characteristics broadest at the Biscayne Bay shoreline  with a spring head, the origin of Snapper Creek, rapidly reduced width westwards, disappearing at the Atlantic Coastal Ridge into to a series of dolines connected to subsurface conduit. Snapper Creek developed in the opposite direction of most valleys. The floor was near sea level. Collapse occurred either evenly in two directions or in one direction more than another ( Figure 9A, north-east side of valley shown in the lower southeast part of the figure). Immature valleys terminated at a series of collapse dolines ( Figure 9A, north of the coalescing pocket valleys) or at swallows ( Figure 9B). A swallow is where surface water enters the subsurface draining a pocket or half-blind valley as a point source, usually a solution hole. Major transverse glades either crossed the Atlantic Coastal Ridge, were terminated at the outer ridge of the Atlantic Coastal Ridge as swallows (Figures 1 and 4), or changed course to disappear at swallows ( Figure 9B). Arches were occasionally still in place where two pocket valleys were incompletely coalesced ( Figure  9C). Hanging valleys were developed along older valley walls.
Hanging valleys resulted in deterioration of older valley walls and valley expansion by sudden collapse ( Figure 9D) or by collapse of a honeycomb of high density, small-scale solution pipes ( Figure 9E). The Snapper Creek outlet was a series of large spring heads ( Figure 9F).

| Stage D
Dissolution and collapse of rock above the cavernous horizons resulted in development of terraces and escarpments between cavernous zones and above the upper cavernous horizons along widening valley walls ( Figure 10A). Three near horizontal surfaces (the transverse glade floor, lower and upper terraces) were visible. An escarpment between the upper two terraces was slightly more than a metre in relief ( Figure 10B). Each terrace had an irregular surface from solution of limestone, solution holes and the presence of remnant boulders. Terraces developed as limestone above a cavernous zone were lost by dissolution and collapse creating a flat area littered with boulders. Hanging valleys along the main valley margins became common in some areas ( Figure   10C, Inset A documented two hanging valleys). These hanging valleys were younger than the main valleys and were also formed by collapse ( Figure 10D). Hanging valleys in their early development were suspended above the main valley floor but eventually developed to the lower cavernous zone, the floor of the main valley ( Figure 10E, A-A′). A collapse doline was displayed at the head of this valley. Many hanging valleys had the appearance of swales, as found in ridge and swale complexes associated with barrier island depositional environments ( Figure 10F). These valley floors developed a series of solution holes and dolines, perhaps originating in precursor swale topography ( Figure 11A). Stage D valley floors ( Figure 11B, Profile B-B′) were uneven, the roughness less than in Stage C valleys but more than found in later stages. Valley width expanded as the steep walls continued to fracture ( Figure 11C) and collapse ( Figure 11D). This was the first stage when valleys occasionally traverse the entire Atlantic Coastal Ridge. Most valleys however were blind or half-blind, many ending at swallows prior to reaching the coastal plain (Figure 4). Stage D valleys were frequently compound valleys displaying two parallel channels merging into a single valley with maturation ( Figure 3B, Profile D; Figure 3C and D, profiles A-D). Valley walls at this stage were very irregular ( Figure 10C). The area between the two valley channels consisted of remnant outcrops, boulder fields and exposed upper cavernous zone.

| Stage E
Valleys that had widened and deepened to near their full potential or reached near total collapse of the underlying cavernous zones were considered mature, Stage E valleys. Valley walls were much less steep with less relief. Reduced relief was the result of valley floor elevation rising in response to wetland sedimentation and the final stages of wall collapse and continued epikarst development ( Figure 12A). Hanging valleys had broadened, in many areas greatly reducing the valley wall height and slope ( Figures 9E and 12B). Occasional large remnants were found a metre or more in elevation above the valley floor ( Figure 12C). Hardwood hammocks developed on these isolated remnants surrounded by wetland soils. Valley floors are more level with boulders only observed along the valley walls. Some wetland sediment filling had begun as the valleys were wide enough that shading from trees no longer prohibited wetland vegetation growth and marl or peat soil accumulation. Terraces and escarpments were welldeveloped along valley margins or were almost unrecognizable as they decomposed by dissolution and filled with organic debris ( Figure 12D).

| Stage F
Continued erosion and valley filling resulted in reduced valley relief in the final stage of valley development. Most transverse glades exhibited some sediment filling along reaches, especially in the west, and others were filled from wall-to-wall with marl ( Figure 13A). Surface erosion frequently left behind small scale but high relief pillars ( Figure 13B). Epikarst processes reduced the valley margins in both elevation and relief burying the lower caverns with boulders, organic debris, sand and marl ( Figure 13C). At this stage rapid, continued valley development became a localized activity. Cessation of active karst processes and sediment filling began at the margin of the Everglades Basin and developed eastwards because valley formation first began at the edge of the Everglades Basin and moved eastwards with groundwater. The slope of the valley wall generally decreased with increased age of the valley.

| Atlantic coastal ridge
The Atlantic Coastal Ridge is of depositional origin (Halley et al., 1977) the question is how much of the present topography  Figure 8A, line A). The Everglades Basin development was by limestone dissolution at the expense of much of the original Atlantic Coastal Ridge (active shoal) limestone body. Limestone loss was at the surface, one layer after another, as well as in the subsurface. Limestone remnants in many cases exhibit the loss of four to five distinct depositional strata in a step-like manner, perhaps analogous to escarpments and terraces at a larger scale along valley walls ( Figure 10B) documenting the lowering of the basin by 1.5 m or more (Figure 1, Site 7). The base of many remnants in the Everglades Basin approached the average depth of the Everglades Basin. The present Atlantic Coastal Ridge also extended further east under Biscayne Bay to a distance of at least 1,000 m from the western shoreline (Byrne, 1999). The presence of this oolitic limestone has not received attention. Cross-bedded strata overlie mottled strata along the Atlantic Coastal Ridge (Neal et al., 2008). Because crossbedded sediments grade westwards into mottled sediments, the presence of the lower mottled strata suggest the presence of cross-bedded strata to the east based upon Walther's Rule. However, the only oolitic sediments present east of the present Atlantic Coastal Ridge is the thin strata found under Biscayne Bay suggesting considerable transport and erosion. Hoffmeister et al., (1967) first postulated oolitic limestone loss from in front of the Atlantic Coastal Ridge. Cross-bedded oolitic limestone remnants up to 1.5 m in height above the background elevation of 2.5 m are found in the eastern Big Cypress National Preserve and further document the loss of considerable limestone (Duever et al., 1986). Therefore, the present narrow Atlantic Coastal Ridge is the remnant of a precursor, broader, active oolite shoal both to the east and west and the entire Everglades Basin has broadened and deepened through karst processes. The Atlantic Coastal Ridge is made up of two ridges subparallel to the coastline, best defined north of Site 6 ( Figure  11). The outer ridge is interpreted as a barrier-bar system (Halley et al., 1977). In some areas along the Atlantic Coastal Ridge, ridge and swale deposition occurred (Figure 1, Site 3) which focused karst processes to the swales resulting in development of a series of dolines forming early stage karst valleys. These valleys, with near vertical walls and floors littered with collapse boulders, highly alter the depositional topography. These early stage karst valleys intersect larger karst valleys forming hanging valleys which expand by collapse resulting in the expansion of valley length and width.

| Transverse glade
The abundance of data suggests that the transverse glades are karst in origin or have been so modified by karst processes as to have removed all morphologic evidence of tidal origin. Original tidal channels have been removed by surface erosion of up to 3 m of the upper Miami limestone. Everglades Basin documenting a minimum loss between 2 and 3 m based  Figure 10D). The relief along the valley wall is <35 cm, marl sediment is in the foreground and organic litter is beginning to accumulate. Lateral valley expansion by collapse ceased as the terminus of the cavernous system was reached ( Figure 6F, Inset 25) | 571 MEEDER anD HaRLEM upon remnant outcrops within the Everglades and in the eastern Big Cypress Swamp (Dutton, Carlson, Long, Milne, & Raymo, 2015).

| Large scale observations
The Atlantic Coastal Ridge is dissected by large karst valleys, transverse glades, that are in most cases half-blind valleys (Figure 1). Nearly 35% of the Atlantic Coastal Ridge length is actually occupied by low-lying transverse glade topography ( Figure 2). The association between foot caves and associated collapse features with the transverse glade valley walls suggests that the transverse glades are of karst origin.

| Transverse elevation profiles
Transverse glades generally have the flattest, widest profiles in their western reaches adjacent to the Everglades Basin ( Figure 1). The transverse glades deepen and narrow towards the axis of the Atlantic Coastal Ridge. Usually along a central reach, two channels can be found in the transverse glade valley. Valley wall slopes are highly variable but, in general, are steepest in the reaches through the highest areas of the Atlantic Coastal Ridge. Valley walls are frequently step-like because of the presence of terraces. Tidal creeks do not form terraces and escarpments. The changing elevation profiles suggest that the karst valleys originated to the west and developed towards the east. The lower relief profile indicates a more mature and more reduced surface ( Figure 4B

| Longitudinal topographic profiles
The variability in longitudinal profiles is a response to karst valley maturity and is not the result of tidal action process over a shoal ( Figure 5). Termination of valleys at sinks occurs in eight of the 12 transverse glades studied. Arches or evidence of past arches are found at several localities as well as along Arch Creek, to the north. The floors of the transverse glades generally are at 1-2 m above msl, decreasing southwards at the margin of the Everglades Basin and about 0.9-1.2 m at the coastal plain. This coincides with the elevation of the lower cavernous zone. Ten out of 12 transverse glades slope towards Biscayne Bay the tidal source, in contrast to modern ooid shoals that slope towards the platform interior, (Harris & Purkis, 2019;Hoffmeister et al., 1967). Additionally, no transverse glade exhibits tidal deltaic deposits at their terminus.

| Orientation
Transverse glades form a radiating pattern from the axis of the Everglades Basin ( Figure 5) following the original groundwater movement from the Everglades Basin towards base level and forming along groundwater flow lines as conduits collapsed. A few small scale linear depressions run from NE to SW and form hanging valleys along a few transverse glades and may be the remains of tidal creeks ( Figure  8A, dashed line D). Most of these possible tidal channels are unavailable for observation because of urbanization.

| Scale
Tidal creeks in most ooid shoals are less than a few metres in depth with respect to the top of the active shoal. Since limestone loss of up to 3 m can be documented in large areas and up to 5 m locally, it is assumed that evidence for most of the active tidal channels is now missing. In addition, the elevation of the top of the active shoal is usually within a few metres of that of the adjacent inactive shoals, whereas the difference in elevation in the Miami Limestone is as great as 6 m. The widths of tidal creeks in modern oolite shoals vary with the setting, but usually do not make up 35% of the surface area of a shoal as transverse glades do along the Atlantic Coastal Ridge (Harris & Purkis, 2019). In contrast to transverse glades, tidal channels in modern oolite shoals slope towards the interior basin and produce sub-tidal sediment deltas (Hoffmeister et al., 1967).

| Karst valley origin and development
Six developmental stages of the karst valleys are presented ( Figure 6). Most individual karst valleys exhibit two or three stages of development along their reaches; however, the sequences of development are repetitive between transverse glades. The development of karst valleys is demonstrably tied to prior development of shallow subsurface cavernous conduits connecting the Everglades Basin to the coastal basins. Sea level, geology and freshwater supply determines the position of the freshwater lens which control cavern development.

| Origin of cavernous horizons
The karst development of southeast Florida topography is tied to freshwater availability and sea level which controls the carbonate platform groundwater table (Figures 14 and  15). Freshwater is the agent of dissolution and in limestone regions rapidly infiltrates bedrock moving downward to the water table. Two sides of the Biscayne Aquifer are exposed along the platform margins to ocean conditions. The Biscayne Aquifer is an eogenetic karst environment (Cunningham, Renken, et al., 2006;Vacher & Mylroie, 2002) which is represented by the 'simple island' model (Jenson et al., 2006). Carbonate island karst develops in young, relatively unaltered limestone units with high-primary porosity. The Miami limestone has relatively high primary porosity (ca 25%) which has increased through dissolution and diagenesis to >40% exhibiting an extremely high transmissivity (Fish & Stewart, 1991). Carbonate island karst is further defined by the effects of differential dissolution associated with the contacts between vadosefreshwater phreatic groundwater and fresh-marine water mixing zone. Caverns are best developed along the vadosephreatic contact where fresh rain water is under saturated in terms of Ca (Plummer, 1975). Solution conduits are used to indicate palaeo sea-level positions (Mylroie & Carew, 1988). The migration of the mixing zones in response to glacio-eustatic and tectonic variations in relative sea level is recorded by the elevation of cavernous zones developed during corresponding sea levels. The freshwater lens is elevated by the denser sea water, therefore the upper surfaces of cavernous horizons are slightly elevated above the groundwater sea-level stand during which they developed and have a slight seaward dip in areas with a significant freshwater lens. The timing of cavern development is based upon: (a) rapid Late Pleistocene sea-level regression; (b) sea level never reaching elevations high enough to affect platform near surface water table during the Early Holocene; (c) very minimal debris or sediment filling; and (d) a lack of 'dripstones' or other cave formation development suggesting immature Middle and Late Holocene caves. The timing of cavern development is linked to the periods of time when the groundwater surface was above present sea level.

| Geological history
The six stages of karst valley development are related to changes in geological environments from the original F I G U R E 1 4 Key hydrologic conditions for the development of South Florida topography and karst valleys. (A) Post-depositional low sea-level stand (11,700 yr bp) and the Late Pleistocene (120,000 yr bp) depositional surface. The Miami Limestone (orange) is divided by a line representing the present land surface and the postulated overlying post-depositional surface. Rainfall (light blue lines above illustrations) infiltrates the porous bedrock and travels vertically as groundwater (subsurface light blue arrows). Groundwater moves along bedding planes and exposure horizons prior to reaching the low water table associated with the last glacial low sea-level stand. (B) Middle Holocene (6,000 yr bp) sea-level rise and inundation of the Everglades Basin. Sea level at the beginning of the Middle Holocene was within 6 m of present sea level which was sufficient to raise platform groundwater to the point of Everglades Basin inundation (light blue) and wetland marl deposition (black). Present day sea level is represented by 0 in the elevation scale bars. Prior epikarst processes resulted in the development of the Everglades and Biscayne Bay basins and reduction of surface along the Atlantic Coastal Ridge as suggested by limestone remnants (Note the limestone loss in A). Groundwater movement became near horizontal (light blue arrows) as the freshwater lens developed over rising denser sea water. Horizontal groundwater movement along the vadose-freshwater phreatic boundary resulted in intensified dissolution and cavern development from the base of the Everglades Basin thru the Atlantic Coastal Ridge to the coastal plain (light blue dashed arrow labelled 1). (C) Continued sea-level rise and resultant historic high freshwater stage. As sea-level rose to near present levels, the freshwater stage rose to its historic high of 3 m above present sea level (light blue) in the Everglades Basin. A second cavernous zone (light blue dashed arrow labelled 2) developed along the new higher vadose-freshwater phreatic boundary) depositional setting followed by a major post-depositional drop in sea level exposing the carbonate platform. The Holocene transgression began 11,700 yr bp raising the platform water table until the maximum stage was reached. Karst valleys developed during the latter 6,000 yr bp prior to drainage which began in ca 1880 (McVoy, Said, Obeysekera, Van Arman, & Dreschel, 2011) lowering the water table and exposing many karst features.
Sea level was at ca 8 m above present during the last interglacial ca 120,000 yr bp inundating the entire platform during deposition of the Miami Limestone strata (Moore, 1982) although this elevation does not consider the lost elevation of the Atlantic Coastal Ridge. There must have been a strong cross-bank current analogous to the 'Quick Sands' area (Shinn, Lidz, & Holmes, 1990) and the mid Pliocene (Klaus, Meeder, McNeill, Woodhead, & Swart, 2017). Currents were, at least episodically, sufficient to produce cross-bedded ooid sets 30-80 cm thick west of the Atlantic Coastal Ridge and to transport ooids into Collier County (Figure 1, Sites 6,  8). The ooid shoal developed behind a fringing coral reef, the Key Largo Limestone (Hoffmeister & Multer, 1968). The ooid limestone body begins in northern Broward County and extends ca 34 km west of Florida City, the quartz sand content rapidly decreases southwards.
Sea-level dropped rapidly to >120 m below present after deposition, not to rise higher than 6 m below present until approximately 6000 yr bp (Wanless, Parkinson, & Tedesco, 1994). The dominant post-deposition processes were epikarst and vertical dissolution processes with minor development along bedding planes ( Figures 14A  and 15). During this time interval, the Everglades Basin and Biscayne Basin formed by epikarst processes and the Atlantic Coastal Ridge was reduced in width and height (all limestone above the dark line within the Miami Limestone is lost, Figure 14A). There was little standing water although rainfall may have been abundant because of rapid downward percolation.
Sea level was at least 120 m below present during glaciation and epikarst dominated the Florida Platform between deposition and ca 6,000 yr bp when the rate of sea-level rise slowed and when sea-level approached the platform margin (Robbin, 1984). This period of surface erosion lowered the limestone surface everywhere and produced the Everglades and Biscayne Bay basins ( Figures 14B and 15). Rising sealevel elevated the freshwater lens inundating the Everglades Basin providing a marl depositional environment (Gleason & Stone, 1994). The marl soils deposited are a product of precipitation of calcium carbonate from dissolved limestone (Gleason & Spackman, 1974). The marls form a leaky seal on the Everglades floor ( Figure 14B) slowing water infiltration and storing water, increasing the hydroperiod and providing an environment suitable for peat deposition which started ca 4,500 yr bp (Gleason & Stone, 1994) at elevations between 1 and 1.3 m above present sea level (Wanless et al., 1994). Groundwater flow patterns became horizontal, moving south and eastwards resulting in the development of complex conduits, or horizontal cavernous zones, along the vadose-freshwater phreatic contact forming a slight seaward dip along the upper surface of the freshwater lens. This surface and the subsequent cavernous zone ran from the base of the Everglades Basin to ca 1 m above sea level along the seaward margin of the Atlantic Coastal Ridge, a drop of ca 1 m with respective elevation increases northwards.
Early observations document the Everglades' stage was at 3 m inland from Coconut Grove (Shaler, 1890). The historic high-water stage occurred prior to drainage when the water stage was between 0.6 and 2 m higher than present in the study area (McVoy et al., 2011;Parker, 1975;Parker et al., 1955). This high stage was the product of abundant rainfall F I G U R E 1 5 Relationship between sea level, platform margin, freshwater table and cavern development. The dark blue line is the late Holocene sea-level curve (modified from Wanless et al., 1994). The freshwater stage or table of the platform is in light blue. This curve is developed from radiometric dates of coastal and Everglades Basin wetland soils (Gleason & Stone, 1994). The timing of five major events is indicated by circles. Periods of epikarst development (red arrow), vertical (green arrow) and horizontal (orange arrow) groundwater movement are indicated along the top. The major periods of cavern development are indicated by dashed lines. The period of karst valley development is indicated by the dashed arrow. The sealevel and groundwater table curves nearly parallel one another after 6,000 yr bp with ca 4 m of separation 6,000 yr bp and ca 3 m 1,000 yr bp and sea-level rising to within 1 m of present mean sea level 3,000 yr bp at which time the rate of sea-level rise declined to <1 mm yr −1 (Figures 14C and 15). During this short interval of 3,000 yr, the upper cavernous zone developed ( Figure 14C, dashed light blue arrow labelled 2) by the same process as the earlier, lower cavernous system ( Figure 14B, dashed light blue arrow labelled 1), at both a higher sea level and freshwater table. The maximum Everglades stage first occurred ca 1,200 yr bp (Brooks, 1974) and existed until drainage defining the period of most intense upper cavernous horizon development. The upper cavernous zone is not as well-developed as the lower cavernous zone, probably because the lower cavernous zone was still active during upper cavernous zone development, pirating under saturated water.
Karst valleys formed by collapse of shallow subsurface groundwater conduits. These conduits developed along the vadose-freshwater phreatic boundary forming cavernous horizons. These conduits presumably developed along groundwater paths of least resistance from the Everglades Basin to the coast. Solution pipes and dolines are found in lineaments overlying the subsurface conduits. It is difficult to pinpoint the timing of collapse and valley development, but it is soon after conduit development. Prior to karst valley development, the southern Everglades stage was controlled by southward surface flow and eastward groundwater flow through the Atlantic Coastal Ridge. Groundwater flow supported scores of springs along the Atlantic Coastal Ridge, coastal plain and under Biscayne Bay. After karst valley development, the Everglades stage was controlled dominantly by surface water discharge through the karst valleys and secondly, by groundwater. Water flow via the subsurface conduits occurred until drainage in the 20th Century. Based upon weathering of clasts (decreased size and angularity) and the presence of large mature trees ( Figure 9C), the period of active collapse has passed. The following three patterns of cave collapse are recognized: (a) the uppermost strata collapse into the upper cavernous zone forming a terrace; (b) the entire rock section collapsing into the lower cavernous zone forming vertical collapse walls; and (c) the intermediate rock strata collapse into the lower cavernous zone with the upper strata forming the ceiling to a cavern or arch. The floors of most solution pipes are at the same elevation as the floor of the lower cavernous zone.
Drainage of the Everglades has exposed both sets of cavernous zones, now separated by between 1 and 2 m of rock. Exposure of caverns has created air pockets potentially rejuvenating collapse over existing cavernous horizons.

| Review of karst valley development stages
The documentation and description of the six stages of karst valley development are presented (Figures 7-13) and used to produce a model ( Figure 6). Karst valley development is summarized in Table 2. Karst valleys developed as subsurface caverns formed along palaeowater tables collapsed, collapse usually beginning at the Everglades Basin margin then progressing eastwards. The majority of karst valleys never reached Biscayne Bay but disappeared into swallows. These valleys originated entirely by karst processes.

| CONCLUSIONS
The Atlantic Coastal Ridge is highly modified by epikarst and subsurface collapse karst processes. Up to a few metres of upper strata has been removed and much of the western Atlantic Coastal Ridge reduced to form the Everglades Basin. Valleys that cut across the Atlantic Coastal Ridge were formed by the collapse of conduits between the Everglades and coastal basins. Epikarst and collapse of shallow subsurface karst have altered the depositional landscape and freshwater delivery from the Everglades to the coastal basins. Valley formation takes place in six described stages, from the origin of subsurface conduits to their collapse and later sediment filling ( Figure 6). The best evidence of karst valley origin is the collapse nature of valley walls, exposure of subsurface conduits, the presence of arches indicative of incomplete collapse, the presence of terraces, sinks located at the blind end of many valleys, orientation and profiles inconsistent with tidal creek morphology. No evidence for the tidal origin of the transverse glades was found with palaeotidal channels possibly removed along with the upper 2-3 m of limestone by dissolution.
These karst valleys have formed within the past 6,000 yr in response to rising sea level and a corresponding rise in the freshwater table. Cavernous horizons with a slight dip seaward formed along the vadose-freshwater phreatic contact at two different intervals of sea level. These cavernous zones, a complex network of conduits, radiated outwards from the Everglades Basin axis towards base level. Their collapse and evolution into valleys changed the water delivery patterns to the near shore environment and controlled the Everglades stage.